Jim,
Yes, the annual measurements include any intermod products that are detected.
If the intermod is not being radiated from any of the included stations’
antenna systems, its presence is noted and any direction or directions it
appears to be coming from. There are 2A – B and 2B – A mixes, here, between
two radio station transmitters, located a couple of miles apart. Neither
intermod product can be nulled toward either transmitter site with the Faraday
shielded loop antenna incorporated into the field intensity meter. The
intermod appears to be coming from multiple, multiple sources. Probably,
utility lines, mostly. The level of the mixing products varies greatly,
especially on a windy day. It would be virtually impossible for the stations
to locate and mitigate every source of the unwanted products. If the mixes
were occurring in the transmitting equipment, they could be filtered out. The
stations involved are 40 kHz apart, so these mixes are in the broadcast band
and do not cause any interference to any other stations. This intermod has
been around here as long as I can remember. I have spent much time trying to
track down a source and, finally, threw in the towel. The mixes can no longer
be heard 20 miles from town. It’s unfortunate, but sometimes there is no
practical solution.
Gary, K0CX
Yes. But do those spectral measurements also include intermod products?
I started out in ham radio in jr high school with two 5kW non-DAs within
a mile of each other and about 1.5 miles from me, on 930 and 1470 kHz
producing a strong product at 3870 kHz. There may have been others
products, but that one gave me a lot of grief.
Jim K9YC
> Every AM broadcast station is required to perform annual measurements,
> searching for these types of problems. I contract to perform spectral
> measurements for most of the AM stations within a 100 mile radius of my home.
> I have the necessary equipment and travel to within approximately 1
> kilometer of each transmitter site. I measure each station?s occupied
> bandwidth, measure harmonics, up through and including the 6th and search for
> spurious signals, below, in and above the broadcast band.
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