Understanding image rejection and harmonics

[dietermoreno]

On my TV radio, which might be broken because I bought it for only $2 at a thrift store, of course there is no sound on the TV part of the dial anymore... accept for at CH 13!

At CH 13 it is receiving station 105.5

CH 13 is 210 MHZ, which is double 105 MHZ.

So is CH 13 the first harmonic of 105 MHZ and that's why its able to receive it?

Keep in mind that this only works for very strong local stations. The 105.5 station is only 5 miles away from my house and I can hear it in my guitar amp if I don't use a noise gate pedal.

So in theory, with a bigger antenna perhaps, could I receive station 95.1 MHZ on CH 10 at 192 MHZ?

In the high VHF TV band it is making all sorts of beeping and buzzing digital noises all over the place, but CH 2 (CH 11) is the only digital VHF station in Chicago!

Perhaps, the digital broadcasts in the high VHF TV band I am receiving are actually harmonics of the digital broadcasts actually at UHF?

 

[(*steve*)]

Do you still have analogue TV where you live?

 

[dietermoreno]

In the U.S. analog TV is allowed to continue at low power on all TV bands (but usually only on VHF because the low power signal travels farther at VHF than UHF) until September 2015, presumably to make room for a digital FM band in the old (current) low VHF TV band. Which can be presumed from the FCC accepting comments in January 2014 for a new FM band in the current TV channels 5-6.

However, CH 6 is the only low power analog TV station in the Chicago area remaining. Right now its a sports-talk format. I've never actually been able to get any pictures on my TV from it, just the audio, but presumably there is a picture signal accompanying the audio signal to please the FCC.

All full power analog TV stations stopped in the U.S. in 2009.

 

[(*steve*)]

At CH 13 it is receiving station 105.5

CH 13 is 210 MHZ, which is double 105 MHZ.

So is CH 13 the first harmonic of 105 MHZ and that's why its able to receive it?

It is possible that the station has some very small radiation at the first harmonic and you're receiving that. I really can't comment on whether that's likely, but you'd have to be pretty close -- and it sounds like you are.

So in theory, with a bigger antenna perhaps, could I receive station 95.1 MHZ on CH 10 at 192 MHZ?

Possible, but maybe not very likely.

 

[dietermoreno]

Yes, the Star 105.5 station is only 5 miles away.

No, I was not able to receive 95.1 on Channel 10, which 95.1 is about 15 miles away.

So we have proved then that it is possible to receive the first harmonic of an FM station in TV frequencies if the station is less than 5 miles away and is a commercial transmitter.

So I suppose that an FM trap for my TV would be a very good idea.

Well anyway, funny thing is, when I changed the TV radio antenna from a clothes hanger facing a wall to a log periodic TV antenna facing the window at an attitude looking at the sky, now CH13 is overwhelmed with DTV CH 12 ( CH 2-1) and you can't hear the radio station anymore on CH13.

Much the same with hearing several different digital signals and the low powered CH 6 analog TV on the low VHF dial, when I changed the antenna now the whole low VHF dial is spammed with DTV CH 5 (CH 68-1), except for it becomes quiet at CH 3 when I turn on the digital converter box that has an RF modulator on CH 3.

So it appears that adding more gain to an antenna can actually make weak stations go away because they become overwhelmed by new stations!

So maybe that's why my digital converter box receives CH 68-1 but not CH 2-1 even though CH 68-1 transmitter is farther away because of the first harmonic of that local FM transmitter overwhelming it!