2 Transmitters

[XXL]

What would happen if you put 2 transmitters of maybe 100w each, on the same frequency, just 10 or 20 meters apart? Would there be some kind of feedback ?

[Krakatoa]

If they don't share anything in common (supply, ground, audio cable, antenna system) they will just interfere each other and cause a mess as their unsynchronized carriers will mix, sum and substact power from each other.
But, there is a catch, and it may give you some fun idea: if you have 2 transmitters working at almost the same frequency and their difference is some hundreds of Hz or a couple of kHz, their signal will mix and generate an audible tone when they encounter some non-linear semiconductor junction in their way. It was this way when an individual was able to destroy a powerful car audio system by just transmitting nearby with two handy Tx's at different but very close frequencies. They mixed at the inside part of the amplifier and a very loud tone made it go bang!

[fmuser877]

I tried this was an interesting experience in the car getting one then another every few secs I put a small car TX it went over the local station almost till we got right by its TX site. its a handhold one I have

https://www.ebay.co.uk/itm/313841309976 ... BM8smA8ctg

[Albert H]

One install we did in Eastern Europe (where the government tried to close down all pirate stations and arrest their operators) used to phase- synchronised transmitters a few kilometers apart. Both were synchronised to their common UHF link. One was much more powerful than the other. The listeners just received the transmitter that gave them the strongest signal (because of "capture effect"), but locating the rigs was a nightmare. If you tried to take bearings on the location (if you didn't know that there were two sites), you'd get all sorts of weird results. Later on, the station ran as many as 5 transmitters - all on the same frequency - to really confuse those people trying to find them!

[Albert H]

Incidentally, running two FM transmitters on almost the same frequency (separated by and audio frequency difference) is unlikely to have any effect at the receiver, because capture effect (look it up) means that the receiver will only "hear" the stronger of the two signals.

The situation is different on AM - both would be audible, and you'd get a "heterodyne" tone which is the difference in their frequencies. This is the reason that aircraft communication is still AM - with FM only the strongest signal gets heard, but with AM, a weaker co-channel signal will be audible: This has proved to be crucial in emergency situations.

[yellowbeard]

Aah the capture effect. Back in the day we used to run 50 Watts on 90.6 MHz into a folded dipole up a 60 foot mast from a site 150 feet above sea level - piss I know, but we thought it was strong dark piss. One Saturday morning we got a very nice phone call from RTE themselves saying they were putting on a 12Kw transmitter on 90.7 into a fat stack of panel antennas from a site 4 or 5 miles away, and 1500 feet above sea level - we had a couple of hours to announce a new frequency. Very civil of them, they coulda just raided the granny out of us. When they slapped on the Kilowatts I went walkabout with a radio before we moved, we were getting out 20 feet in some directions, 30 in others - DLR became 2FM like flipping a switch. 30 whole feet I says - that was halfway down the back garden... We did laugh, and probably RTE laughed too.