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I live near an AM transmitter, and the input of my RTL-SDR becomes overloaded. I've added an AM filter and it's really helped, however I still get a very strong AM signal with some IMD all over different bands.

Would daisy chaining another AM filter help?

I have to admit I'm concerned that the harmonics from the transmitter are strong enough to cause IMD also, which of course the AM filters wouldn't prevent.

My aim is to receive HF ham radio as well as shortwave radio from around the world.

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Yes, multiple filters in series will combine “like they ought to”.

However:

  • You will also see more loss and ripple in the passband (all the imperfections of the filter design are also multiplied). This probably doesn't matter for your application; you're not designing a receiver or spectrum analyzer.
  • Even a perfect filter doesn't help at all with interference that's not coming in through the antenna but directly impinging on the receiver itself, or the shield on the computer side of things, etc. If this is an issue, it may or may not help to ensure that your RTL-SDR is one of the metal-cased models.
  • No filter helps with interference that is within the passband, such as the harmonics you mention. However, from my own experience and understanding of RF standards, I would not expect there to be substantial harmonics (in the specific sense of multiples of the carrier frequency) from a properly operating AM transmitter.

    Signals at the wrong frequency, whether harmonics or intermodulation products, appear due to the signal entering a nonlinear process/device. If that's your receiver, then the filter will help. If that's the transmitter, nothing will help for you but your local regulatory authority will care.

    A third possibility is another object that happens to be in the vicinity, or bad connections in your receiving system, that act as nonlinear elements (the "rusty bolt effect"). Therefore, it can be useful to apply direction-finding techniques to try to locate undesired signals and eliminate their source.

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  • $\begingroup$ Thanks for your detailed answer, it's very helpful! Regarding point 2), yeah, my RTL-SDR and the Ham-It-Up converter are both in metal cases. My 9:1 "balun" is not though, but the filter(s) come directly after this. Regarding 3) I do indeed hear harmonics at frequency multiples, although tbh it's the IMD (mixture of multiple stations) that cause more problems. Could it simply be that I live so close and it's quite high power that this is happening? Yes, there are actually 2 or 3 stations broadcasting from the site. $\endgroup$ – user14615 Apr 4 '19 at 15:09
  • $\begingroup$ All of the above aside, if I disconnect the antenna and hear nothing at all, is that "proof" that the IMD isn't coming from within the receiver? $\endgroup$ – user14615 Apr 4 '19 at 15:10
  • $\begingroup$ @user14615 No, because you need the input signal for it to be distorted. It is weak evidence against my second point (interference coming in through paths that are not the antenna) but you're also disconnecting part of the shields/grounds of your system so that's not the whole story. $\endgroup$ – Kevin Reid AG6YO Apr 4 '19 at 16:03
  • $\begingroup$ Sorry, I worded my question totally wrong. I guess what I meant to ask was, if I disconnected the antenna and received nothing, is that proof that any non-linearity that my receiver is exhibiting is due to signals received by the antenna? Please correct me if I'm wrong, but my theory is that the IMD is being caused by the input being overloaded, and by reducing the input, hopefully the IMD won't happen. $\endgroup$ – user14615 Apr 4 '19 at 16:16
  • $\begingroup$ @user14615 Nonlinearity is a property of something that a signal is applied to. It doesn't make sense to say that it goes away when the signal does; the distortion products go away and so you're no longer in conditions that the nonlinearity can be observed, but it's still a feature of the system. Perhaps you are interested to know that you may still see spurious signals with no antenna connected; those are not the same as distortion products. $\endgroup$ – Kevin Reid AG6YO Apr 4 '19 at 16:37
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Cascading more filters may help. Also simply adding an attenuator may help, as long as the RF noise floor is above the receiver's internal noise floor.

However, spurious emissions from the station can also be an issue. In the US, spurious emissions are regulated by 73.44, which states:

Emissions removed by more than 75 kHz must be attenuated at least 43 + 10 Log (Power in watts) or 80 dB below the unmodulated carrier level, whichever is the lesser attenuation, except for transmitters having power less than 158 watts, where the attenuation must be at least 65 dB below carrier level.

Say this is a 1000W transmitter: that means spurious emissions more than 75 kHz (which would include all harmonics) would have to be attenuated by 46 dB. 46 dB less than 1000W is 25 mW, which is still quite a lot if you are very near the transmitter.

Unfortunately, there's not much you can do about spurious emissions from the transmitter, except attempt to orient your antenna to null them out.

You can determine if a spurious signal is from the transmitter, or generated by your receiver, by switching an attenuator in and out. If you have a 3 dB attenuator, and it makes a 3 dB difference to a spurious signal, that's most likely something being received by the antenna. But if adding or removing the attenuator makes a 6 dB difference or more, that signal is generated by your receiver.

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  • $\begingroup$ The transmitter is in the kilowatts lol. I'm pretty sure the signals causing problems are the original signals (which overload the input), and the IMD generated within the receiver, due to it being overloaded. My second filter hasn't arrived yet, but I'm hoping it will help. Thanks! $\endgroup$ – user14615 Apr 4 '19 at 16:12

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