Is it a mag loop? Lets find out!

Here is a preliminary review of a recent antenna toy acquisition thanks to some spare Amazon points.

If this antenna proves to be a good option for MF waves instead of EF waves, this is likely going to become a popular product pretty quick. Let's dig in here, shall we?

MF, EF - What is it? A round antenna duck?

Here the antenna is on Amazon and it comes in BNC, SMA and SMA-J variants so it will pretty much work with most radios if you buy the right one.  

The vendor on Amazon does not call this a magnetic loop and I have not taken this antenna apart to see if a hidden capacitor is in that black bubble at the top of the loop but I suspect there is.

For me, I chose the BNC connector antenna version since using it with an adapter like the ones shown in the "Portable RF Connector Emergency Kit" are always on hand.

Where to find this?   Look on Amazon

How does this compare to the original factory antenna?

The radios tested with the HYS antenna pictured include the Alinco DJ-MD5XTG, the Kenwood TH-D74 and the Lanch HG-UV98 all using the original factory antennae.

A second round of testing was done with all three same radios but with the same aftermarket Diamond SRH320A tri-band antenna.

The test scenario was to communicate via a 2m repeater 30 miles away with all three different antenna options and at all available power levels for each radio. 

The results were from an audible report from different users on the repeater without telling them what I was doing over the span of a few hours on different days.

Results of Test: Non- Scientific you say?

The reason why I chose to use different radios is that different antennas will work differently based on the radio mass offered as the reflective or counterpoise part of the antenna system. What?   Here, look into counterpoise or "Tiger Tail" if you must learn more...

This test allows us to see more uniform tests in a real world environment instead of lab style results.

For each transmission, the radio was held the same way and at the same height.

Signal reports in green were all positive and as good as one could expect to come from an HT 30 miles from a repeater. Yellow reports included comments of fading or weak signal but still audible.  The red reports were scratchy or not very pleasant sounding.

At first pass, we can say that the HYS "Loop" antenna and stock antenna are all pretty much about the same results.  As expected, the longer and higher "gain" Diamond antenna worked the best.

Now with some baseline curiosity under control, lets do something more scientific in terms of radiation patterns and reception tests.

Is this an EM or RF antenna?

To test if the HYS functions in the "electro-magnetic" wave or the "electric" wave function,  I used a field strength meter to check radiation patterns of all three antennae at three distances outside in my backyard away from anything before it snowed this morning.

Based on the test data as expected, stronger signals from the stock and Diamond antenna were strongest when they were vertically oriented which matched the polarity of the sense antenna used with the field strength meter.   

When transmitting with the radio held horizontally, the field strength meter dropped to more than 70% less than what was shown in the vertical test.

Here is the interesting thing with the HYS antenna though. No matter which orientation (horizontal or vertical) the same signal strength was recorded at the field strength meter.

MATH TIME  (1005/146.500 MHz = 6.8 * 12 = 82.3 inches)

Rotating the HYS antenna to have the edge of the hoop or broadside of the hoop aimed in phase with the sense antenna did not show any major differences unless I moved to the furthest test location which was five full wavelengths  away from the sense antenna.  

At the one and three wavelength distance, there was no major difference with all three radios transmitting at highest power levels.

What does that mean?

I think it is safe to say that the polarization of the HYS antenna does not conform to the normal EF wave portions and this seems to function like a true magnetic loop when used for transmitting since its polarity and field strength are very different.

For reception, there is a very sharp null off the edge, but it is very sharp.  Testing this with an attenuator built in to the Kenwood TH-D74 while tuned to the local repeater or NOAA Weather station which are known locations was the only way I could tell there is a sharp null.

Magnetic waves work differently compared to electric waves.  When a true EM or MF antenna is used, signals received may be lower, but with less noise.  This means that the "signal to noise" radio is higher and better for reception usually.   This is what makes a true MF antenna interesting.

This  becomes important especially for HF frequencies where there is more man made noise than ever.  Even still, today more man made noise also happens in the VHF spectrum and this small loop might come in handy. 

If you want to learn about EM antenna, especially for HF, use your Google subscription to discover more about that. Many options exist, including overpriced commercial products and many DIY projects that are fun to look at if you are interested in this for HF spectrum communications.

Further Testing Needed

There are not many people local to me involved in casual non-contest 2m SSB or FT-8 activity to see how well this will work as a portable antenna for SOTA with the Icom IC-705, so that will have to wait.  

Some testing on APRS however has been interesting but will need to drive around with the antenna on the roof for more testing too.

Overall, I think this antenna is worth a purchase if you have some Amazon points or other disposable funds anxious to leave your ownership. 

Your mother told you NOT to stare, right?  Here are some of the things used during the test
(Source:  Steve Bossert K2GOG)

Basic Overview: Tracking Radio Signals

Finding the source of a radio signal is sort of a fun thing to do for the modern electronics hobbyist.

Confirming the location of a local FM band music broadcaster, amateur radio station or even your cell phone and maybe your car keys is possible with relatively inexpensive equipment since they all transmit RF energy!

With the much anticipated KerberosSDR nearing availability, which will provide unique radio based location capabilities.

Something unrelated recently reminded me to go back to have a look at RTLSDR_Scanner and its latest developments since it offers a lot of great functionality for those interested in radio signal location options available today.

Both solutions use a software defined radio or SDR rather than a traditional radio. This is one reason that makes both of these solutions really exciting!

The conclusion of this article will show you how to generate a map such as the one below that will show signal/power measurements called an "RF Heat Map" using RTLSDR_Scanner.

Kerbo What?

The well funded IndieGoGo campaign of the phase coherent software defined radio that Carl Laufer of the excellent RTL-SDR.com blog, the design team behind the now named Othernet Project and Tamás Pető who is studying electrical engineering at Budapest University.

This trio has created something that will be able to do some amazing things, but let's first take a look at basic radio signal location theory and another modern radio signal location tool.

What is phase coherence?

Let's first look at some basics of radio signal location through some easy to understand math.

If two people (1 &2) knew exactly how far apart they were  (A) and had directional antennas that could help find the maximum signal strength of a transmitter (3), they can use the angles of the signal direction and the known distance between them (A) to guess pretty closely on the approximate direction and distance (B&C) of the signal source (3).

This method of signal location is called triangulation.Radio signals travel at about the speed of light which is about 982 million feet per minute. 

If our two friends standing at location 1 and 2 had identical and synchronized clocks and had radios tuned to the frequency of the transmitter at 3, they could also determine the direction of the signal by moving around a little to see how the signal strength fluctuates, they could also determine the general location of the transmitter with a little more help by use of doppler theory.

Doppler works by sensing how a received signal's frequency (2) fluctuates up or down based on the speed it is traveling and how long it takes to go from source to receiver (1). with multiple antennas (A,B, C, D) .

If multiple receivers/antennas are used at the same time, the difference in time it takes to be received at each can be used to calculate a direction of the signal.

Some form of very fast analog or digital computing and comparison that is part of the receiver is needed since we are talking about nano or millisecond differences.

Phase coherence combines theory behind the speed of which radio signals travel against a known time source along with triangulation in order to find where a signal is coming from.

Police departments have been using a phase coherence system to locate stolen cars for many years called LoJack.  

If you look closely at the roof of the New York State Police cruiser above,  the four antennas on the roof spaced in a square pattern roughly about a foot apart help perform the triangulation and time difference of arrival (TDOA) measurements to quickly locate the stolen vehicle. 

All the police officer generally needs to do is view a display not too different from a no longer in  business company that offered a device called the Ramsey DDF-1 Doppler Direction Finder.

The doppler method of signal location involves the ability to visualize the arrival angle of signals relative to one another of equally spaced antennas through the use of a series of LED lights spaces in a circle pattern.

A special circuit compares the received signal strength at each of four antennas relative time or direction of travel in order to give the direction towards the signal

As the police car travels, the LED lights would blink in the direction where the signal is coming from, if it is in one location. This would tell the officer (or amateur radio operator) which way a signal was coming from and they could try to get close to its location.

Once close enough to a signal, other methods could be used such as a field strength meter to find the smallest of hidden transmitters or chopped out hidden LoJack units. 

HVDN Field Strength Meter Monday is coming back. 

Here is where it started and where it is headed next

Twenty or more years ago, doppler based analog solutions were almost as fancy as one could get in locating transmitters.

In 2018, however, things have come a long way thanks to SDR and even embedded computing devices like the raspberry pi, which can also be used to run RTLSDR_Scanner and the KerberosSDR.

Heat Maps and GPS

The RTLSDR_Scanner application is a little more simple than KerberosSDR. An inexpensive software defined radio (SDR) receiver along with a GPS USB dongle can be purchased together for less than $40 USD.

KerberosSDR, is essentially four SDR's combined into one unit, so should also be able to use the RTLSDR_Scanner software too.

The SDR and GPS along with one antenna, a computer (laptop or Raspberry Pi) and the appropriate software can perform some interesting signal location applications.

GPS provides the function of providing accurate location of the receiver along with a stable time reference.

The locations coordinates, exact time and signal strength of the signal can be combined to provide stunning visualizations of how strong or weak a signal is on mapping programs such as Google Earth

Lets get SDR_Scanner working

In 2016, this program was only operational under a linux computing environment, so was not easy to set up unless you were very involved in computer "stuff".

This program has been available as a much easier to install Microsoft Windows version since last year and is finally what this article is about!

You will need the following hardware and software along with a little patience and clear mind.

Hardware

• SDR dongle (Suggest the RTL-SDR v3, but pretty much any will work)
• USB GPS with NMEA output (Suggest the uBlox7 Gmouse)
• Modern Windows Computer (Meant to run Win7 or later!!)
• Antenna for frequencies of interest (Most SDR come with a basic antenna or add on options)

Software

• RTLSDR_Scanner Software (Versions are clearly explained on Al's website)
• ublox u-center test software (Optional, but nice to have anyway)
• SDR# Receiver Software (Optional, but you should have this already, right?)
• Google Earth (If you want to see the real time location of signals on a map, you need this)

First, lets make sure your SDR receiver is functional.  The best tutorial on this can be found here.  

Step 1:  Get your SDR working:  https://www.rtl-sdr.com/rtl-sdr-quick-start-guide/ 

Try listening to some FM broadcast music between 87-108MHz,  weather broadcasts in the 162.4 to 162.6 MHz range or for local amateur radio or business/first responder activity between 420-500 MHz.

Now, let's get your GPS working

The USB GPS should automatically recognize and install drivers for most people on a Windows 7 or 10 computer.

After installing the ublox center program and just playing around with it to show it can receive location data, make a note of what COM port the GPS has decided it will use. 

Note:  The GPS obtains a virtual serial port over USB. You DO NOT need any outdated USB to 9 Pin serial adapter.

The instructions for getting RTLSDR_Scanner are cracking fantastic, so give them a read here and you should be up and running fairly quickly

Step 2:  RTFM = RTLSDR_Scanner Instruction Manual

Try tuning to local weather or music broadcasts to see what activity looks like in a one megahertz wide segment would look like.

The above image shows from 162 to 163 MHz, with a very powerful signal located at 162.475 MHz (Local weather broadcast) and how the signal fades slightly over just a few seconds from 12:04:19 to 12:15:50 due to driving around at a slightly variable speed.

A pinch of GPS and a cup of Google Earth

Now that you have a feel for how the GPS and SDR with RTLSDR_Scanner function, lets combine the two now by enabling GPS data to get combined with RTLSDR_Scanner output.

Below is what exporting the RF signal data looks like over the same 11 minute period when combined with GPS location data as output against Google Earth map.

Step 3: Use this opportunity to install Google Earth if you do not have it already. 

You will need to enable GPS under the "Edit" menu of RTLSDR_Scanner and ensure you change GPS type to "NMEA (Serial)" and select the COM port your USB GPS has.

When everything is configured correctly, you can use RTLSDR_Scanner to show what GPS satellites are being received and also when you get a location lock, your GPS coordinates and altitude will appear in lower right part of the application.

All that is left now is to go drive (or walk?) around with your laptop and start taking some measurements.

Depending on the refresh rate (dwell setting) and resolution (FFT Size setting), the output image on Google earth may vary.

Helpful Note:  Be sure to set mode to "continuous" and not try to sweep too wide a frequency range. Keep it to the smallest based on the signal you are looking for.  YOu also need to indicate how many sweeps to perform or otherwise, it will just keep overwriting the previous sweep which is not very valuable. The minimal setting is 1 MHz. This will help create the best results viewed on the map. To generate an the images in this article required 115 continuous sweeps.

Major benefits with RTLSDR_Scanner 

Here is a list of possible real life user cases that this application will enable:

• Creating a coverage map for amateur radio repeaters in the 144, 220, 440, 900, 1200 bands

• Determining general radiation patterns of a mobile or home amateur radio antenna installation

• General location awareness of commercial radio or broadcasters

• Interference source location finding

• Figuring out the range of your garage door opener or other pulsed mode transmissions

• Basic passive radar system

Sadly, RTLSDR_Scanner is not really designed as a Wi-Fi mapping tool, but by playing around with the dwell setting and using a device like a LimeSDR Mini, HackRF One or ADALM Pluto which are more expensive could give interesting results.

These other SDR options can go to about 6 GHz and not covered by the inexpensive SDR dongles that usually do not go too far past 1.9 GHz plus monitor wider bandwidth of up to 30 MHz wide at one time compared to the 3 MHz wide capable RTL SDR v3.

So what is special about KerberosSDR?

For the price of under $150 USD, users will be able to use the hardware and software along with four antennas to generate a heat maps just like RTLSDR_Scanner and also show signal direction using phase coherence theory through new software that Tamás Pető is focused on where an early version is demonstrated here:

Hope you found this article about basics of radio signal location interesting and something to keep you occupied while HVDN awaits its pre order of the KerberosSDR which will then be reviewed here later this winter.

HVDN [reset] = Is the "Feather" better?

Embedded computing and open hardware in the form of Raspberry Pi, Arduino and a few others have propelled creativity in the maker, electronics hobbyist, STEM education and amateur radio communities in recent years.

A new format standard called Feather has recently come to market and is very exciting, so lets find out why.

Embedded computing you say?

Since 2012, there have been over 19 million Raspberry Pi sold when combining all the different (and improved) versions sold over the years.

Perhaps the biggest competitor to the Raspberry Pi has been the genuine Arduino family of micro-controller boards with an estimated few million shipped.

Both "embedded computing" ecosystems have spawned various forked versions that illustrate the success these two standards have demonstrated both for hardware and software compatibility.

HVDN [reset] is a sub-series of content that compares
different enabling or disruptive innovations relevant to the
maker, electronics hobbyist and amateur radio community.

One reason that has made both devices popular for add-on device or "hat" accessory development has been standard sizes and pin or GPIO configurations.

This has helped developers create exciting applications quickly & reliably

A new standard called "Feather" has arrived recently and promoted by Adafruit and Sparkfun, both known for providing a range of original add-on products to build even better Raspberry Pi and Arduino based projects.

18 different Feather based products using standard sizing and connections

The [reset] moment

Thanks to Limor Fried and her team at Adafruit along with a number of different vendors like Particle, the new Feather and related terms such as "Feather Board" and "Feather Wing" are poised for a lot of success.

This HVDN [reset] article will explore the "Feather" open hardware ecosystem in  comparison to Raspberry Pi and how it simplified development of our latest project by standardizing on this new small form factor wireless enabled embedded computing standard.

Different sizes of Pi for all appetites

Let's look at how the size of the most well known and current Raspberry Pi offerings look next to each other.

• Raspberry Pi 3B  = 85mm X 56mm
• Raspberry Pi Zero = 65mm X 30mm
• Raspberry Pi Compute  =  67mm X 31mm
• Feather Classic = 51mm X 23mm

Pictured from top left clockwise are the Pi Compute 3 Module,
Raspberry Pi 3 and Raspberry Pi W Zero

Raspberry Pi has been chosen by HVDN to compare against the Feather format because they all offer wireless connectivity.

Arduino has too many variants to compare that have a similar size to Feather but do not offer wireless connectivity. ESP32 and ESP8266 boards have too many versions.

A major benefit with many of the smaller Feather based products
compared to the slightly larger Raspberry Pi Zero is built
in battery charging, power options and connectivity options.

Arduino versions such as the Nano, Micro, and Lilypad along with the Adafruit Trinket are left out of this review since they lack wireless connectivity.

Plucking Feathers

Specifications help drive standards and adoption, so Adafruit has everything documented on its website to make development easy for most everyone.

Standard dimensions of the Feather help
create easy product development and compatibility

Key Feather Physical Dimensions:

• The 'classic' Feather and Wing size is 0.9" x 2.0" with 0.1" holes at each corner.   

• There is one 16-pin breakout strip on the bottom side, centered 1.0" from the left edge.  

• There is one 12-pin breakout strip on the top side, 1.2" from the left side.  

• The spacing between the two strips is 0.8"  

• Don't change the GPIO spacing or location, or you will not maintain compatibility with Wings! 

Key Feather "In/Out" Specifications

Using the Particle.IO "Argon" product as the example, here are the connection details copied from the super helpful datasheet.  Many of the elements found in certain Particle products were even part of the attendee badge at this years Open Hardware Summit to show example of an embedded application in the real world.

So, what is the project?

In 2017  HVDN started to develop a modern field strength meter that has remote sense and analytic capability as featured in the Field Strength Meter Monday series.

After working out the actual sensor part of the project and offering for sale a limited run of stand-alone sensors, we then set to work to redesign the RF signal strength sensor unit to fit as a "hat" onto a popular embedded computing board.

The initial plan was to use the Arduino Nano because it was low cost and easy to program but was soon abandoned because certain versions of it used non-standard USB drivers to update or interface to the device.

A later version looked at standardizing around the STM32 based ESP32 and ESP8266 based embedded computing devices because they both offered integrated battery charging and wireless functionality. They also used the very flexible microPython programming language.

WEMOS ESP32 based Wi-Fi/BLE
4MB microPython IoT device

Quality and version control of these different ESP32 or ESP8266 based boards proved hard to develop easily replicated results around, but working prototypes showed the proof of concept was sound and could be improved upon.

Looking at Feather based options again seemed ideal due to a price drop and additional features. 

It was decided to build this project around the Particle Argon (Wi-Fi + Mesh)  and Xenon boards (BLE + Mesh), with potential support for the Boron (LTE + BLE + Mesh) board at a much later date.

Making projects simple with Feather

The Adafruit Feather based options helped simplify our project design and also allowed even more features to be added!

Future articles will cover microPython coding basics, computer-aided circuit design and fun applications for this exciting project.

Here is the HVDN-FS1....

Field Strength Meter Monday: Remote Monitor F.S Meter

Next Monday, June is June 20th and National Wi-Fi Day. 

HVDN will celebrate by releasing our Smart Field Strength meter that has been the lead up to the "Field Strength Meter Monday" series.

Along the way of the project design, a few changes were made which includes:

• Shifting from using an Arduino Nano as the "smart" element of the project to the WEMOS ESP32

• Redesigning the HVDN FS1 RF sensor board to stack directly on top of the ESP32

• Simplifying the power source by using the ESP32 with integrated lithium battery charger

• Learning some of the coding needed to make a self served web page from the ESP32 instead of an Android application for remotely reading the RF sensor data.

The project software coding and board layout designs will be made available as a reference. Completed versions will be available for purchase in the HVDN store which has not officially been launched yet, but can be found here.

HVDN FS1 LT5507 RF sensor unit top view of stand alone PCB board

Field Strength Meter Monday: Let's Calibrate Baby!

If you are catching up on the HVDN Smart Field Strength Meter project or coming across this for the first time, here are past articles in the "Field Strength Meter Monday Series"

• Field Strength Meter Monday: 3 Real World Applications For A F.S Meter
• Field Strength Meter Monday: General Overview
• Field Strength Meter Monday: A New Modern Low Cost F.S Meter
• Field Strength Meter Monday: Say Hello To A Smart F.S Meter Part 1

Frequency Calibration Comments

With the brief US Memorial Day weekend break over, the first post of June in the series will focus on ensuring the RF power sensor is calibrated for somewhat accurate measurements useful to the amateur radio community. Key design frequencies are:

The frequencies chosen between the 1 MHz and 25 MHz were chosen because they are in the sub bands for digital text transmission modes such as PSK31 and FT-8.  Both modes have great signal to noise ratios and permit very long distance communication with low transmit power. Ensuring optimal antennas are used, especially for portable or temporary operation, is one use case for the field strength meter to get the most out of these most modern modes for those interested in HF communications.

Frequencies chosen between 28 MHz and 925 MHz were chosen with a little less formality. The 28.120 MHz  and 50.290 MHz were chosen because of similar use cases as those in the HF band.  146.000 MHz and  223.000 MHz were chosen as good approximate "middle" frequencies in the 2m and 1.25m bands. The three frequencies in the 70cm band were chosen since its the widest and most used amateur band. Lastly, 925 MHz was chosen for representation of the 33cm band which is not very well used, but can help determine the top end of the field strength meter range.

Antenna Calibration Comments

With such a wide design range, it would be hard to use a 1/4 wave antenna for each frequency for calibration, even though that would be optimal. Only 3 antenna lengths will be used to calibrate the field strength meter.

• 6in/15.3cm = Chosen for small size and near 1/4 wave length for 70cm UHF testing
• 19.2in/48.5cm = Chosen for mid length size and near 1/4 wave length for 2m VHF testing
• 39.2in/1m = Chosen as a standard metric length to be used for general HF and VHF testing

The same exact material diameter will be used for all three antenna lengths

Unit Measurement Calibration Comments

Using the tables found in the "RSSI, dB, Oh My!" article is where we will get our calibration targets of 50 uV for the lower 8 frequencies chosen and 5.0 uV for the upper 8 frequencies chosen.

The upper frequencies will be easier to calibrate based on easier to set up test sites compared to the lower frequencies which require larger reference antennas, so calibrations will be more general for HF compared to more accurate for the higher frequencies.

Calibration Goals

The general theory in ensuring an S9 signal level can be generated at specific frequencies from a chosen distance from the transmitting antenna is how we will arrive at our calibration figures. The transmit power from the signal source will also be uniform at 37 dBm or the equivalent of 5 watts for all measurements to gauge the distance needed to generate the S9 signal level.

Field Strength Meter Monday: Say Hello To A Smart F.S Meter Part 1

 As part of the continuing "Field Strength Meter Monday" series, here is part of the schematic of a finished product to be released later this summer.

Detecting RF

Use of 1N34 germanium diodes have been the standby detector for a long time, but does not offer linear detection across a wider frequency range or sensitivity.   PIN diodes with a voltage bias increase sensitivity, but still do not have regulated output across a wide frequency range. This is why a new and low cost modern "smart" field strength meter is needed.

The HVDN designed Smart F.S Meter has the Analog Devices LT5507 as its RF detector.  The range is 100 kHz to 1 GHz, so this will find a lot of use for the amateur radio community.  Its also possible to use the LT5534 as a direct replacement to offer 50 mHz to 3 GHz range if desired for those interested in things like detecting 2.4 GHz Wi-Fi, bluetooth and more.

The LT5507 is small, in case you did not know. 

Moving to the LT5507 provides additional benefit as well compared to other older detector solutions. When RF signals are received by the LT5507 (or LT5534) the output signal is a calibrated voltage from -0.3 to +6.5 volts which  is easy to translate into RSSI values. This is important since it can easily be read by most analog to digital converters that operate up to about 5V, such as the embedded capability of an ESP8266 or Arduino.

Here are the datasheets for the LT5507 and LT5534 power detectors.

Basic schematic for LT5507 based RF power detector.

This uniform voltage compared to frequency received is what makes this a building block of  a smart field strength meter.

However, the LT5507 and LT5534 are not very sensitive for this purpose, so adding an operational amplifier (OpAmp) is needed.  The common LM324 is used for this purpose as illustrated in below schematic.

OpAmp expanded LT5507 RF FS Meter

In the "OpAmp expanded LT5507 RS FS meter" schematic, the "signal output" can be fed to a simple analog 150 mA meter, A/D converter as part of a ESP8266 or even a LM319 bargraph add on circuit for more visual readouts.

The 47K potentiometer can either be a regular analog one or a digital potentiometer which can then be manipulated by interfacing it to GPIO on an Arduino or ESP8266.    The function that the 47K potentiometer controls is sensitivity of the OpAmp up to 2x or to even reduce sensitivity by 1x.

Adding the "Smart" to the FS Meter

Now that there is a sensitive, broad banded RF detector that can output a signal that is easy to interface to, now we can start to look at integrating this with the "smart" aspects of the project.

Stay tuned.....

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Field Strength Meter Monday: A New Modern Low Cost F.S Meter

Most relative field strength meters employ an analog meter. Some newer models use LED bar graphs or LED numerical displays.

Both types of indicators offer different benefits and is why a modernized relative field strength meter is needed to bring all the benefits of these display types together. 

Additionally, having the ability to "listen" to how a signal has changed would be nice to have.  The human ear can detect much smaller changes compared to those that are visual.  So, how about we make our own modern relative field strength meter?

Key design considerations include:

• Offer somewhat linear sensitivity across the entire design frequency range.
• Provide amplification and attenuation for different use cases
• Allow the user visual and audible customization indicators 
• Enable remote monitoring capability
• Balance cost and benefit to potential users

Evolution: Software Defined Relative Field Strength Meter

As covered in previous "Field Strength Meter Monday" articles, the components are pretty simple, but if we add more of a programming element thanks to a raspberry pi or arduino, we can do some interesting things by loading custom code and taking advantage of the different GPIO connections offered by these embedded computing devices.

Major Design Challenge

It would be nice to include the ability to remotely monitor RF signal strength since the human body can influence reception when standing in the RF near field.  Connecting a field strength meter to send its readings over Wi-Fi to a mobile phone could be an interesting feature to have.

Building This Thing 

It will be better to share a final product with HVDN readers to as to not confuse based on some half built, semi-functioning product or one that just works "theoretically".

When Can I See It?

Because HVDN wants to build a community with major benefits available to them first, the official schematic, parts list, code and other construction information will only be available to HVDN members for an embargo time period from the public.


Please join HVDN (Free) to learn more about this great project

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Join HVDN as a member and get much more!

Field Strength Meter Monday: 3 Real World Applications For A F.S Meter

Last week saw the first article in the "Field Strength Meter Monday" series with a general overview.

Here are three practical applications to use any type of relative or calibrated RF field strength meter for.

• Check antenna radiation patterns
• Measure approximate transmitter power power or antenna efficiency
• Locate hidden transmitters

Which way does your antenna work best?

From a simple dipole for HF frequency operation, to directional beam antennas at UHF ranges, knowing which way your antenna is sending out the most RF energy is helpful to know.

Using a simple relative field strength meter will help you make your own charts somewhat accurately and very inexpensively. 

While the RF source being measured transmits a signal, the relative field strength meter will show a measurement that can be plotted on a piece of graph paper.  As you walk around the transmitting antenna with your field strength meter and note the readings, you can arrive at a chart like the one pictured to better understand your antenna characteristics.

Is my radio transmitting?

A relative field strength meter will also provide some idea as to if your radio is actually transmitting. This may be helpful to know if you hear other stations, but they do not hear you. Some radios will indicate a signal is being sent or tell you how much power it is supposed to be transmitting, but how do you know that for sure?

Using a relative field strength meter wont show you any calibrated power output measurements. For that sort of testing, a watt meter that measures specific levels of RF energy is needed, but that only shows actual RF power output and not how well the antenna is projecting your signal. A watt meter also needs to be connected inline with the transmitter and antenna. A field strength meter does not. 

Perhaps you want to see how well a new hand held radio transmits with the included antenna and another one purchased as a premium accessory? 

You can use a relative field strength meter to see which antenna works best and see how much of a difference you really get when changing from "extra low power" to "low power" and then to "medium" or "high" power output when using different antennas.

Showing the transmit output levels on the Kenwood TH-D74. The manufacturer claims 5 W, 2 W, 0.5 W and 0.05 W on the 3 bands the radio operates on which is 144, 220 and 440 MHz.

Where is that signal coming from?

A relative field strength meter can also be used to find very close by transmission sources.  Using a simple whip type of antenna will help the user find the signal source within a few hundred feet or less depending on how the relative field strength meter was designed.

If combining some form of directional antenna with a relative field strength meter, that creates an even more interesting tool for finding the strength of a signal as well as the direction from which it is coming.  This combination forms a basic radio direction finding tool that is inexpensive and easy to use.

Whats Next?

Next Monday, HVDN will introduce an updated relative field strength meter that blends both analog and digital technologies to create a very interesting tool with even more potential applications.

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Please subscribe to HVDN Notebook for additional details on the Field Strength Meter Monday series and other great content along the way too.

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Field Strength Meter Monday: General Overview

Having an RF field strength meter on hand is perhaps one of the best and most multi-purpose piece of test equipment any electronic hobbyist, ham radio operator or not, should own if they are doing anything with wireless signals under 1 MHz to over 3,000 MHz!

In 2006, the Britain based Shefford & District Amateur Radio Society released an LED bargraph relative
field strength meter kit for its members and others to copy. Clones of this kit can now be found on E-Bay. 

Brief Overview

Calibrated and relative field strength meters serves different purposes. A calibrated one is much harder and expensive to design unless its for a narrow frequency range and calibrated against a known signal source at all critical design frequencies for linearity purposes. 

The output measurement of a calibrated meter would best show the user in a standardized measurement unit such as decibels per meter (dBm), micro-volts (mV) or milli-watts (mW).

Analog meter showing dBm and micro-watt measurements

A relative field strength meter differs in that the user would first take a first measurement of a baseline signal reading before making any changes to the signal source or measuring device which may impact an increase or decrease in signal visualized on the field strength meter.

Measurement units can be a range scale, such as 1 to 5 on an un-calibrated or relative field strength meter.

Having a broad banded relative field strength meter is very useful, but also having some basic form of calibration makes it slightly more useful for certain applications.

A basic "unbiased"  and un-calibrated relative field strength meter

There are very few components needed to construct a basic relative RF field strength meter and there is not even a  power source needed to worry about to make it work. The below circuit will detect RF signals up to and slightly over 500 MHz.

The circuit theory is rather simple. Signals being picked up by the antenna are coupled through a 100pF capacitor (C1) to the junction point formed by two matched 1N34 diodes. The 10K ohm resistor (R1) and .001pF capacitor (C2) provide the function of a voltage divider to "smooth" out the signal generated by the diodes. Variable resistor R2 provides a level of control to prevent the ammeter from being fully deflected by a strong signal. It also provides some attenuation functionality when dealing with ovwrly strong signals. A value of 50k ohms for R2 and a 50 mA meter are suggested to round out a parts list.  None of these component values are critical and is what makes a relative field strength meter fun to experiment.  The antenna length should be close to a 1/4 wavelength of the signal being monitored or some user standardized random length, like 1 foot.

Essentially, all the above circuit does is takes RF energy, converts it into a rectified voltage and is then read on a visual meter. 

This circuit and many like it have been around for over 100 years. The only major advancement with this basic circuit concept has been in the type of diodes used based on sensitivity.  1N34 or any similar small signal germanium based diode has been the choice of many, but is not the only option out there.  

The other change is how to visualize the signal. You could use a moving analog meter, a digital meter, an LED bargraph or anything else you can think of to show the signal visualized in some way.

Using a very common 1N914 diode will provide a usable circuit, but is not as sensitive as the 1N34 diode.  You could even use 2 low voltage LEDs or certain transistors like a generic 2N2222, which essentially act as two diodes connected together in the circuit above. One transistor to experiment with would be the 2N2222 NPN transistor.  There are also PIN diodes and Schottky diodes that can also be used which are thw most sensitive when properly biased.. You may be surprised what else can be used to detect RF signals.

An RF signal will need to be much stronger to power the junction of a 2N2222 transistor or PIN diode, so a more advanced circuit is needed that adds in a 1.5 volt or greater power source to "bias" the transistor and thus creating a slightly more sensitive circuit compared to the 1N34 based circuit. 

This circuit is more sensitive because of the 2N2222 acting as an amplifier after the single 1N34 detector diode. This circuit will detect signals from a further distance when using the same length antenna compared to the simpler unbiased circuit.  This more advanced circuit will work up to about 500 MHz, but may show other readings as high as 3000 MHz.  It will likely NOT be useful for measuring WiFi at 2.4 GHz because those signals are NOT the same as traditional ones broadcast by those involved in ham radio using modes like FM, SSB and CW, but there are ways to design a circuit to detect 2.4 GHz signals as well as 5.8 GHz.

Calibrating a relative field strength meter

There are ways to calibrate circuits such as the above by using some filtering before the signal detector to limit the frequency range before it is detected by the field strength detector, but it gets a little complicated and is why calibrated measurement tools are somewhat expensive since they need to show the same measurement across the entire frequency range accurately. Last year, HVDN released an article discussing measurement units which closely ties to different frequency ranges.

What values and circuit design works at 7 MHz, 14 MHz or 27 MHz will not work the same as at 146 or 440 MHz.

Calibrating a field strength meter makes it almost closer to a wave or watt meter which can measure much stronger signals directly. A field strength meter is typically used for indirect measurement.

Build, purchase or modify?

If shopping for a used field strength meter on Ebay for example, do not confuse a meter that has a knob showing different frequencies and think it is calibrated. It is still a relative field strength meter, even if the meter shows mW, dBm, etc.  At some point, some commercial models, such as those sold as CB SWR/FS meters may have been calibrated against a known signal source to provide a full scale reading based on common industry levels, but how do you know if it is accurate still?

The QRPGuys Digital Field Strength Meter can detect RF energy over the VLF-500MHz range with sensitivities from -80dBm to +10dBm. It uses the popular Analog Devices AD8307 logarithmic detector/amplifier

There are certain meters that have well documented modifications to increase functionality or accuracy.  There is alot to learn about design, use, purchasing of F.S meters.

The HVDN Field Strength Series

Beyond this and the other article mentioned, future articles will cover:

• Part 2: 3 things to do with a basic relative field strength meter
• Part 3: Updating a basic field strength meter for the modern world
• Part 4: Building an Arduino Nano or ESP8266 based modern field strength meter
• Part 5: Calibrating your Arduino Nano based modern field strength meter
• Part 6: Using your Arduino Nano based modern field strength meter
• Part 7:  Remotely monitoring a modern field strength meter
• Part 8:  And what about Wi-Fi and drones plus signal measurement?

This series will be updated every Monday, starting April 30th 2018, so please subscribe to the HVDN Notebook for updates about field strength measurement and other articles.

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