Strange behaviour on 20m

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Sasan
Posts: 96
Joined: Sat Jun 27, 2015 1:15 am

Strange behaviour on 20m

Postby Sasan » Sun Feb 07, 2016 1:32 pm

I've got a very strange problem occurring only on 20m (14MHZ). I haven't seen this occur on other frequencies:
The shape of spectrum and waterfall changes for some time (less than a minute) and then gets back to normal.
In that period, I can't receive anything. It's just noise.
It occurs with different behaviours and shapes and is not predictable. (I mean, I can't reproduce it intentionally. It's random)
I've uploaded some pictures of it: http://imgur.com/a/Y9VgM

My system info:
SDRSharp v1361 and HDSDR on a PC (Windows 7 SP1 x64)
ExtIO plugin installed: 3.7 (build 0109)
Installed API: 1.8.1

I'm using a 14MHZ Dipole as antenna and a shielded USB cable (with ferrite)
I can tune to local FM stations, commercial shortwave stations, local AM stations and VHF radio without problems.
I've tested all combinations of LNA and AGC (I usually use 1.536MHZ IF bandwidth and 2M sample rate and Zero IF)
Could it be a problem in the tuner? Because the problem shows only in a particular frequency.
I tested it on a tablet too (Windows 10 Pro 32 bit) but the results were similar to the PC.
Any ideas?

Thanks in advance
Last edited by Sasan on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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g1hbe
Posts: 228
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: Strange behaviour on 20m

Postby g1hbe » Sun Feb 07, 2016 8:07 pm

Hello Sasan.
The third screenshot looks very much like the shape of Tetra (or similar) which is used in the 300 - 400 MHz area. As the SDRPlay uses up-conversion to cover the HF bands, it is possible that harmonics of the conversion oscillator are heterodyning UHF signals into the 20m band. This is a common problem with any RX that uses frequency conversion. Although the SDRPlay has front-end filters, they will not completely stop this.
You could try opening the EXTIO panel and changing the conversion oscillator frequency. This may move this response away from 20m.

Andy G1HBE.
Last edited by g1hbe on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Andy

Sasan
Posts: 96
Joined: Sat Jun 27, 2015 1:15 am

Re: Strange behaviour on 20m

Postby Sasan » Sun Feb 07, 2016 10:12 pm

g1hbe wrote:Hello Sasan.
The third screenshot looks very much like the shape of Tetra (or similar) which is used in the 300 - 400 MHz area. As the SDRPlay uses up-conversion to cover the HF bands, it is possible that harmonics of the conversion oscillator are heterodyning UHF signals into the 20m band. This is a common problem with any RX that uses frequency conversion. Although the SDRPlay has front-end filters, they will not completely stop this.
You could try opening the EXTIO panel and changing the conversion oscillator frequency. This may move this response away from 20m.

Andy G1HBE.


Thanks. The problem is that this behaviour is unexpected and random. It comes and goes away without knowing in advance.
Last edited by Sasan on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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g1hbe
Posts: 228
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: Strange behaviour on 20m

Postby g1hbe » Sun Feb 07, 2016 11:49 pm

OK Sasan. It could be that the interfering signal is coming on and off. Try tuning your SDRPlay around the 300 - 420 MHz area to see if you can find a signal that looks like this.
Last edited by g1hbe on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Andy

CaptainNemo
Posts: 126
Joined: Mon Jan 05, 2015 1:22 pm
Location: Genoa - Italy

Re: Strange behaviour on 20m

Postby CaptainNemo » Mon Feb 08, 2016 1:19 pm

In my QTH (I live in a block house - five floors) I have a problem similar to the pattern in your third picture.
In my case it is due to an elevator (to change the speed of the elevator there is an inverter and this generate noise on large part of HF).
In the 3rd picture it seems you got the moment when the elevator stops.

Recently -- since the motor was 20 years old -- we had the service supplier to change the motor, inverter and cabling.
The noise was reduced but still is there.
Bye
Last edited by CaptainNemo on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Sasan
Posts: 96
Joined: Sat Jun 27, 2015 1:15 am

Re: Strange behaviour on 20m

Postby Sasan » Sat Feb 13, 2016 5:47 pm

CaptainNemo wrote:In my QTH (I live in a block house - five floors) I have a problem similar to the pattern in your third picture.
In my case it is due to an elevator (to change the speed of the elevator there is an inverter and this generate noise on large part of HF).
In the 3rd picture it seems you got the moment when the elevator stops.

Recently -- since the motor was 20 years old -- we had the service supplier to change the motor, inverter and cabling.
The noise was reduced but still is there.
Bye


Thank you. I think, the problem may be the elevator, but it's a new one.
I live in a five floor apartment (in 3rd floor)
I experienced this behaviour with two different Yaesu transceivers. For some seconds, I couldn't hear anything on 20m, except heavy noise.
So the SDRPlay is not the cause.
Last edited by Sasan on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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