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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 26th Mar 2020, 9:40 am   #1
crackle
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Default Radio Direction finder

I have an old RDF from my boating days. https://www.radiomuseum.org/r/electr...eafix_200.html
I have been thinking if it would be possible to use it as a RDF to locate interference. It may work well as it is on the current frequency range of about 150kHZ to 500kHz picking up harmonics from the source of the interference.
But I have been wondering if it could be converted so it could be tuned to the specific frequency of the interference.
Can anyone recommend a simple and small scanner type kit that could be tuned to cover from 1MHz to 500MHz.
It could maybe coupled to the internal ferrite rod with a loop of wire.

Thanks
Mike
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Old 26th Mar 2020, 9:55 am   #2
Andrew2
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Default Re: Radio Direction finder

Hi Mike. I think you need to go down in frequency to get to the fundamentals, which will almost certainly be switch-mode power supplies running in the range 30 to 120 kHz.
In my experience, this type of noise tends not to radiate directly from the offending gadget, but creeps out onto the mains connection and into the house wiring and out into the wider world where it uses the mains cables to radiate.

I've found that harmonics (especially the higher ones) can seem to come from a different direction than the lower ones as the mains wiring has a filtering effect and distorts the radiation pattern.
You also run the risk of confusing one noise with another as they all tend to have the same buzzy sound.

I don't know what others think, but I reckon you'll get a very confused picture if you work on anything but the fundamental and maybe the low harmonics.
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Old 26th Mar 2020, 10:27 am   #3
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Default Re: Radio Direction finder

We believe the interference in question is coming from a mains substation transformer.
I thought it would normally be impossible for these to give interference but I suppose the consumers connected to it could insert noise from their items that are connected. The noise would likely be stepped up onto the high voltage primary winding.

Mike
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Old 26th Mar 2020, 11:31 am   #4
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Default Re: Radio Direction finder

Something has changed on the interference front around where I am. With current circumstances, the local amateur radio club has started up a nightly net on 144MHz. I haven't used that band in years, so I've been out of touch with what's happening there. There simply hasn't been any activity, it's been an empty wasteland. But on Monday I heard a broad mains-modulated noise at a very strong level across the whole band. Hmm, not heard that before. Then I noticed the waterfall spectrum on my HF rig (tuned to 7.1MHz showed mains modulated noise across a lot of that band. The two effects come on and off together.

Whatever the source is it covers MF to VHF

The mains related modulation clearly says it's power supply related, or mains distribution. I've been madly powering down everything here, suspicion inevitably starting with that new microwave-combi oven fitted on Monday to replace the one that packed in several weeks ago. Nope!

Still haven't found the source, but others a few miles away can hear the noise too.

Now isn't really the best time to go wandering around on a QRM-hunt. I have a Hacker Helmsman to restore. That would make a fine DF receiver with its ferrite rod, and looking just like a posh transistor portable, it won't look too threatening.

To track down QRM source, anything that can hear it, or any harmonic of it, will do if it has a loop antenna or ferrite rod. If using an external antenna, better screened sets are advantageous. Once you get close you suffer from overload. There's not much point in using an external antenna and attenuator if the set lets signals in through the cabinet and receives them bypassing you waving the loop around. (been there, done it!)

You can put a small radio in a metal biscuit tin, just the external antenna cable goes in, and some little perforations let the sound out.

Some SMPS now run a lot faster than 150kHz. A few MHz is fairly common driving 1v supplies for domestic PCs.

David
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Old 26th Mar 2020, 5:38 pm   #5
David Simpson
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Default Re: Radio Direction finder

Mike,
A couple of years back or so I used my portable BEME Marine DF LW/MW Set to trace noise on my ADSL Broadband. Worked a treat. Located it to a pole 3 spans away in the company of an Openreach "Engineer". ( My opinions on Openreach's use of the term engineer, and my internet problems at the time, can be found via "Search".
Its over 20 years since I retired from our local utility, but have heard over the years that much development has gone into using the distribution network to carry superimposed sub-station switchgear controlling signals. Over 40 yeas ago, I used to do PTW manual switching of air breaks & racking-in of oil switches. A lot of this has been automated nowadays. At sub-stations these days, one can also see VHF & UHF Antenna's & microwave dishes.

Regards, David
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Old 28th Mar 2020, 5:52 pm   #6
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Default Re: Radio Direction finder

So no one is aware of any simple little SW kit radios then?

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Mike
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Old 29th Mar 2020, 9:45 pm   #7
m0cemdave
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Default Re: Radio Direction finder

One sector of the amateur radio hobby is known as "fox hunting". No furry animals are involved in this, er, pursuit - it's about finding hidden radio transmitters. Often now known under rather more PC terms such as ARDF or Radio Orienteering.

Although much of it takes place on the 2 metre VHF band, 80 metres is also used.
There are various kits available for small handheld receivers and the 80m types are very useful for tracking down interference, even though they only cover a small frequency range.

A typical kit is this one, which I have used successfully for tracing sources of noise from computers, video screens, SMPSU's etc:
http://www.nationalradiocentre.co.uk/ardf/80mrx.html
I don't know how old that page is, so if you consider ordering one, check with the supplier first...

Another version here - online plans, but in the Dutch language:
https://www.qsl.net/pa3fdc/tech/hrx80/

There are plenty of others available but, as always, be aware that the quality of cheap Chinese stuff sold online can be very hit or miss.

I know this is not exactly what you are looking for, but some may find it helpful.

Last edited by m0cemdave; 29th Mar 2020 at 10:05 pm.
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Old 30th Mar 2020, 12:53 pm   #8
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Default Re: Radio Direction finder

During the weekend evenings I like to listen to the SW bands, but after 10.30 pm I change to MW. There is always strong interference on MW that sounds like a car ignition running at fast idle.
At 11.10pm on the dot the interference disappears and the band is clear.
I often wonder if it is someone’s heating system shutting down for the night?
Lynton
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