Nearby strong AM broadcasting station

[VE2VAE]

Hello folks !

I am the new owner of an RSP2. My main interest is listening to distant AM broadcast station and beacons (NDB) in the low frequency, wich I am doing since a while with an old (1980) Kenwood R-1000 receiver.

So, the Hi-Z antenna input of the RSP2 will be great.

I have not tried my SDR yet, but I am getting more and more concerned about the "nearby field" of a local strong AM broadcast station.

I live 6 km (3.5 miles) from 730 AM, Radio Circulation (french canadian), a 50 Kilowatt AM broadcasting station. On the S-meter of my kenwood, it shows a 60 dB over S-9 ! Wich is a lot ! By using 60 dB of attenuation, on my Kenwood, it still gets in at S-7, S-9.

So... yeah... I am a bit concerned now.

Chris from Montreal area, Quebec, Canada.

[g1hbe]

Hi Chris. If you are worried about harm to your RSP2 from 730 AM, I doubt the signal will come anywhere near strong enough to actually damage your front end, but you will almost certainly get some trouble with images of the station appearing within the NDB band. I live about 10 miles south of a big TX site which is host to three AM stations in the 100 to 250 kW power level. Boy are they strong. I also have a local broadcaster site just 3 miles to the NW which has a 4 kW, a 1kW and a 100 W TX. The 4 kW puts in a whacking signal here.
No damage, but I have to use a low-pass filter which keeps out everything above 500 kHz if ever I want to listen on LF or VLF. It stops the images and lowers the noise level.
Listening in the MW when there is a BIG unwanted signal also in the MW can be interesting. Use as low a sample rate as you can as this will select a narrow IF filter internally, and tune so that the big signal is well off the screen. If you do get an image in an awkward place, try clicking the tuning cursor to a new spot on the 'dial' and re-tuning. This should move the image. Keep the RF gain low, you don't need super-sensitivity on the BC bands. And as always with this type of SDR, don't tune a station right in the middle of the display (ie on the same frequency as the local oscillator) as this will cause distortion. Keep to one side or the other. Or select LIF mode and it won't matter.
I use the Hi-z input for VLF. LF and SW. 50 r inputs for VHF/UHF.
Sorry, I've gone on a bit.

[mike0agner]

Hi Chris. I'm sure you're not the only one to have to deal with a strong MW station within striking distance of your home. You have basically 2 options - install a filter or use a passive preselector which is much like a tunable filter. It will allow only a certain range of frequencies to pass while heavily attenuating others.

There are several distributors of filters, and a couple of passive preselectors (the MFJ 1046 and the Cross Country offering, which are just 2) which are listed here in the RadioReference wiki under this article;

https://wiki.radioreference.com/index.p ... _Reception

I would also search the other forums; I'm 99% certain this topic has been discussed in detail before. They may have other links that are not in the above article.

When looking at filters, make sure you examine the response curve to see if your MW station will be attenuated.

At such high signal levels you might even need to start looking at any overly long connecting cables that *might* end up as unintended re-radiators. I would also consider using a loop to try to get away from some of that RF. At such signal levels, it's entirely possible that the loop might be overwhelmed, but it is worth a try. In this case, a passive loop (such as the Chameleon series or the DoxyTronics) might actually be a better choice, as these loops are tuned every so often. This article, again from the RadioReference wiki, has lots of such links...

https://wiki.radioreference.com/index.php/Loops

This is a tough nut to crack, for sure. GL...Mike

[mike0agner]

Mea Culpa. I thought you were tuning on HF, not MW and LW. Nevertheless, the Cross Country and MFJ-956 passive preselectors would be a better bet for you, rather than the other MFJ one.

Many of the loops I have on that page really don't cover LW all that well; however I'm willing to bet the Longwave Club of America would be right up your alley insofar as the LW beacons are concerned. As the name suggests, they cover LW almost exclusively and I'm willing to bet your problem is one they've seen before.

https://www.lwca.net/

Still though the comment on overly long leads still works

That R1000 has a VERY hot front end; I don't think you mentioned anything about what antenna you are using, but that is a factor to be considered

Mike

[ernestyn]

Dear VE2VAE, please use a notch filter narrow with the notch on the strongest AM station near. The notch stops it and you can listen far away MW. Perhaps use a low noise small as shown by Mike 1961 in The antenna chapter.

[ernestyn]

Sorry I forgot. Use a piece of coax 1 quart wave. Leave open other end and cut small parts until strong station goes down a lot no more

[mike0agner]

Using a quarter wave stub works fine on VHF, but on MW it's very long and unwieldy. Better to use a tunable notch, and I know of only 1 commercial unit...

http://www.scott-inc.com/html/notch.htm

Mike

[VE2VAE]

Hi Chris. If you are worried about harm to your RSP2 from 730 AM, I doubt the signal will come anywhere near strong enough to actually damage your front end, [...]

Hi Andy ! aaah, that is good news.... Yes, I wanted to be sure that it wouldn't damage my SDR2.

What, you live near 100 kw and 250 KW station ??? Wow ! That is a huge amount of power, for sure !!!

Thanks for the tips with low sample rate, narrow IF filter, low gain. and tuning the stations that or off-centered on the screen. Some notion learned as an amateur radio operator will help me, but I surely have to learn how to use that "new toy". HI, HI.

Thanks again Andy,

Chris. 73

[VE2VAE]

Hi Chris. [...] You have basically 2 options - install a filter or use a passive preselector which is much like a tunable filter. It will allow only a certain range of frequencies to pass while heavily attenuating others.

Hi Mike !

Yes, filters ! I bought one, last year, from DLW associates. That was an High-pass filter, to stop and reject all the AM broadcast band, and allows only signals from above (especially lower ham bands such as 160m and 80m. Doug, the owner, was kind enough to make a custom on for me, with the strongest null (-60db) centered on 730 KHz.

But, for listening the AM band, or NDB, you are right... the best would be a tunable filter. I might be able to build one... winding some coils and soldering variable capacitor... probably a good project for next winter nights, hi hi. Thanks for the links on radiorefence and Longwave club of America.

Yeah, I am planning to use loops, the kind mounted on a wooden frame, about 4-6 foot in diameter, about 150 foot of small wire coiled around it, with a variable capacitor to tune it, and a second loop, for the coupling. One advantage of those loops, is that they have nulls, so that you can null out an unwanted signal. And by tuuning them, you can also de-tune the interference station. I think that is the way to go.

Thanks Mike.
73

[VE2VAE]

use a notch filter narrow with the notch on the strongest AM station near. The notch stops it and you can listen far away MW.

Hi ernestyn.

Interesting those narrow notch filter. I will have to try that. Thanks

Chris.