Is a "Poor Man's 705" possible?

While I am proud owner of multiple radios from Icom over the decades (and still own all of them) I am having a hard time cost-justifying an expensive and new shiny Icom IC-705 as part of my QRP kit, like Steve K2GOG already jumped in on. 

My Yaesu FT-817 has served me well the past twenty years which includes many ARRL Field Day's and HOPE conferences and with the 705 coming in at ~US$600 over the cost of a Yaesu FT-818ND, ~US$400 over a Elecraft KX-2, and even US$750 over a used Yaesu FT-817ND, what the premium that the IC-705 delivers is really just the same core capability as all these other fine radios, but with some fancy user interface (UI) and an improved overall user experience (UX)

The Shiny

The top three attributes often discussed about the Icom IC-705 are:

• Color touchscreen display
• Bluetooth and Wi-Fi connectivity
• Compact size

To make my FT-817 or a new FT-818ND "shinier", I could buy a CAT-to-Bluetooth Adapter (US$20), use a Amazon Fire HD10 Tablet (US$15) as a touchscreen display running the 817 Companion app (Free.) This approach would allow for control of the radio, but not much else like how the Icom IC-705 integrates Wi-Fi to also offer full remote access which is very different compared to the basic bluetooth connectivity also present.

But the article is about a "Poor Mans 705." A FT-817/8 approach is still too much for your blood at ~US$700 to US$820.  We need to go much lowah ..... lowwwwaaahhhhh!

Cheap(er) Building Blocks

I recently completed building a uBitX. A QRP project with a sublime simple design blending analog and microprocessor technology. 

It is designed with experimentation in mind which can also combine a touchscreen display just like the one found on the expensive Icom radio. I figure this display upgrade would be a good start for a Poor Mans 705.

uBitx Architecture Model

The Block Model above is the uBitx with touchscreen modification. It is broken down into Analog, Digital, and Human Interface layers. 

It is important to note that the uBitx is a software-controlled radio as opposed to a software defined radio. 

In software-controlled, a microcontroller is used to control analog stages of the transceiver and interact with the user interface that makes and display changes/status. 

A software-defined radio (SDR) takes the  analog signals, converts them to the digital domain where the power of math (and coding) let us manipulate the signals to the desired analog output whether transmit or receive. The 705 is a SDR which also has software control, the best of both and you pay the price for it! 

With Arduino Nano as the microcontroller and all the uBitx code open sourced, this should help with the software learning curve.

User Interface and Experience

For those new to touchscreen projects, the Nextion HMI Display will expand your horizons when it comes to building/designing the user interface experience.  Many radio amateurs became familiar with these displays either through a uBitx project or something involving an MMDM digital hotspot. 

The Nextion has its own processor and memory that is programmed with its own editor software for developing the GUI you interact with. 

Nextion Editor with uBitx Main screen loaded

A web search will yield libraries of Amateur Radio screens made for the Nextion. The development experience is analogous (no pun intended) to front-end web development with your radio being the back-end infrastructure.  A robust community of users exists via various Facebook and Groups.io pages too.

It should be noted the Nextion does heavy processing and may introduce radio frequency interference (RFI) that may disagree with some less thoughtful HF QRP project designs. 

There are other TFTs to go with out there but few are simpler to program and addressing the RFI through the Nextion being in its own metal sub-enclosure will often suffice.

Wireless Connectivity

Anyone who has had a long history with digital modes commonly used on amateur HF spectrum is no stranger to the nest of wires and peripherals that would be built between their radio and a PC. In essence there are two services you are trying to make wireless; audio connectivity and control connectivity. 

Adding Bluetooth audio to a transceiver is a well trodden path of simple solutions  as long as you choose VOX over PTT. 

The same can be said for Bluetooth Serial device adapters that can plug into serial ports of common radio manufacturers. 

The Bluetooth serial connection is transparent to the radio control software and manufacturer protocols running across it. 

But many QRP builds do not include a serial connection nor a protocol to be used across one. The few that do like the QRPver Minion have standard on Yaesu FT-817 CAT Library.

One approach is for connectivity to be its own subsystem at the digital layer built with an ESP32. ESP32 have WiFi and Bluetooth built-in. 

Code would be built for the ESP32 to run network services (Bluetooth and WiFi) that understand various vendor protocols and give commands to the Arduino via I2C for control changes.

A Poor Mans Approach

Poor Mans Architecture

Based on what has been covered so far targeting the top three attributes of a 705, I come up with the above architecture. 

As of this publishing, I have yet to find a better source for the analog foundation than a uBitx kit which includes the microcontroller. 

For connectivity the Bluetooth Audio module and ESP32 can be readily sourced from Amazon. For the touchscreen I will go with a Nextion 4.3 inch - the same real estate as the 705.

Going larger than the available 3.5 inch touchscreen upgrade for the uBitx is going to require learning UX design with the Nextion Editor Software and the nuances of the controller running on the Nextion.

Once that is successful, adding connectivity require writing code on an ESP 32 and a routine added to the Arduino to look for I2C messages from the ESP32.

You would be correct in thinking I am planning to turn this into a fall/winter project.

 Be sure to keep an eye on the HVDN Github repo as ideation develops into design and prototype.

Any Pi?

At the Four Days in May (FDIM) 2021 annual QRP conference, the creator of the uBitx Ashhar VU2ESE announced work on the sBitx - an Open SDR using a Raspberry Pi. A PDF was published providing in-depth technical detail (that included code snippets) on their approach as well as a repo shared of code developed to date. 

Just like many of us saw with the expensive RS-BA1 remote control software that Icom sold, but has since been phased out due to competing lower cost (and better) options by the community itself, only time will tell if a combination of the sBitx and people like me will create a competitor via an open source low cost "poor man's 705".

Will be tracking this closely.

73,

- Joe, NE2Z

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.....

Subscribe for HVDN Notebook updates only 

Join HVDN as a member and get much more!

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.

Subscribe for HVDN Notebook updates only 

Join HVDN as a member and get much more!