Sharing antenna between KiwiSDR and HF transceiver

With some trepidation I am about to install one of the inexpensive (about $65) RF sense antenna sharing boxes with one of my Kiwis, so that I can occasionally use my ICOM 7300 transceiver without having to swap cables and completely disconnect the KiwiSDR. The switcher box has both a PTT connection to the rig as well as an RF sense, so IN THEORY, it should be OK. However, given how precious a KiwiSDR is (even more so, now that production has ceased), I am following a "trust, but verify" approach, outlined below for any comments that might be had, or flaws that are in my reasoning...

1) Connect everything up, with a dummy load in place of the KiwiSDR, then use an oscilloscope to measure the p-p voltage across the load (if any) during a continuous carrier transmission from the transceiver, set at its full 100W output on each of the 80-10 meter bands. Do this in a 15 second on, 15 second off cycle for about 2 minutes (i.e. use FT8 software to control the rig). Convert the measured VRMS across 50 ohms to a dBm value...make sure it does not exceed 0 dBm.

2) Swap out the dummy load with a "sacrificial" SDR such as an RTL-SDR dongle, old SDRPlay RSP1, etc., and repeat the transmitter output test. Verify the SDR has not been damaged.

3) Connect the KiwiSDR and repeat the transmitter test, starting at very low power level ("0%" setting on the ICOM; about 0.5 watt) and gradually increase the power to 100W while observing the KiwiSDR signal strength during transmission, to ensure it is still "OK".

Aside from this test plan, it occurs to me that adding a couple of diodes (recommended type?) across the Kiwi input would be a good insurance policy. I should also mention that I already have a 10 dB attenuator in place before the KiwiSDR, in order to keep a local MW signal (-15 dBm@1340 kHz) from overloading it, while still preserving the MW signals. This may also reduce the risk somewhat, barring a catastrophic failure in the switcher.


  • edited January 19

    I've recently addressed the same problem and though I've built and tested what appears to be a sollution it is a bit scary to push the limits. My application was connection of my IC-7300 to the 0-2 GHz transverter I've also designed and built. My solution was a Transverter Interface. This has both T/R connection and a RX-only connection that can drive a separate receiver and which I've used with a Kiwi.

    I used the IC-7300's ALC to control the output even when I forgot to turn the power down after HF operation. The design falls back to a bypass mode if the aux cable to the IC7300 isn't connected or if something goes wrong with the AGC and power goes over about 600 milliwatts. This switching is done with an electro-mechanical relay that claims to open within 5 ms. I'm also using transient suppressors on the receive-only output as additional protection, in much the same way the Kiwi does.

    So far it all seems OK, I've run the IC-7300 power up to 100% while measuring the transverter and receiver ports. The IC-7300 seems to have a very well-behaved ALC with no overshoot.

    But I'm still a bit scared ...

    Glenn n6gn

  • Seems you are setting up a scheme where the antenna switchover follows the TX keying. Anybody listening on the Kiwi will see intermittent signals, ie the KIwi will not be usable. Only reason to use this setup seems to be if you are using the Kiwi as rx instead of the main station.

    I have a switchover going, but are switching as soon as the transceiver is powered on. So if the switchover relay does not work there will be no signal to the transceiver and the tx SWR will be mile high. Seems a safer solution than switching the Kiwi in and out according to the tx keying.

    Olaf LA3RK

  • Maybe I misunderstand his goal. If there is only a single antenna, the kiwi can listen to that as long as the transceiver isn't transmitting, in which case the Kiwi must immediately and safely disconnect, ALC is present for safety back up and additional transient protection too along with that already in a Kiwi. That's a Kiwi-as-receiver use.

    If the transceiver is the one with occasional but priority usage, don't use PTT at all, use a switch to enable the bypass, only leave in the ALC/RFsense &protection on the Kiwi side for the case where the operator forgets to throw that switch.

    I wasn't suggesting that he use this exact PCB, just that all the pieces are present for T/R, ALC and overdrive sense. and have been tested on an IC-7300. OD can be used to drive an emergency changeover. All the other protection can be applied for extra safety until switching is complete.

    This require a logic change but all the same HW pieces which do work.

  • Glenn/Olaf,

    Thanks for the information and comments. Indeed my goals are very modest here - to share access to an HF antenna that has better S/N than the majority of public SDRs out there...even though it means that every time I am transmitting, any listener on the Kiwi will be effectively cut off for a few seconds. Given that my use of the transceiver is very sporadic, and the public use of the KiwiSDR is infrequent, the probability of a "use collision" is fairly low.

    Glenn, I currently have the antenna switch box connected to the "send" outlet on the ICOM 7300...which seems to work fine as a "PTT" signal to the box. I do not have a proper understanding of what the ALC port is used for (amplifier control?), so I definitely need to study this more. Connecting the PTT line from the switcher to the SEND RCA phono port just seemed easier than working with the multi-pin accessory connector to access the "official" PTT signal.

    Lastly I'll mention that I am part way through the test sequence outlined in my original post, and am currently at step #2, with an RTL-SDR dongle connected while I use the transceiver for some 100W continuous tests (FT8 mode). So far the dongle seems fine. In step #1 I was satisfied that even with a 100W continuous RF output from the 7300, the typical RF level seen at the SDR port was around -30 dBm, in the very worst case reaching about -10 dBm on the highest frequencies (10m & 6m bands I recollect). While the -10 dBm will overload the KiwiSDR it should not do any damage....especially since I will eventually have a -10dB attenuator in place before it. I also made sure that the PTT connection was working, i.e. with the ICOM RF disconnected if I activated the transmitter the box would switch the SDR port off properly. Also, being somewhat paranoid, I verified that the switcher box "fails safe"....i.e. if it is not supplied the 12V it requires to operate, the SDR port is inactive, as it should be. In a few days I will graduate to step #3 and put the KiwiSDR back on the map, literally and figuratively.

    FYI, this is the current link to the switcher box I am testing...

  • An additional update from some additional testing of the RF sense feature of this box. I disconnected the SDR from the box, as well as the PTT cable, in order to see if the RF sense would properly switch things in the presence of RF coming out of the transceiver. I was startled to find that it took around 2 WATTS (33 dBm) of RF output from the transceiver to activate the relay. This would likely "fry" any SDR had it still been connected. It also means that connecting a 200 mW WSPR transmitter to share the antenna with the Kiwi is completely out of the question, unless I find a way to fashion a PTT connection as I did with the ICOM 7300.

    It's possible I have a defective unit, and some oscilloscope measurements at the SDR port across a dummy load are in order to confirm my observations. Glad that I realized this would be an important test to make before firing up the Zachtek WSPR transmitter....though I could still safely run it with attenuation down to 0 dBm, of course.

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