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:
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
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...
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.
Hi, Are nearby transmitters in the VHF and UHF frequency bands likely to cause issues for the KiwiSDR 2 like HF transmissions?
I'm thinking say a 25 Watt transmitter in either the 144 to 148 MHz or 430 to 450 MHz bands is the antennas a close, like 10 to 20 feet away from the Kiwi antenna?
I'll make sure the Kiwi is off and not connected when I fire up on 28 MHz. Thanks for the heads up on this one.
The kiwi has a low pass filter ahead of the attenuator. That should give some extra protection for frequencies above 30 MHz.
Maybe John has measurements or simulations, but a quick simulation that I ran gave -24 dB on 50 MHz and -92 dB on 144 MHz.
Yes, I suggested that configuration to John, to alleviate the problem of an antenna being too close to a VHF/UHF antenna.
The attenuator would have most likely survived, but I was concerned about IMD being generated whilst RF was present.
Regards,
Martin
I've just installed a Kiwi 2 and was concerned about how much power it maybe exposed to from my main transmitting antenna which is parallel to the Kiwi loop and only about 100ft away.
So ran the tx up to 200 watts output on each band in turn from 1.8MHz to 30MHz and measured the power at the Kiwi. The power varied from 0dBm to 10dBm (1mW to 10mW) depending on band.
This is with the loop preamp turned off - had it been turned on the power would have been some 10dB higher so worst case 100mW or 20dBm. The preamp has a high OIP3 and can easily produce 27dBm output - hence my concern for the Kiwi input.
As a precaution the homebrew linear PSU which runs both Kiwi and preamp has an RF detector such that if the Kiwi sees more than 10dBm a 20dB attenuator is switched in. This means that when I transmit the attenuator would just about activate on every band reducing the signal to 0dBm or less.
73 Tim
Good evening, I'm sorry, can we have details or a diagram of how you built this attenuation system? I'm interested in doing one. Thanks and 73 de IK8SUT Antonio.
Lately I have been testing an excellent product from SV1AFN.
It works as advertised and might be a very good solution if you insists on placing your Kiwi`s close to strong RF fields.
If you are using a active antanna on your Kiwi, be reminded of the possibility of very high levels from the amplifier in the antenna. As an example a LZ1AQ loop can measure up to 6Vpp, +20dBm or 100mW. I usually prefer not to expose any of my receivers of signals higher than -10dBm.
Please see the enclosed link. BTW I am just a happy customer and not affiliated with SV1AFN.
73 de Hans LA9LT