Author Topic: Why do weak signals boost strong ones on HF? See explanation below  (Read 293 times)

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Offline Beamin

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That's kind of a vague question but this would always happen: When I lived out in the county an upper class suburb of DC I had a long wire antenna set up with a balun good ground and went horizontal from the 2nd story to a tree. A few miles down the road there was the worst thing ever: An AM radio station. When I would see a signal coming through on the HF bands with my SDR you could hear the AM radio station while they were transmitting then they would stop and it would go back to the noise. If someone a few hundred miles away had a HF rig with 100 watts, you could hear them and the AM station or sometimes just the AM station. How is it that the AM station "hijacks" their signal and is completely gone when they stop transmitting? Is that a problem in my SDR? It was an SDRPlay so not some junk TV dongle.
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Online langwadt

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Online radiolistener

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Re: Why do weak signals boost strong ones on HF? See explanation below
« Reply #2 on: November 09, 2019, 01:40:25 am »
dither doesn't related to this issue at all. This issue caused by receiver input overload and IMD distortions. 

So, the short answer is your receiver issue due to bad dynamic range. Your receiver has small dynamic range and when it overloaded you got high IMD distortions, which means that you will receive more images mixed from different frequencies.

These AM stations that appearing in presence of strong station are actually images. They are transmitted at different frequencies. Their frequency are shifted due to mixing effect caused by high IMD distortions of your receiver.

And regarding to your question, on the topic title, the answer - NO, there is no boost, no amplification of weak signal. These signals that appears in presence of strong signal is just phantom signals which is generated in your receiver, they are transmitted at different frequency, their frequency is shifted in your receiver due to high distortions.

You can find these stations on different frequency and will be able to receive them on real frequency even when strong station don't works.
« Last Edit: November 09, 2019, 02:43:13 am by radiolistener »
 

Online radiolistener

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Re: Why do weak signals boost strong ones on HF? See explanation below
« Reply #3 on: November 09, 2019, 01:57:41 am »
It was an SDRPlay so not some junk TV dongle.

I didn't tried SDRPlay, but I tried Chinese clone MSi.SDR which actually almost the same as SDRPlay, but with no preselectors on the input. And I can say it's performance is even worse than RTLSDRv3 TV dongle.

SDRPlay 2 has several bandpass preselector filters on the input, it should be better, but not so much.

If you want really good SDR performance, don't look at these junk SDRPlay, RTLSDR, hackrf and other stuff. They all have very bad dynamic range and IMD performance. Their performance is even worse than cheap low-cost analog receivers.

If you want really good SDR performance, you're needs device with fair separate high speed ADC with direct down converter on FPGA. But the price for such devices starts from 200-300 usd. And the price for middle class SDR will be about 500-800 usd.

Also, such SDR should use ultra low phase noise oscillator, otherwise dynamic range will be reduced and you will get the same effect as you have on your SDRPlay.

With such device with good performance external ADC and ultra low phase noise oscillator you will get much-much better performance. But it will cost you about 1000 usd.

For example you can see on receivers such as ELAD FDM-S2:
http://ecom.eladit.com/epages/990298944.sf/en_GB/?ObjectPath=/Shops/990298944/Products/%22ELAD%20FDM-S2%22

There is also cheap low cost project Hermes Lite 2: http://www.hermeslite.com/
« Last Edit: November 09, 2019, 02:33:40 am by radiolistener »
 

Offline w2aew

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Re: Why do weak signals boost strong ones on HF? See explanation below
« Reply #4 on: November 11, 2019, 08:52:51 pm »
In addition to the input overload and IMD issues, which certainly can be happening, another likely cause would be the AGC in the receiver.  AGC, or automatic gain control, is a circuit that adjusts the receiver's RF/IF gain inversely to the received input signal strength.  Thus, strong signals can result in reducing the RF/IF gain, often making weak signals "disappear" into the noise.  In the case of many SDRs, there is often very little filtering on the front end, so large signals like your AM broadcast signal, can overload the front end, causing distortion across a wide range of frequencies, but can ALSO result in reducing the gain due to the AGC.

In short, you'll likely need to build/buy an AM Broadcast reject filter to keep the high power AM broadcast from sledge-hammering your SDR front end.
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Offline profdc9

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I have used this AM broadcast filter

http://www.kf7p.com/KF7P/Morgan_Filters.html

I have the M-400X, it is good up to about 100 watts.
 


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