Many thanks to Philip (KA4KOE) for the following guest post:

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Review of the FX-4CR Mini-Transceiver: What’s Old is New Again.

by Philip (KA4KOE)

Introduction

My start in Amateur Radio began in 1979 as a newly-minted, 16 year-old Novice. At the time, my resources were slim. I worked in a country store; pushing a broom, putting up stock, bagging groceries, etc. I was paid $23.46 every two weeks. At this time, Heathkit was selling kits to the ham radio community that ran the gamut from simple to elaborate. Looking over the catalog, my young eyes were drawn to the HW-8. What drew my attention was not the radio’s features, but the price.

The Heathkit HW-8 was listed for $129.95, plus shipping. I did not even notice that the power output was specified at 3.5 watts on 80 meters. The green gem was a bonafide ham radio transceiver!

In due course, the kit was acquired, assembled, tested, and put on the air with an Elmer’s aid. I made a bunch of contacts with that CW-only transceiver. The term “QRP” never entered my mind. I don’t have any photographs from my early days on the air: the only photograph of the HW-8 dates from 1981 during my freshman year of college. By that time, I had acquired an HW-100 from a fellow ham. See Photograph No. 1: note the chrome Vibroplex Lightning Bug, homebrew “T” network transmatch, and of the course the ‘8 on far right (see red arrow).

Rationale and the radio

Without getting into my entire life’s story, I will start this paragraph by stating that my ham “career” has now come full circle as I’m back to using compact, lightweight, low power gear; this time by choice.

I had the bad habit of deploying too much gear during POTA activations. I came across internet chatter with regards to a relatively new product; the FX-4CR SDR mini-transceiver. The base specifications are as follows:

• Dimensions: 107mm (4.2”) L x 65mm (2.6”) W x 43mm (1.7”) H.
  Weight: 0.46 KG (1 LB).
• Bands/Power:  80 meters thru 10 meters, inclusive: 20 watts. 6 Meters: 5 watts.
• Display and Panadapter Span: 50mm (2.0”) TFT, Approximately 24 KHz +/- (48 KHz total) and waterfall. See Photograph No. 3.
• Modes: LSB, USB, CW, FM, AM, Digital.
• Receive Current Draw: 210 mA.
• Transmit Current Draw: 3.0A to 4.0A at full rated 20W output.
• Input Voltage Range: 9 VDC thru 16 VDC (do not exceed). The optimal supply voltage is 14 VDC.
• Connectivity: Bluetooth for CAT/Audio. USB Port for CAT/Audio and Firmware Updates. Integral audio soundcard input/output.
• Output Power: Continuously variable from 0.1 thru 20W. Recommended maximum wattages are SSB – 20W, CW and Digital – 10W.
• No internal battery or antenna matching unit.

Expectations

The prospective buyer should realize what this radio “brings to the table”. As others have previously stated on the various internet chat groups and social media, do not expect Elecraft performance at the $550 (including shipping) price point for this radio. All radios have flaws and this one is no exception. Issues of which I am aware are as follows:

1. CW operation has improved dramatically with the latest firmware dated 25 August 2023. There are still some bugs accompanied by occasional software crashes, requiring the user to power down the radio for a reset. Additionally, CW is semi-break in only. The transceiver is now quite usable in CW mode.
2. Bluetooth connectivity via Windows exhibits audio quality issues. In my opinion, this issue is not the fault of the radio. Windows identifies the radio under the category of “handsfree audio” during pairing. While the transmit waveform is satisfactory, “audio-in” is bandwidth-limited and compressed: this limits digital decodes. Internet research reveals that degradation of Bluetooth “audio-in” is a long-standing problem that others have noted under Windows for years. FT8, for instance, works best if one uses the hardwired USB TRRS female port on the right-hand side of the radio for audio and CAT control. The radio has a built-in sound card; a discrete interface is not required. Bluetooth connections for FT8/FT4 are very good using an Android device with the application FT8CN, which is available for internet download at no charge.
3. The noise blanker has not been implemented yet in firmware. Digital Noise Reduction (DNR) is activated and working.
4. Firmware updates are issued at regular intervals; on average approximately at one-month intervals. The designer Yu, BG2FX, recently made the firmware open source in order that third-party software developers can contribute.
5. The supplied hand microphone is small in part due to the size of the electret condenser capsule inside. The transceiver also has an integral microphone that works well; PTT is accomplished by changing the function of the RIT button in the menu system. Others, myself included, have tried using larger “handy talky” condenser speaker microphones to good effect. One solders a 1/8” TRS to the end of the cord. The input impedance at the microphone jack is 2.2 KOhms.
6. The manufacturer has issued directions via the Facebook FX-4C group for a user-performed modification. An internal surface-mounted chip bypass capacitor in the microphone input circuit is removed in order to improve transmit audio quality. No soldering is required and Yu demonstrates the procedure using a very small pair of needle nose pliers. The capacitor prevents RF from getting into the radio via the microphone cord. I soldered a small 5.7 nF capacitor inside the 1/8” TRS connector on the third-party microphone to provide RF bypass outside the radio enclosure but close to the chassis. The 5.7 nF capacitor, in combination with the input audio impedance, has better audio low-pass characteristics and yet still keeps RF out.  The microphone I chose is an Abbree AR-780 and is available economically from Amazon. I am receiving good reports with regards to my transmit audio quality. I do not know if subsequent production runs will incorporate a differently-sized internal bypass capacitor. The transmit SSB audio bandwidth is adjustable in the menu system in six (6) increments from 1.5 to 3.0 KHz.
7. The FX-4CR is supplied with an attractive foam-lined carry case, hand microphone, fused power cord, 1/8” TRRS Male to USB-A cord for connection to a PC for CAT/Audio, instruction manual, and two spare fuses. The radio accepts an XT-60 style female power plug; the XT-60 is popular in the RC vehicle community.
8. Firmware updates are listed on Yu’s website. The procedure involves the use of the supplied 1/8” TRRS/USB-A cord and following the firmware update directions exactly. I use the most current firmware version dated 25 August 2023.

Various Musings and Conclusions

Operationally, the radio appears to be very sensitive. The CW DSP filter works well and has several bandwidths available from 50 to 800 Hz. Only the narrowest bandwidth exhibits some “ringing” in the receive audio. Personally, I prefer using the 200 Hz filter. I have had enjoyable CW QSOs up to 25 WPM. The T/R delay is adjustable via the menu system.

The radio will receive all the traditional shortwave bands from 90-11 meters via menu selection. Receiving in-between the listed shortwave bands is currently not possible with the most recent firmware. Transmit is disabled while receiving on these frequencies.

In normal use, the radio does get warm if using higher power, especially during CW and WSJT-X transmissions. The radio is software-limited to 10W while using digital modes. BG2FX will soon be offering an optional upgrade kit that includes a ventilation fan with a temperature-sensing control board for those wishing to run near the design limit of the transceiver. The radio will shut down as a protective measure if it overheats. The kit also includes two new chassis end pieces with ventilation slots and integral handles. The price point is aimed at around $50.00.

The maximum power output of 20 watts is the sweet spot for effective SSB transmissions. The difference between a 20 watt transmission and a 100 watt one is approximately 1.2 S-Units on the receiving end, using the Collins standard of 6 dB = 1 S-Unit. The US Military understood this relationship: most HF manpacks are rated at 20 watts maximum output power. This power level balances communications effectiveness versus how many batteries an infantry soldier must lug into the field in addition to the rest of his kit. Additionally, the FX-4CR provides a selectable level of transmit audio compression. On-air reports using the Abbree microphone have been very favorable.

I have thoroughly enjoyed using this radio during my three weeks of ownership as of the time of this writing. I have not yet conducted a POTA activation.

For the price asked by the maker, you get a lot of “bang for the buck” combined with extremely low weight for excellent portability. Yu is very responsive to requests for assistance from his customers. Expect approximately two (2) months lead time from the time of your order to delivery. The manufacturer should offer the FX-4CR again for pre-order soon.

Ham radio transceiver selection is a very personal thing and akin to picking out a pair of shoes. The radio should fit your operating style and what you wish to accomplish with your ham activities. I chose this radio as the “fit and finish” is superior, in my opinion, to other compact field radios currently on the market. Is the FX-4CR the “end all” for mini-SDR transceivers? No, it is not. Certainly, the low receive and transmit current requirements are a big plus for those who do SOTA, POTA, and other activations in the field. Moreover, I cannot over-emphasize the outstanding support provided by the manufacturer. Yu takes great pride in his work and is a talented electronics engineer.

And…my daughter Sarah thinks the little radio is “cute”.

Finally, I eagerly await cooler fall weather soon (I live in Georgia and air conditioning is a requirement similar to spacecraft life support) so that I can “play radio” again in the great outdoors. If you decide to acquire an FX-4CR, be aware that there are a lot of counterfeits floating around on the internet which are not endorsed or supported by BG2FX. Order only from https://bg2fx.com.

Good health and fortunes to you! POTA ON!!

Respectfully Submitted,

Philip Neidlinger, PE, KA4KOE

Other Internet Resources:

• Facebook Group:  FX-4C. This group is very active with many posts by members.
• Groups-IO