AM signal interaction?

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Shane
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AM signal interaction?

Post by Shane »

A friend noticed an interesting phenomenon that I, too, noticed once but never looked into.

2 AM stations 140 kHz apart. The one higher in freq. is 10000 watts most of the day, non-D; the one lower in freq. is 1000 watts day, 2-tower directional.

Likely none of that matters. Or does.

These sites are about 3000 feet apart. When the lower power/freq. station is off - no carrier - the higher station can be heard on the lower’s freq. It does not appear on adjacent channels nor any of the channels in between the two, nor any other freq that we know of.

An interesting side: this would always happen on the lower station’s mod monitor. We always assumed it was overload due to proximity. But the audio was pretty clean as it was when heard on a regular car radio at greater distance (miles, I believe).

I think there is a filter network at the higher station to reduce mixing products but no such filter at the directional.

If you could follow this without actual call letters and freqs. (thinkin’ this group will figure out the who’s and what’s), what could be going on here?
Mike Shane, CBRE
— — Omaha — —

kcbooboo
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Re: AM signal interaction?

Post by kcbooboo »

I run spectrums for a 1kw station that's 40 kHz and 3000ft from a second 1kw station. When mine was running IBOC, I'd manage to get the other station to shut down for 10 minutes so I could get a decent trace, because they'd show up at -40dBc at the IBOC sample port on my TX.

Signals more than 75 kHz away should be 80dB below the carrier level for any station 5kw or higher. Mod monitors are pretty insensitive as they're usually connected to a sample port on the antenna system. It's certainly possible that the daytimer's antenna system is helping to receive the 10kw signal. However if the mod monitor is receiving the signal off-the-air at the studio, then it could be picking up the 10kw signal and there may not be much you can do about that other than add a trap or filter.

Has anyone scanned the band with an FIM to check the signal level at +/- 140 kHz? Is there also another station 140 kHz above the 10kw station that might preclude checking the AM spectrum? I have that in my situation with the two being only 40 kHz apart and the signal must be at least 45dB below the carrier. It barely exceeded that level but with the other TX off, everything was clean. It was better to eliminate the other station rather than try to explain such a strong signal on the spectrum.

140 kHz sounds suspiciously like twice a PDM frequency of around 70 kHz. Maybe just a coincidence.

Bob M.

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kkiddkkidd
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Re: AM signal interaction?

Post by kkiddkkidd »

Shane wrote:
Wed Feb 12, 2020 10:28 pm
A friend noticed an interesting phenomenon that I, too, noticed once but never looked into.

2 AM stations 140 kHz apart. The one higher in freq. is 10000 watts most of the day, non-D; the one lower in freq. is 1000 watts day, 2-tower directional.

Likely none of that matters. Or does.

These sites are about 3000 feet apart. When the lower power/freq. station is off - no carrier - the higher station can be heard on the lower’s freq. It does not appear on adjacent channels nor any of the channels in between the two, nor any other freq that we know of.

An interesting side: this would always happen on the lower station’s mod monitor. We always assumed it was overload due to proximity. But the audio was pretty clean as it was when heard on a regular car radio at greater distance (miles, I believe).

I think there is a filter network at the higher station to reduce mixing products but no such filter at the directional.

If you could follow this without actual call letters and freqs. (thinkin’ this group will figure out the who’s and what’s), what could be going on here?

Several random and fleeting thoughts come to mind...

Other than the mod monitor, where are you hearing the ghost signal?

If the DA doesn't have filters, that may be a problem itself.

Do both stations pass NRSC?

Have you ever listened with the DA station TX completely off? As in dead? ND station still there?

Or try connecting/disconnecting various parts of the DA system? IE, pull j-plugs in various points or short across tower bases, etc. Any change in level or sound?

Do you hear a spur on the ND station 140khz on the other side of their carrier? I can't think of a single instance where I've done an NRSC test where a TX had PDM filter problems that doesn't show up with symmetrical spurs. Harris/Gates SX and Gates transmitters are notorious for their +/- 60khz when the PDM filter caps go bad. I have seen a few early Nautel TX's with the same spurs at about +/-70khz.

Later,
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

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Deep Thought
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Re: AM signal interaction?

Post by Deep Thought »

Does the ghost signal go away when the 1 KW transmitter is completely off...as in disconnected from power?

I don't know if this is the case in your example but BE AMs have a funny way of transmitting mix products even when they are "off". Jerry Westberg described them to me once as being a very efficient mixer and since there is only about 20 dB return loss from the transmitter output terminal to the PA *any* excitation which ends up there will be amplified and sent back out to the antenna. I had a mix product spur in an AM-6A once which increased in intensity when the transmitter was turned off. The only way to get rid of it entirely was to turn off the breaker.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Shane
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Re: AM signal interaction?

Post by Shane »

Thanks much. This is all very helpful. There is also a station right smack dab between the two - 70 kHz away from both in different directions - which would mask the existence of a 70 kHz spur. On the other side of the high station there are no locals 70 and 140 kHz out but there is one 60 kHz out. I told him to check up that end, too.
Mike Shane, CBRE
— — Omaha — —

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