Why do weak signals boost strong ones on HF? See explanation below

[Beamin]

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.

[langwadt]

https://en.wikipedia.org/wiki/Dither ?

[radiolistener]

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.

[radiolistener]

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/

[w2aew]

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.

[profdc9]

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.

[Beamin]

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.

Unfortunately I am living in the city now so I'm guessing I'm not going to have much luck with shortwave. On the plus side I have access to a fire escape to the roof 4 stories up so I could string a long wire out it. I'm still setting my electronics up as I lost a few bits moving like my huge box of coax. 


When I read the reviews of the SDRPlay it scored as well as 300$ models and seemed like it had a really good adc with 12 bits of sampling. TVdongles don't even work below 30 mgh but that makes sense as there is no tv down there..

[radiolistener]

When I read the reviews of the SDRPlay it scored as well as 300$ models and seemed like it had a really good adc with 12 bits of sampling.

It has very bad ADC, which parameters can be compared with 7-bit ADC.

SDRPlay is overpriced. This is more marketing product.

It has 12 bit ADC, but it's performance is even worse than 8-bit ADC of RTLSDR. SDRPlay and RTLSDR have almost the same performance in average. SDRPlay is a little less sensitive to strong signal, but has a little worse IMD than RTLSDR. SDRPlay has less sensitivity and more gaps in frequency coverage. Also SDRPlay has 8 MHz bandwidth against RTLSDR 3.2 MHz. Wide bandwidth helps to see wider frequency span on the waterfall, but this is not required for radio listening.

The bad thing with SDRPlay is that it has worse NCO noises than RTLSDR. Also it has terrible gain control in comparison with RTLSDR.

If you try to compare SDRPlay with fair 12-bit ADC receiver, you will find a huge (very huge!) difference. When you can see that difference you will realize that the real SDRPlay performance is the same as RTLSDR and even sometimes worse. They trying fix it by adding filters and amplifiers on the input, but you can do the same with RTLSDR. You cannot fix bad dynamic range and bad IMD performance with filters and gain tweaks.

The good SDR receiver with high dynamic range don't needs filters on the input (at least cheap like used in SDRPlay), because they cannot improve something and just add losses.

TVdongles don't even work below 30 mgh but that makes sense as there is no tv down there..

Regarding to short wave, RTLSDRv3 works very well from 100 kHz to 30 MHz, I compared it with SDRPlay and didn't find any difference. With RTLSDRv3 you can see and receive all short wave stations that you can see and receive with SDRPlay. But SDRPlay has more dead frequencies and spurs which are missing on RTLSDRv3.

Here is comparison for SDRPlay and RTLSDR tv dongle, both are working simultaneously on short wave (1413 kHz).



Do you see any advantage against RTLSDR? And this RTLSDR tv dongle cost just 21 usd

So, don't believe to that marketing stuff. You can do the same with RTLSDRv3 dongle for less price and even with better quality

If you want better SDR quality, try Hermes Lite 2. It's price about 200-300 usd. But it has much better performance.