CAT vs antenna tuner, automatic coax switch, etc

Discussion in 'General Technical Questions and Answers' started by N4VDI, Apr 22, 2021.

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  1. N4VDI

    N4VDI Ham Member QRZ Page

    Is there some relatively standard (or at least, de-facto Yaesu/Icom/Kenwood standards) way for a transceiver to directly announce its desired frequency to something like a connected external coax switch or antenna tuner?

    Offhand, I can think of a few good ones:
    • second (output only) serial port... god forbid, multidrop RS-485, so multiple things like an antenna tuner & coax switch could all passively watch it for broadcasts. Maybe even get fancy, and implement an additional line that's normally pulled high, but another device can pull low to indicate that the radio should NOT transmit, because something is unready.
    • I2C
    • Dallas 1-wire
    • low-frequency low-power signal directly mixed into the antenna feed to communicate the desired transmission frequency. Maybe even something like Bell-103, modulated onto a VLF carrier for downstream observers to see, before it ultimately got filtered out by high-pass filter before the antenna
    • Ethernet. Ok, I'm dreaming, unless it's a SDR, in which case it would be absolutely trivial to do... ;-)
    Example use case:

    Arduino-controlled coax switch with trio of cascaded SPDT RF relays to switch coax between 3 antennas (40m, 30m, and 20m) and a dummy load. When radio is tuned to something like 7.070mhz, the switcher notices & sets the relays to connect the radio to the 40m antenna. If the frequency gets changed to something like 14.071 or 10.138, it changes the relays to connect the 20m or 30m antenna. If the radio gets changed to something like 3.560 or 28.180mhz, it connects to the dummy load, and pulls the error line low to tell the radio it shouldn't transmit.

    The same station might have an automatic antenna tuner connected the same way. It sees that the radio was tuned to 7.070, checks to see whether the error line is low or high, pulls it low *itself*, temporarily switches the radio's output to a dummy load & takes over the antenna itself using another RF relay, acts like a headless NanoVNA to recursively adjust itself to optimize SWR within a second or two, then gives the antenna back to the radio & lets the 'unready' line go high again.

    Does anything like this exist? Or, is CAT strictly a single-client polled interface, and doing something like this would require the host pc (or software running on it) itself to act as a proxy & communicate the radio's state to interested observers (like the coax switch, antenna tuner, etc)?
  2. KC3PBI

    KC3PBI Ham Member QRZ Page

    I'm still new at this, but I've worked out a bit.

    Icom has a system where the transceiver outputs DC on a pin in the amp/accessory socket at a certain voltage to indicate the band in use.

    I don't know which Icom radios actually have this output.

    Yaesu uses a 4-pin, 4 bit matrix. I don't know what Kenwood does.

    I have seen hambuilt converters and interfaces, plans and finished gadgets available too. Some passively sniff the CAT connection, just listening. That seems to be fair game.
  3. K6RUV

    K6RUV XML Subscriber QRZ Page

    The answer is yes. Any transceiver that has CAT will do that but each manufacture has there own code as to what freq is what. Im assuming that but i believe im correct.. My Kenwood would return a series of numbers and letters that indicate its set freq. Those numbers can be read by your Log book or any other piece of software that is designed to read it and is sharing the the transceivers communications port through a port sharing utility. It could be designed to set the transmit antenna or perhaps tune a mag loop. well maybe tune a mag loop. let your imagination zoom
  4. N4VDI

    N4VDI Ham Member QRZ Page

    Hmmm, I guess that might explain why the more advanced screwdriver antenna controllers all come in 3 different flavors (Yaesu/Icom/Kenwood).

    I guess I'm going to have to grab the source code to FLdigi & poke around a bit to see how it handles CAT. Since the radios themselves all handle it in different ways, and those ways are mostly limited to "just the band", it looks like I'd be better off trying to just graft proxy functionality into FLdigi (or maybe a library it uses, like Hamlib).

    I did more reading last night about CAT on the Yaesu FT-1000mp (my dad's radio, which is the one I've used lately when visiting him). It looks like it HAS a CAT command that might return a byte value corresponding to measured SWR (though probably only while actively transmitting, after some unknown sample delay, and an as-yet unknown mapping of measurements to values.... probably something like "divide the byte value by 10 to obtain a value between 00.0 and 25.5, then add 1.0 to get a SWR between 1.00:1 and 25.5:1... with a practical ceiling somewhere around 10:1 before it becomes too high for the radio's measurement circuit to really quantify".

    With something like that, I could try to implement a scheme kind of like this:
    • Before transmitting, FLdigi polls the "readiness" line
    • Periodically, it polls the radio via CAT for band & frequency. When transmitting, it also polls SWR.
    • Periodically, it notifies observers (like a coax switch, antenna tuner, screwdriver controller, etc) of the current receive freq & band and the likely transmit freq & band (since, for a mode like ft8, tx freq might differ from rx freq by 1000hz or more), plus the current state (tx + swr, or rx). It outputs this via a port with flexible USB-FIFO bridge like FTDI's that can be programmed to support almost any semi-modern protocol from uart/i2c/1-wire to ECP/EPP/SCSI and more.
    • When initially tuning to a freq, it has the antenna tuner choose a preset if it already has what appears to be a sane one available, or forces a tune-up if it doesn't.
    • Each time it transmits, it observes the measured SWR, and does tiny experiments to tweak the antenna tuner up & down a step to see whether it makes SWR better or worse. It might even attempt the tune-tweaking during the transmission itself, just like a human looking at the swr meter with one hand on the dial might do.

    The main problem I can see with an existing scheme that only communicates a few discrete values to indicate 'band' is that on a band like 80m, a given antenna-tuning combo might only give you 20-30khz of low-swr bandwidth centered on a single frequency. Without some way to slice the band up into smaller tuning chunks, merely knowing the radio is tuned to "80 meters" (or even 75) is nowhere close to being adequately granular. Even on 40m, merely dividing the band in half would probably compromise SWR, and leave you with 2.5:1 or worse on a frequency the tuner COULD get down to 1.04-1.18:1 if it were explicitly tweaked to it.
  5. KC3PBI

    KC3PBI Ham Member QRZ Page

    I imagine flrig (or a modified fork) could do quite a lot of this.
  6. NG1H

    NG1H XML Subscriber QRZ Page

    IIRC there used to be a product that sort of did like N4VDI describes. It plugged in the radio (an Icom in my case) and when you hit the tune button it put the radio in low power mode, transmitted a carrier, then while reading the radio's SWR report adjusted the antenna for best SWR. It worked very well. Even K0BG used to lament its passing.

    Implementing something similar using the CAT interface shouldn't be too hard. The only issue I see is that for the model in question (FT-1000MP) I don't see a command to set the power level and so if you didn't want to tune with full power you need to manually lower it.

    BTW - you may already know this but just in case - there are Terminal programs that support hexadecimal. Just connect to the com port connected to the CAT line and test away.
  7. N4VDI

    N4VDI Ham Member QRZ Page

    Well, ultimately, it probably wouldn't be for a FT-1000mp... that's my dad's rig, and the one whose manual I've spent a lot of time reading. I personally don't have a rig yet, and at the moment, have no idea what I'll even ultimately end up getting. At this point, it's more like I'm exploring lots of options and scenarios so I can have them ready to go when I finally either find a magic-unicorn SDR (non-QRP 100w output that ALSO costs less than a thousand dollars) OR stumble over a good deal on something like a used FT-757GX(2) at a hamfest to use for a couple of years until affordable non-QRP SDRs actually become available.

    Part of my motive for exploring the "read the frequency" idea is because my medium-term antenna plan (the point where I'm tired of having to use temporary antennas on a daily basis, but can't afford $2,000-3,000 for something like a roof-mounted Scorpion screwdriver with Tarheel motorized tilt-mount and automatic controller) anytime soon) would be to put a 40m hamstick and Diamond K9000 motorized tilt mount up on the roof (so I could casually use at least 40m with minimal ceremony), and use a secondary antenna mounted to the screen enclosure over the back yard as my "other bands" antenna. If affording a screwdriver antenna, big motorized tilt mount, and electronic controller is utterly hopeless within 2 years, I might augment it with a second K9000 and a 20m hamstick up on the roof as well (further reducing the times when I'd need to use the semi-temporary backyard antenna).

    The thing is... I know, beyond doubt, that if I had to juggle at least two different antennas, it would only be a matter of time until I accidentally transmitted 100w on 20m using the 40m antenna, or 100w on 40m using the 20m antenna. So... I need to come up with some (probably homebrew) way to safeguard against that situation. I'm kind of neurotic about burning out transceivers by transmitting with no (or the wrong) antenna... it happened to me ~30 years ago(*), and was basically why I fell out of ham radio in the first place, so I'm determined to make sure it can't ever possibly happen again.


    (*) August 23, 1992. Early afternoon. I'd spent a few days moving into my first real apartment prior to fall semester starting in a few days, and was in the middle of setting up my 2m radio and TNC when my mom called, crying, to warn me that some hurricane named "Andrew" had suddenly turned towards Miami overnight, was heading towards me, and I had to drop everything and evacuate. She called right as I was powering up the TNC to change some settings (including turning off its beaconing), and it distracted me long enough to forget that the radio was turned on & had no antenna connected. By the time she finally hung up & I made it back to my bedroom, the magic blue smoke was gone from the radio & it never worked again.

    Making matters worse, the next few times I tried using it, I couldn't even tell for sure whether it was fried, because half of Miami's packet network was gone, and the other half was semi-dysfunctional, so I couldn't even tell for sure whether it was due to a problem with MY radio, or a problem with the station I was trying to connect to. At the time, I had almost no real idea where any of the digipeaters I'd used daily pre-Andrew were even physically located, so I couldn't tell whether one that used to be strong was now weak because my own radio was destroyed, or because its own antenna was in tatters and it was hanging on by a metaphorical thread. Sigh... that was the story of life in post-Andrew Dade County for pretty much everything for a few years. :-(
  8. KF5FEI

    KF5FEI Ham Member QRZ Page

    Yes. Some radios have a pin with a stepped voltage that indicates the band they are on. You can also poll the radio via the CAT port and read the frequency from the radio and do the math. Both easy-peasy with an Arduino -- been there, done that.
  9. W4NNF

    W4NNF XML Subscriber QRZ Page

    Yep, you are exactly right. I wish they had standardized early on, but they didn't and after this long are unlikely to do so. Icom uses their "CI-V" standard, the others have theirs.
  10. NG1H

    NG1H XML Subscriber QRZ Page

    You are doing a good job of thinking things through. So good that someone already stole your idea(s) in the past ;). That's what the "intelligent" antenna switch controllers do. Also the controller for the SteppIR antennas. But those all cost a lot of money. By doing it yourself you can save some money, possibly expand on the features, and have more bragging rights than "look at this fancy gadget I that someone else made."

    Do it!

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