Behavior of AM broadcast images in longwave spectrum?
Posted: Sun Feb 10, 2019 8:03 pm
I'm scratching my head a little over the behavior that I'm seeing with AM broadcast images in the longwave spectrum, and would be interested in any help or ideas.
I have an RSPduo attached to a Wellbrook ALA1530LNP mag loop antenna, controlled by SDRUno 1.24 on a Windows 7 PC. Normally the longwave spectrum is totally overwhelmed with noise and entirely unintelligible, due to numerous strong MW AM signals in our urban Los Angeles area.
I just received a low-pass filter I bought from Dale Parfitt at Par Electronics, which does a really nice job of knocking back the noise. Thanks to it, I was able to decode a number of WSPR signals on the 630m ham band last night.
Today I was tuning around to see what if anything I could receive during daylight hours in the longwave spectrum. Around 200-300 kHz I noticed images from a couple of the stronger MW AM broadcasters located nearby. This didn't surprise me, as the filter attenuates but does not entirely remove medium-wave RF; the stations I'm seeing typically put out 50,000 watts and are just a very few miles away.
My question: I noticed as I tuned that each broadcast image changed frequency as I moved the tuning cursor and the LO changed. For example, when LO was 427.2 kHz, a broadcaster transmitting at 1430 kHz appeared in SDRUno at ~278 kHz. If the LO was increased by only 11 kHz to 438.2 kHz, the frequency of the broadcaster image increased by ~45 kHz to ~323 kHz.
I'll confess that I haven't yet paid much attention to the inner workings of the SDRplay RSPs. I assume this has to do with an intermediate frequency or some such issue. Could anyone explain what causes the frequency behavior I'm seeing?
I have an RSPduo attached to a Wellbrook ALA1530LNP mag loop antenna, controlled by SDRUno 1.24 on a Windows 7 PC. Normally the longwave spectrum is totally overwhelmed with noise and entirely unintelligible, due to numerous strong MW AM signals in our urban Los Angeles area.
I just received a low-pass filter I bought from Dale Parfitt at Par Electronics, which does a really nice job of knocking back the noise. Thanks to it, I was able to decode a number of WSPR signals on the 630m ham band last night.
Today I was tuning around to see what if anything I could receive during daylight hours in the longwave spectrum. Around 200-300 kHz I noticed images from a couple of the stronger MW AM broadcasters located nearby. This didn't surprise me, as the filter attenuates but does not entirely remove medium-wave RF; the stations I'm seeing typically put out 50,000 watts and are just a very few miles away.
My question: I noticed as I tuned that each broadcast image changed frequency as I moved the tuning cursor and the LO changed. For example, when LO was 427.2 kHz, a broadcaster transmitting at 1430 kHz appeared in SDRUno at ~278 kHz. If the LO was increased by only 11 kHz to 438.2 kHz, the frequency of the broadcaster image increased by ~45 kHz to ~323 kHz.
I'll confess that I haven't yet paid much attention to the inner workings of the SDRplay RSPs. I assume this has to do with an intermediate frequency or some such issue. Could anyone explain what causes the frequency behavior I'm seeing?