[RFI] Poor man's portable spectrum analyzer

[David Robbins]

Loop antennas do not have to have a capacitor to resonate them.  you can make a simple loop with just a couple turns of wire... for hf I use about 12" diameter and 2 or 3 turns, whatever I had handy... the one thing that really helps the directionality is to choke the feedline near the loop, as many turns as you can get through whatever ferrite happens to be handy... then go hunting!  No need for computers, spectrum analyzers, or other fancy stuff just a receiver that can hear whatever noise you are hunting and go find it... as I noted before, a grundig g5 has a built in antenna that has nice nulls, and its very inconspicuous walking around.


David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net:7373



-----Original Message-----
From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of Charles Gallo
Sent: Friday, July 13, 2018 17:22
To: David Eckhardt
Cc: Rfi List
Subject: Re: [RFI] Poor man's portable spectrum analyzer

On Fri, July 13, 2018 1:06 pm, David Eckhardt wrote:
> I have never been able to display more than 8 MHz with the SDRPlays.
> However, with the AirSpy using the SpectrumSpy application which comes 
> with a download of SDR#, I am able to display on one screen from 
> roughly
> 25 MHz
> through 2 GHz.  Of course, you can display less if desired and zero in 
> on specific 'signals' when desired.  I have no connection with AirSpy 
> other than owning 3 of them.
>
> Dave - WØLEV

My question is not the SDR (I have one running), or even "How the heck did you get it running on an rPi (software etc) but...

I always see references to loop antennas, and have even seen a few links to plans.  They all use a variable cap to tune the resonance, but they all seem to use different values etc.
Assume I'm going to a hamfest this weekend, and I'm walking around and the usual vendors have bins of variable caps, everything from small little things, to huge ones with 20-30 plates that look more like what you see in an amp or an antenna tuner.  They don't seem to have markings similar to what a fixed cap has (either XXXmfd/pdf or 103/104 - which I understand). 
How do I pick a cap, and if necessary, tweak the design of the antenna

Right now, I'm not very radio-active (need to get my replacement rotor installed, but I can get on 17m and 80m without that), but I like to help my friends with RFI.  I DO have various receivers, and even have an Arrow 2m beam and attenuator, but something for HF/MF isn't in my kit


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