Medium Wave Coaxial choke

Noise and interference issues?
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daveB
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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
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Patrick73
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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
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Patrick73
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Re: Medium Wave Coaxial choke

Post by Patrick73 » 18 Nov 2018 11:43

Yesterday evening, I ran a short test with a FT240-31 toroid core. I was able to wind up to 16 turns, not more due to the diameter of the coax.

Very effective down to 2 MHz, but useless below, whether it was placed on the SDR or the antenna side or used around the USB cable. Maybe more turns are needed, but it would then require a thiner cable / wire I don't currently own.
I'm now looking for a different kind of core that offers better permeability on LF / MW (material 73, 75 or 77), hoping for some improvement.

I also gave the GI300 another try and, as prevously reported, it brings even more noise ...

:|
Patrick
Main interests : NDB, MW & FM DX'ing
REU Editor http://www.classaxe.com/dx/ndb/reu

satnipper
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Re: Medium Wave Coaxial choke

Post by satnipper » 18 Nov 2018 11:50

The two cores mentioned here are what you need - largest for power cables and smaller for data leads.
http://www.ifwtech.co.uk/g3sek/in-prac/

pe1etr
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Re: Medium Wave Coaxial choke

Post by pe1etr » 18 Nov 2018 11:53

Patrick73 wrote:
18 Nov 2018 11:43
Yesterday evening, I ran a short test with a FT240-31 toroid core. I was able to wind up to 16 turns, not more due to the diameter of the coax.

Very effective down to 2 MHz, but useless below, whether it was placed on the SDR or the antenna side or used around the USB cable. Maybe more turns are needed, but it would then require a thiner cable / wire I don't currently own.
I'm now looking for a different kind of core that offers better permeability on LF / MW (material 73, 75 or 77), hoping for some improvement.

I also gave the GI300 another try and, as prevously reported, it brings even more noise ...

:|
Patrick you need RG-174 coaxial cable. You can get more than 30 turns around a ferrite torroid.

Peter

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Patrick73
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Re: Medium Wave Coaxial choke

Post by Patrick73 » 18 Nov 2018 12:11

Thank you both Peter and Stuart for the tips.
Patrick
Main interests : NDB, MW & FM DX'ing
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KnobTwiddler
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Re: Medium Wave Coaxial choke

Post by KnobTwiddler » 18 Nov 2018 14:21

Watch this video on YouTube, it helped me with common mode interference (I also found an improvement by running my Mini Whip on a leisure battery) >> https://www.youtube.com/watch?v=0JnBX_7usl0
AirSpy HF+ SDR + Full-wave Loop for FM + Home-brew Mini Whip for LW/MW/HF or 9:1 Balun + Whip Aerial
Yamaha T-D500 Tuner For DAB - Alinco DX-R8 Comms Rx - Tecsun PL-880 & ICR-110
Yaesu FT-897D - Icom IC-2200H - Anytone AT-588 for 70Mhz

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Patrick73
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Re: Medium Wave Coaxial choke

Post by Patrick73 » 18 Nov 2018 14:36

I watched this video a few days ago ! Mark did a good job to reduce RFI.
I also own a 12V / 7A battery but noise level is the same when using an AC adpater (the one provided with the Weelbrook loop) whose cable is wrapped around a ferrite rod.
Patrick
Main interests : NDB, MW & FM DX'ing
REU Editor http://www.classaxe.com/dx/ndb/reu

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Patrick73
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Re: Medium Wave Coaxial choke

Post by Patrick73 » 18 Nov 2018 16:18

Further tests this afternoon in order to maybe lower the noise from my home side :

- Tried different USB ports - front panel : no effect
- Tried different USB ports - rear panel : no effect
- Tried another USB cable : no effect
- Switched off the internet router : no effect
- Switched off the monitor : no effect
- set the FT240-31 core on the antenna side : more noise !

It seems everything is OK on my side (the TV was also off and unplugged).

The main culprit is actually an interference from one of my neighbors, with about 240 kHz spacing (712, 949, 1185, 1423 kHz, etc), quite wide (the 1200-1700 kHz segment is almost unsable when weak signals DXing). Any idea ?
A TV set I suppose, as it often starts around 18-19z during the week.
Attached is a screenshot of this interference, showing the 240 kHz spacing (with the loop pointed to most favourable azimuth)
Attachments
Capture.JPG
Patrick
Main interests : NDB, MW & FM DX'ing
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satnipper
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Re: Medium Wave Coaxial choke

Post by satnipper » 18 Nov 2018 20:35

Plasma tv QRN is normally every 15k IIRC. Could be tv or a chinese switched mode power supply for cheap set top box.

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KnobTwiddler
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Re: Medium Wave Coaxial choke

Post by KnobTwiddler » 19 Nov 2018 02:06

satnipper wrote:
18 Nov 2018 20:35
Plasma tv QRN is normally every 15k IIRC. Could be tv or a chinese switched mode power supply for cheap set top box.
If it's a Plasma TV you can use a null steerer (better known as a phasing unit) to null out the interference if it's on a spot frequency.
I have a massive amount of electrical noise at my QTH across the whole of MW and HF, unfortunately a null steerer has little effect on this type of interference.

Easy to remember:
QR(M) = Man made noise from all of the crap gizmos that people use to cause interference to us radio enthusiasts (oh and F*****g British Rail in my case). :evil:
QR(N) = Natural noise and atmospherics from nature which you can't do anything about and annoys our John in Skegness.
This made me laugh >> viewtopic.php?f=31&t=5101&hilit=when+is ... ng+to+stop.
AirSpy HF+ SDR + Full-wave Loop for FM + Home-brew Mini Whip for LW/MW/HF or 9:1 Balun + Whip Aerial
Yamaha T-D500 Tuner For DAB - Alinco DX-R8 Comms Rx - Tecsun PL-880 & ICR-110
Yaesu FT-897D - Icom IC-2200H - Anytone AT-588 for 70Mhz

satnipper
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Re: Medium Wave Coaxial choke

Post by satnipper » 19 Nov 2018 08:45

That was the original definition over a hundred years ago. However the usage of QRN and QRM has evolved. I use QRN for " to mean any noise that interferes with propagation or reception of transmissions" and QRM for deliberate or unintentional interference from another station.

Stuart

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KnobTwiddler
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Re: Medium Wave Coaxial choke

Post by KnobTwiddler » 19 Nov 2018 12:08

Well when I did the Radio Amateurs course in 1987, that's how I was taught to interpret the Q code - I must admit the guys who taught me were old, but I don't think they were a hundred years old. :lol:

KT.
AirSpy HF+ SDR + Full-wave Loop for FM + Home-brew Mini Whip for LW/MW/HF or 9:1 Balun + Whip Aerial
Yamaha T-D500 Tuner For DAB - Alinco DX-R8 Comms Rx - Tecsun PL-880 & ICR-110
Yaesu FT-897D - Icom IC-2200H - Anytone AT-588 for 70Mhz

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