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Please protect your KiwiSDR 2 from the high-level RF fields of nearby transmitters

We've had one person blow up three KiwiSDR 2s now. Antenna ports wired in parallel and connected to a large antenna. With another ham radio transmit antenna operating a few hundred feet away. No disconnect before transmit. No automatic antenna switch.

The symptom is that one of elements of the electronic attenuator on the Kiwi-2 is now shorted "on" giving a constant 20 dB of attenuation. Interestingly, the remainder of the elements seem to function normally.

So the Kiwi-2 may be somewhat less forgiving than the Kiwi-1 in this regard. The Kiwi-2 has increased protection against impulse and static charge compared to Kiwi-1. But it probably doesn't help the large RF field case very much.

Please be careful.

Comments

  • John,

    Does the transmitter output power matter? Reason I ask is I have a 24/7 10 meter CW beacon at 5 watts output. Does this mean I should relocate TX antenna?

    JP

  • Well, sure, power matters. And size of antennas and how much of the transmit field they are coupling. But I can't definitively say what's okay and what isn't.

    Is there some reason you can't be 100% safe and just switch the Kiwi out of the receive antenna when transmitting?

  • Special care should probably be taken at low frequencies.

    From the PE4312 datasheet:


  • jksjks
    edited March 27

    I was going to post that. But that begs the question: How do I know if I might exceed 10 to 100 milliwatts on my Kiwi's antenna?

  • It's hard to get better coupling than 20 dB with reasonably spaced antennas.

    But what I do with Kiwi1's: Transmit first with very low power and check the signal level on the kiwi. And then calculate how much it can take.

  • Wow, what sort of power level must have been present at the KiWi input to do that ?

    Even with relatively close spaced antennas, the coupling between them isn't usually that great.

    Most receivers will withstand RF levels up to about +10dBm, and I would expect the KiWi to be similar.

    There was recently a bit of discussion about receiver protectors on my websdr IO group.

    This was a simple design I've previously used with other receivers.


    The low wattage 'pea' bulb goes progressively higher resistance as it heats up. This is a graph I plotted of a couple of bulbs I tested.

    The KiWi can be set to auto-mute at excessive signal levels, and this combined with an RF limiter provides a useful TX/RX switch.

    If you look on-line you should be able to find various circuits of commercial receive protectors. Most of which consist of a mini-circuits transformer which saturates to provide soft limiting, followed by a pair of diodes to provide hard limiting.

    And another type using a similar principle.


    Regards,

    Martin

  • Well, as happens all too often on this forum I may not have been told the full story.

  • Thanks for the reminder/warning, jks, even if it wasn't quite the full story.

    Just to note what I'm choosing to do: I just picked up a used DXE Receiver Guard on QRZ.com for $60. I plan to put this inline with my KiwiSDDR2 IN ADDITION to using a MFJ1708B-SDR switch that switches the Kiwi antenna to ground on transmit. This way if the 1708 fails, or my PTT fails, or whatever, the Receiver Guard still limits power to the Kiwi to about 13 dbm from my QRP transmitters. (I plan to share 1 antenna with my transmit equipment.)

    Also, not sure if anyone has ever had an issue with the GPS antenna, but I bought a cheap ($18 USD) GDT static/lightning "protector" on Amazon to help shield any static or nearby strike on the GPS coax. We'll see if it works/interferes with the GPS signal: https://www.amazon.com/Lightning-Arrestor-Coaxial-Cellular-Protector/dp/B07K24RTCW


    -Nate

    N8BTR

  • edited March 28

    With the question about figuring out what your Kiwi might see:

    Not exact, but you can find approximate V/m calcs online for field strength based on antenna gain, distance, power, etc.... Then you can use that field strength and your receive antenna gain, etc to approximately see what your Kiwi might experience in dbm here: https://www.ahsystems.com/EMC-formulas-equations/power-density.php


    -Nate

    N8BTR

  • As a suggestion, if you are curious about coupling between antennas, and you have a VNA, connect the port 1 to one of your antennas and port 2 to another. You should be able make some relative measurements as long as you know the output of your VNA.

  • Shouldn't really need to know level of VNA...

    • normalize the P1 to P2 to 0dB as you normally would
    • P1 to Ant1, P2 to Ant2
    • read dB loss
    • apply that loss to your TX power to find reduced level at other antenna


  • Wouldn't knowing the output level improve accuracy?

    RX gain type antennas, and really even active antennas in general react differently to near field that get into them. What about common mode?

    Knowing what is to be expected as a maximum would certainly help.

  • if you're just measuring loss, that with be in dB and regardless of dBm from source, dB is the same relative loss.

  • Just anecdotally I have had my two Kiwi I's on and connected to a dipole about 90 feet from a triband yagi sometimes powered with 1100 watts at 10/15/20M for years, without damage.


    I would not recommend doing this, but I mention it only to indicate the extreme tolerance of the Kiwi 1 to strong RF fields.

  • Hi, maybe quite off topic, but could probably help explaining some mysterious failure. Some consideration in where RF power actually takes effect. Not even on transmitter's antenna, as it should be. Me performed a testing on CB radio, and after 5 seconds after pushing the transmit button on mike magic smoke came out of car antenna's attenuating coil. Reason: No good fastened connector. The RF vagabundates uncontrolled. And forward reverse ratio detector does not switch off transmitter as soon as possible.

    Would be better, as suggested to take care and protect any receiver equipment. In particular if directly assembled on transceiver with no safely working TX/RX switching inbuilt.

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