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Front end overload from HF transmitters

Does anyone have direct experience running the KiwiSDR in proximity to HF transmition?

My Kiwi antenna (50 meter EFLW 9:1 unun) is about 50 meters from my transmitting 20/40/80 Meter dipole. They're running the same direction, but not overlapping. I am only putting 100 watts into the dipole antenna.

I am concerned about excessive RF hurting the front end on the Kiwi.

I could install a remote antenna switch to ground the Kiwi antenna while transmitting but it would be a little complicated. The Kiwi is on another barn and I am using a router extender to bridge the gap. It works well, much better than a 300 meter feed line.

Thanks,

Duncan KL7QT

Comments

  • edited September 9

    The most basic (quick) protection is back to back signal diodes across the green antenna terminals.

    I'll have a look for the threads about overload protection because there was some good information there.

    Stu

    There are commercial solutions but as mentioned in this thread much of the issue depends on the coupling between TX and RX antennas (passive RX antenna assumed).

    http://forum.kiwisdr.com/index.php?p=/discussion/comment/10586/#Comment_10586

  • njcnjc
    edited September 9

    1N4148 diodes in parallel facing opposite directions across the input. The term "back-to-back" has always been a pet peeve since it incorrectly describes what we are trying to do. You want to limit the voltage to Vf in both directions.

    You could also check out limiter diodes from Skyworks. I've used those too.

    ETA: Check out Skyworks SMP1330-005LF

  • in the presence of BIG sigs, when those diodes conduct, they create new problems

  • Indeed they do, but those problems do not result in blown hardware. I think if we are going to be TX-ing in proximity to our kiwisdrs, we have to accept that during tx there will be distortion/overload/mixing products and that the kiwi will probably be unusable during that time. Agree?

    Now if we're talking about a nearby broadcast tx, then attenuation and notch filters could be a solution.

    Nick

  • I have a schematic for a better solution that uses PIN diodes.... I'll dig it out.

  • jksjks
    edited September 9

    The Kiwi has TVS diodes across both inputs (SMA and terminal block, see schematic: http://kiwisdr.com/docs/KiwiSDR/kiwi.schematic.pdf).

    Another big problem can be static charge buildup on antennas, especially wire antennas (from wind or T storms or whatever). For that I would put a 1 meg resistor across the terminal block inputs. Won't effect the signals, but will drain the static charge to some degree.

  • @jks out of curiosity, what is the part number of the TVS used?

    It looks like the absolute max input limits on the LTC6401-20 ADC is specified as a max current of 10mA. If I'm reading the datasheet right the lowest input Z is 170 ohms. At 3.3V that's about 20mA.

    Note 2 says:

    Note 2: Input pins (+IN, –IN) are protected by steering diodes to either supply. If the inputs go beyond either supply rail, the input current should be limited to less than 10mA.

    So does the 10mA limit only apply IF the voltage exceeds the rail?

    Ref: https://www.analog.com/media/en/technical-documentation/data-sheets/640120f.pdf

  • Also, he's right about static charge buildup. In my case, the beverage antenna is transformer coupled so there is no DC but other antennas can and will have static charges from rain/snow/passing storms/wind/alien spacecraft/etc.

  • There's a bill-of-materials linked on the Github page.

  • I've run 100W transmitting in the environs of Kiwis and other receivers on nearby antennas with no damage. With care in positioning/polarizing, I've even been able to run full-duplex from a single QTH that way.

    I think there may generally be far less problem than is feared. If/when one deliberately tries to couple power between two antennas, close together but each in the far-field of the other, it turns out to be very difficult to get even 20 dB coupling. If there is polarization and spatial separation typical to most installations, I wouldn't be surprised if the numbers were 40 dB or greater. Thus 100W TX power may only get milliwatts, not the watts it would likely take to damage components if clamping diodes are present.

    Why don't you just put a bunch of attenuation on the Kiwi such that you can measure the incident power to see if there really is a possibility of damage. If you have less than 100 mW I think you are very safe. You may be fine even into the watts region, though I've never tested a Kiwi to see where the clamping diodes fail.

  • I would certainly not recommend it, but I've left my Kiwi's on when running 900W output . Kiwi has not been (yet) permanently damaged.

  • Howdy. I'm a new KiwiSDR owner and am using a 6-way antenna switch to switch between tranceivers and /or the Kiwi and several antennas. Whenever using a transmitter, the Kiwi is always off the antenna circuit via the switch, but I still see a big RX signal on the Kiwi dashboard due to the RF field even though the Kiwi isn't in line. I'm hoping the limiters in the circuitry will handle this without having to add more equipment. Based on what I'm seeing above, I'm thinking it isn't a problem. 73

  • edited November 10

    your 6-way switch likely gives 60 dB of isolation. a 100W TX would be +50 dBm. So the signal at the Kiwi would be -10dBm, enough to "thump" it but not damage it.

    edited to correct 100W vs dBm error

  • A 100W transmitter directly lnto the switch would be +50 dBm which with 60 dB of isolation (if that's the value) would be -10 dBm into the Kiwi and maybe triggering OV but not hurting a thing. I suspect that a Kiwi can take +20 dBm ( 100 mW) and perhaps even nearer a watt without damage though I've never measured it carefully. Your switch could likely have relatively poor isolation and things would still be fine.

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