TS-850 Panadapter / Bandscope

The BIG Goal

I wanted to have a visual presentation of signals across the band where I was operating. Just like all the new radios. And by "new" I mean pretty much everything in the past 20 years :-O

The First Step - Modify my TS-850's IF-OUT

The Kenwood TS-850 provides an IF-OUT for use with Kenwood's own SM-230 Station Monitor. This operates at the 8.83MHz IF, and offers a view of ±25/100/250 kHz around your tuning-point. It would not provide this sort of coverage in my case, though, because I've installed a narrower 4kHz Roofing Filter in the First IF of my '850 - an SM-230 might give me ±5kHz view :-O Shockingly narrow.

Consequently, I chose another path:

• Modify my TS-850 to buffer and extract the un-restricted 73.05MHz First IF (before the Roofing Filter).
• Add an SDR (Software-Defined Radio) to monitor this fixed- / unfiltered-IF
• Use computer software to view the signals in a band-wide swath of frequency
• Bonus: my TS-850 will perform the TX / RX switching for this SDR configuration, obviating the need for any external switch (or, blowing up the SDR :-O :-( ) Also, because my TS-850 is performing the first up-conversion, the SDR can natively handle this First IF without needing anything further (often HF ham-band-coverage with an SDR will require some sort of up-convertor, depending on the SDR you choose).

I bought and installed a buffer for the 73.05MHz IF: the G4HUP PAT85M. This was routed out Kenwood's original IF-Out RCA-jack, with a label to indicate this update. Rather than strictly follow installation-instructions, I chose not to cut the original cable inside my radio, but rather I saved the original cable and made up an entirely-new cable using new connectors and some RG-174/U I had on-hand. Thus, a back-out plan.

The Second Step - Hardware

I bought a low-cost NooElec NESDR Smart V4 device (Christmas Day 2020, for C$44). This plugs directly into a computer USB port, and has an SMA for RF Input. I had an SMA-to-RCA cable made, to connect this SDR to my '850.

The Third Step - Software

Under (Gentoo) Linux, I tried a couple of different softwares for Panadapter. I liked CubicSDR, but it really wasn't made for this and was un-maintained in my distribution. So, I settled on GQRX.

The Devil's Always in the Details

As an Engineer :-) I got caught up in the SDR specs. It's easy to go looking for faster, deeper sampling. I wanted "excellent", and I was skeptical that a cheap SDR could deliver... But I'm happy to say that this RTL2832U & R820T2 SDR works very well as a Panadapter! For receiving, it's quieter than my TS-850, not quite as sensitive, and it overloads on super-strong signals. But in normal day-to-day use, it's really not very different from the TS-850, and it's quite an excellent panadapter. If I really wanted to critically use it as an HF radio, I might choose better... It's definitely good enough for painting a helpful picture!

GQRX is not made for Panadapter use, really. It wants to directly tune and demodulate your SDR. So, I had to carefully configure GQRX, and add gqrx-panadapter.py to pass actual tuned-frequency data from my '850 to GQRX, and modify GQRX's idea of offset and frequency.

Here's How

• Start FLRig. This will "talk" to my TS-850 via serial-port, and re-serve the connection over XML-RPC (port 12345). Potential clients are FLDigi and CQRLog; multiple clients are supported.
• Verify rig-chirps, and FLRig sync's up to '850 display. Re-start/re-try until this works properly.
• Start CQRLog. It's configured to auto-start rigctld to talk to FLRig, and re-serve the radio using hamlib. Logging and "glue".
• Start GQRX. Hahaha - remember to click the upper-left "Start" arrow-button :-O Remember to enable Remote Control ... internally, GQRX then listens on port 7356.
• change to ~/Downloads/gqrx-panadapter-master/ directory, and run $ ./gqrx-panadapter.py -f 72.6105. This python script talks to rigctld/hamlib using port 4532, then talks to GQRX using port 7356.
• The offset supplied to gqrx-panadapter.py varies by mode, etc. CW-U use -f 72.6105; USB use -f 72.60911.

Bonus

I can use GQRX and the NESDR to actually receive HF signals! It's not bad, really, and offers some nice SDR features (eg. adjustable filtering bandwidths and shape-factors).

The Good, The Bad

• I like how my tune-point remains stationary within GQRX, and as I tune around, the band/spectrum slides around beneath this stationary tune-point.
• I cannot click-tune in GQRX. The rigtcl communication seems only one way. Yet, I can click-tune using CQRLog and FLRig.