Re: New to the list
If I can run the SDR and WD on a single Pi in a shielded box with no LAN then the only external connections will be a short battery lead and the coaxial lead in. The phased pair I described are on 50 ft double shielded LMR-100A feedlines so as short as possible and a lumped element Pi network for the phase delay feeding into a minicircuits ZFSCJ-2-1 inverting combiner. The chokes I made should give about 5k ohms at the low end of HF, will this not be sufficient?
---------- Original Message ----------
From: Glenn Elmore <n6gn@...>
Date: July 14, 2021 at 4:38 PM
Be careful not to underestimate the amount of CM rejection you may need. A Mini-Circuits T1-1 transformer, for example, has around 1 pf interwinding capacitance. That may produce less than 30 dB CM rejection. Similarly, chokes only generate a few k-ohms, at best, broadband and against the few hundred ohm source Z of many long cable/wire CM sources don't do a whole lot. If one injects a signal between the LAN-side 'ground' on a Kiwi and the SMA ground on the other end, the unwanted coupling is only around 80 dB down at midband and not more than 100 dB een down at LF> Considering the kinds of currents that may exist on longisy LAN or antenna cables, it may take a *lot* of rejection to stay below the Kiwi's -157 dBm/1-Hz (or so) noise floor.
I'd not suggest using the Kiwi decoder. It was compiled from a much older and less capable decoder and fed with the same signal does not produce as many successful decodes even if/when it can keep up with the volume of traffic.
Recording/decoding on a separate RPI connected via WiFi gets around the interference noise source problem.
There have been some comparisons of knock-off v 'normal' Kiwis and AFAIK, the knock offs don't seem to do any better and maybe not even as well for most uses. KA7OEI has written about some of this on his blog.
You may decide differently but for myself, I'm not inclined to give freeloaders, those trying to make profit with little or no return on others' efforts my support. I understand, but don't know for sure, that there was even contention between a 2nd knock-off and the first. I suspect you would get better support from a stock Kiwi, should you stay that way.
On 7/14/21 2:06 PM, VE3VXO wrote:
Thanks Rob and Glenn for your comments.
I am doing tests with a pair of phased loops so good current balance and they are each coupled to their feedline through an isolation transformer at the amtenna end and each also have a feedline choke with good common mode isolation at the frequencies of interest as well to guard against common mode noise injection at least on the feedline end of things. I think for the ground loop issues I will try to simplify and eliminate CM noise sources and ground loops as follows. I discovered a clone of the KiwiSDR called flydog which uses a Pi 3B or even 4B (but ships with a 3B). If there are enough resources there to run the Kiwi host and WD concurrently it will help a lot with these issues. I'm guessing I will need a 4B with some extra memory. Since the logging will be done without any internet whatsoever that's another noise source eliminated and I can use a substantial lithium battery which I already have with a linear regulator for the tests. I don't know if this knock off SDR is any good, but I don't have any backup for my bonafide SEED KiwiSDR so this will serve as an exploration of the clone and possible backup for the good one as well and allow it to stay safely at home. I like it too much to leave it out in a field overnight!