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My use of the word “proximity” applies to AM stations that are nearby in frequency. I’ll give two examples of what I’ve observed.
My use of the word “proximity” applies to AM stations that are nearby in frequency. I’ll give two examples of what I’ve observed.
There is a 5kW omni-directional station 1-mile from here at 850 kHz and so strong at this location I hear it on the telephone.
When first testing my SSTran I found 960 kHz was clear, but when my 10-foot antenna was tuned to that frequency it was close enough to the powerful station that their modulation fed into into my final stage and caused a kind of “star wars” sounding impression of their program. When I re-calibrated to 1060 kHz I was clear of their influence.
Today I have two streaming radio stations and two SSTrans, at 1640 & 1680 kHz, respectively.
Even though the transmitters are 75-feet apart, there is a trace of the 1640 audio super-imposed on the 1680 signal.
I notice that in most cases the FCC frequency assignments for licensed stations keep nearby frequencies far apart from each other in geographic space. The “proximity effect” I’ve described seems to be a recognized real-world phenomenon.
Total posts : 45366Hi Carl!
Wasn’t there another person on this
board with 2 transmitters that had
the same observation?
It’s really cool that you are running
2 SS-Trans at the same time. I have
one running and another not built yet
still in the box.
I hope your local omni 5 kW on 850
isn’t an obnoxious station, if it gets
into your phone.
Best Regards
Bruce, MICRO1690/1700
Total posts : 45366In VHF/UHF work on mountain tops where antennas were 10 feet apart on a tower this was a continual problem and we used highly selective cavities and a device called a circulator to prevent RF from an adjacent transmitter from flowing out one antenna and down into another antenna and then backwards through the transmitter going back to the final input or one of the earlier stages and mixing with the local signal, creating intermodulation and other noise etc. and then returning to the RF amplifier being amplified and the returning to the antenna being radiated as garbage raising hell all over the site. To add insult to injury the spurious signal would rob power from the main signal in the amplifier! In the early days before spectrum analyzers became common these were hard to spot. A wattmeter could show that a radio was putting out rated power, say 100 watts, when only 5 watts was on the channel you wanted. A spectrum analyzer would show a spectrum (looked like a comb) of frequencies getting technicians out of their beds for a 50 mile radius. The circulator is a three port device (think RF diode) that only allows RF to flow one way through it. What goes in port A comes out port B, What goes in port B comes out port C, What goes in port C comes out port A. So hook the transmitter to A, Antenna to B, and a dummy load to C. The wanted signal goes in at A and out to the antenna at B. Any unwanted “returning signal” arriving at the antenna goes in B and comes out C being harmlessly burned up in the dummy load. Don’t know if these expensive devices would work at low frequencies or not. VHF FM transmitters would not be allowed on a mountain top without one. Dave
Total posts : 45366In my area, I’ve heard a 50kW Class B at 950 kHz bleed over onto 1710! The clear channel on 760 has done the same thing. I found that quite odd.
Total posts : 45366Driving in the mountains like Shenandoah, or out in western Maryland, you can see transmitters on the tops of mountains all huddled together. I always wondered how that worked exactly. Do they run the transmitter lines all the way up the mountains!?
Total posts : 45366This may be happening in your receiver. Don’t be to quick to judge the transmitters. Many problems with consumer receiving setups can be traced to the original design and quality of consumer equipment. Overloading the front end or any end of a receiver can cause many strange phenomena. Intermodulation mixing can happen in any non linear device (diode, transistor junction, etc.) and overloading can cause transistor amplifiers in a receiver to be biased wrong causing all kinds of problems.
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