Mixing of AM&FM carrier unavoidable?

[Kage]

I am sure not many here have even ran both an AM and FM transmitter at the same time but I encounter this odd problem of the AM carrier mixing with the FM carrier causing signals up and down the FM dial.

For example if I broadcast on 1710Kc and 96.9Mc at the same time I will get a mixed signal at 98.61Mc and 95.19Mc.

This results from the AM carrier signal literally mixing with the FM signal in mid air.

When I walk a block or two away from my house the product signals mostly disappear since they are relatively weak.
However this still worries me a bit since I don't want to piss off neighbors who hear my FM signal up and down the whole damn dial!

I guess this is unavoidable?
I also guess the only way to fix this would be to move the FM and AM transmitting antennas far apart, but that isn't practical in my setup.

Just wondering if any of you experienced this before?

I have noticed this even with our local station here in town as I can clearly pick up a mixed signal of their FM with another AM station that is near by. Sometimes I can even hear their AM signal over FM which is even more bizarre since FM technically shouldn't be able to demodulate AM at all, at least if it's a good quality radio.

[sam]

Feb 4, 2013 23:24:59 GMT -6 Kage said:

I am sure not many here have even ran both an AM and FM transmitter at the same time but I encounter this odd problem of the AM carrier mixing with the FM carrier causing signals up and down the FM dial.

For example if I broadcast on 1710Kc and 96.9Mc at the same time I will get a mixed signal at 98.61Mc and 95.19Mc.

This results from the AM carrier signal literally mixing with the FM signal in mid air.

When I walk a block or two away from my house the product signals mostly disappear since they are relatively weak.
However this still worries me a bit since I don't want to piss off neighbors who hear my FM signal up and down the whole damn dial!

I guess this is unavoidable?
I also guess the only way to fix this would be to move the FM and AM transmitting antennas far apart, but that isn't practical in my setup.

Just wondering if any of you experienced this before?

I have noticed this even with our local station here in town as I can clearly pick up a mixed signal of their FM with another AM station that is near by. Sometimes I can even hear their AM signal over FM which is even more bizarre since FM technically shouldn't be able to demodulate AM at all, at least if it's a good quality radio.

Hi Kage,

I simulcast on both AM and FM as well and I have never experienced what you described. How far apart are your transmitting antennas?

The antennas for both AM and FM are on my property, but they are certainly not next to each other. I have the FM antenna up on the roof of my two story house and the AM antenna is in the backyard at ground level.

For AM, I use an SStran AMT5000 (AMT3000 as a backup TX) and my primary FM exciter is a Ramsey FM35BWT which has a very nice clean output. (Although I do have a plethora of Chinese transmitters that I like to play with I just received an ELE EL-05H that I will review later)

What kind of a setup are you running?

**On a side note: I do process the audio for each feed separately, but I don’t think that would make a difference regarding interference with each other. Mind you, I don't run anything at any significant power anymore due to my past run-in with Industry Canada. I may bend the rules on FM at night, but that's about it these days.

Nice to meet you.

[RFBurns]

Mid-air intermixing is common when examining the signals in the near field. As you already noted and discovered, those intermix signals fade out pretty quickly as you move away from the transmission source/s (antennas).

Basically it is a near field phenomena and has little influence in the far field.

This isn't an issue when your transmitting site is in the middle of nowhere. But in a neighborhood situation, it can be a big problem.

Best solution is to have the AM and FM antennas as far away from each other as possible. Though this will not completely eliminate the problem, it will help. You also have to take into consideration other nearby high powered signals that will mix with yours at the same time yours is mixing and mashing.

One little solution may be to de-tune the FM support structure from the AM signal. The 1710 signal could cause the FM support structure to re-resonate/radiate and aid in the intermixing.

Another helpful tidbit is of course the low pass filtering on both. If either of them don't have enough poles on their respective filters, it could be harmonics well into the 5th or higher that are also mixing together.

Peace!

[Kage]

Feb 5, 2013 8:25:52 GMT -6 sam said:

Feb 4, 2013 23:24:59 GMT -6 Kage said:

I am sure not many here have even ran both an AM and FM transmitter at the same time but I encounter this odd problem of the AM carrier mixing with the FM carrier causing signals up and down the FM dial.

For example if I broadcast on 1710Kc and 96.9Mc at the same time I will get a mixed signal at 98.61Mc and 95.19Mc.

This results from the AM carrier signal literally mixing with the FM signal in mid air.

When I walk a block or two away from my house the product signals mostly disappear since they are relatively weak.
However this still worries me a bit since I don't want to piss off neighbors who hear my FM signal up and down the whole damn dial!

I guess this is unavoidable?
I also guess the only way to fix this would be to move the FM and AM transmitting antennas far apart, but that isn't practical in my setup.

Just wondering if any of you experienced this before?

I have noticed this even with our local station here in town as I can clearly pick up a mixed signal of their FM with another AM station that is near by. Sometimes I can even hear their AM signal over FM which is even more bizarre since FM technically shouldn't be able to demodulate AM at all, at least if it's a good quality radio.

Hi Kage,

I simulcast on both AM and FM as well and I have never experienced what you described. How far apart are your transmitting antennas?

The antennas for both AM and FM are on my property, but they are certainly not next to each other. I have the FM antenna up on the roof of my two story house and the AM antenna is in the backyard at ground level.

For AM, I use an SStran AMT5000 (AMT3000 as a backup TX) and my primary FM exciter is a Ramsey FM35BWT which has a very nice clean output. (Although I do have a plethora of Chinese transmitters that I like to play with I just received an ELE EL-05H that I will review later)

What kind of a setup are you running?

**On a side note: I do process the audio for each feed separately, but I don’t think that would make a difference regarding interference with each other. Mind you, I don't run anything at any significant power anymore due to my past run-in with Industry Canada. I may bend the rules on FM at night, but that's about it these days.

Nice to meet you.

Well I'm not exactly running part 15 legal power lol.
with 10 to 12 watts carrier on AM and 6 watts on FM problems start to pop out that you would never notice from 1/2 those power levels.

The antennas are relatively close, probably around 20' apart. It doesn't help I run both coax cables next to each other out the room but I work with what I have ;D

[Kage]

Feb 5, 2013 9:33:37 GMT -6 RFBurns said:

Mid-air intermixing is common when examining the signals in the near field. As you already noted and discovered, those intermix signals fade out pretty quickly as you move away from the transmission source/s (antennas).

Basically it is a near field phenomena and has little influence in the far field.

This isn't an issue when your transmitting site is in the middle of nowhere. But in a neighborhood situation, it can be a big problem.

Best solution is to have the AM and FM antennas as far away from each other as possible. Though this will not completely eliminate the problem, it will help. You also have to take into consideration other nearby high powered signals that will mix with yours at the same time yours is mixing and mashing.

One little solution may be to de-tune the FM support structure from the AM signal. The 1710 signal could cause the FM support structure to re-resonate/radiate and aid in the intermixing.

Another helpful tidbit is of course the low pass filtering on both. If either of them don't have enough poles on their respective filters, it could be harmonics well into the 5th or higher that are also mixing together.

Peace!

That's interesting. I never thought about the support structure but I do notice my FM tower is hot with AM RF even while the FM antenna is not in use. I guess being that close of proximity it does act like a receiving antenna and to make matters even goofier I am using a J-pole that is grounded directly to the tower so maybe you're right and there is something to that which I should look into.

As you said it's a near field issue. Once I walk away a few blocks the problem is gone. Being as I broadcast often at night I don't think my neighbors care much. They are probably more busy with their smart phones or TV.

There is still a part of me that worries though since I know most peoples radios are POS. There is a good chance that some smart ass has a cheap radio that wants to stir up trouble.
Filtering is not a problem BTW, at least not with my setup