Topband: ADC Overload from MW transmitters

[Steve Ireland]

Hi Jim

Clearly in a large US city, there is going to be a whole larger degree of difficulty than here. 

Perth is still pretty much a small city in world terms, with a population of about 2 million. In addition to the ABC transmitters, we have about half a dozen other transmitters, but only two of these have signals of any size – 6PR (10kW) and 6IX (2kW), with the former of these putting in the largest signal to me, with its transmitter/antenna on the banks of the Swan River estuary about 15km away.

When I used my HPSDR, originally I had no filtering in front of the ADC and had some overload problems on 160m from the local BC stations. However, a simple Chebyshev HPF got rid of this. Later when I added the Alex bandpass filters, which are part of the HPSDR design, there was no longer any need for the HPF.

The main point, as Phil says in his post, is that the amount of protection an ADC is going to need will vary widely, depending on factors such as local AM BC transmitters and how strong they are. In my case, all I had to do was to use the general coverage facility of the SDR to look at the medium wave here, see which of the signals were largest and look for a suitable HPF design accordingly .

Vy 73

Steve, VK6VZ



> That's typical of medium-size cities in the US for high power broadcasters, but major cities typically have twice as many. Both large and medium-size cities, as well as smaller ones, typically have 6-10 stations in the 5kW range, and more in the 1kW range. Chicago is typical of a large city (like New York, Boston, Los Angeles, San Francisco) -- it has 50kW on 670 kHz, 720 kHz, 780 kHz, 890 kHz, and 1,000 kHz. There's also a daytime only station with 50kW on 1160 kHz. Cincinnati is typical of smaller cities like Indianapolis, Detroit, Minneapolis, Cleveland, St Louis, and New Orleans, with 2-3 50kW stations and many smaller ones. Cincinnati 50kW stations are on 700 kHz and 1530 kHz. 

I grew up in a small town in WV, with three 5 kW stations within two miles on 800 kHz, 930 kHz, and 1470 kHz. 

Bottom line -- there's a lot more broadcasting in the US than in most countries. 

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