Medium Wave Coaxial choke

Noise and interference issues?
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daveB
Posts: 1240
Joined: 07 Oct 2014 16:49

Medium Wave Coaxial choke

Post by daveB » 10 Nov 2018 21:26

Quite by accident I have come across a wave to possibly suppress MW interference picked up on the braid of the coax.

As posted previously the endless 62.5 kHz QRM centred on the MW band is noe premanently resident. I decided to move the Loop aerial to anther spot in the garden. To do that I needed to extend the cable. I only had a 30 m coil of coax which I didn't want to cut so just added in-line with most of it coiled up. Expecting some signal attenuation I checked what the spectrum looked like before moving the aerial and after. It didn't make any difference where the aerial was - QRN was jut as bad. But I did notice that the interference level was a few dB lower - but the signal strengthen of the reference stations was the same.

Hmmm....was the 30 m of coiled coax acting as a choke on MW?

So today I tried an experiment. I found out that with the power to the loop aerial turned off I was getting strong spikes of interference at ! MHz with weaker sidebands either side. That seems indicative that the outside of the coax is picking up interference
Loop antenna pre-amp unpowered.jpg
Next I added the 30 m coil of cable at the receiver end. The spikes were completely suppressed suggesting the coil (about 200 mm diameter)
Loop antenna pre-amp unpowered.jpg
Note that the residual spikes in the second picture are actually MW transmitters.

Unfortunately with the loop antenna powered up the spikes were all the way from 520 - 1900 KHz - and there was no difference with or without the coax coil. Except that the noise floor was higher with the coax coil in place - maybe the extra voltage drop degrades the performance of the loop.

Anyway it is an interesting observation - and if a 6 turn coil of 150 mm dia acts as an effective choke it follows many more turns are needed for an effective choke at medium wave.
Attachments
Loop antenna pre-amp unpowered via coax choke.jpg
SDRPlay, AirSpyMini, RTL dongles, SB920s, Yamaha TX930. 5 ele compact yagi on rotator at 18ft agl. FM5 facing east, FM5 facing south, FM3 facing NE, OIRT dipole, 3-ele Moxon vertical facing east.

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Patrick73
Posts: 238
Joined: 22 Mar 2015 12:36
Location: french Alps, JN35am

Re: Medium Wave Coaxial choke

Post by Patrick73 » 16 Nov 2018 09:44

Hi Dave !

To fight RFI on NDB band / MW, last month I purchased a GI300, Galvanic Isolator, from Bonito.
Though quite effective for some people, this device is totally useless for me. Worse still, it brings even more noise whatever the frequency is, from LF to VHF (the GI300 is supposed to work from 30 kHz to 300 MHz). Note : the galvanic isolator is directly mounted on the SDR socket, no cable here to pick up interferences.

Several mails were to Bonito to report this and I haven't received a single reply yet.
A few days ago, I bought a second-hand FT240-31 toroid core but haven't tested it yet.
Attachments
LF MW WITHOUT GI300.JPG
LF MW WITH GI300.JPG
Patrick
Main interests : NDB, MW & FM DX'ing
REU Editor http://www.classaxe.com/dx/ndb/reu

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