AM RF Combiner

[liradioprez]

I have two 20 watt AM transmitters, both on our frequency, 630khz. I was wondering if it would work if I connected the output of both transmitters and connected them into a 'T' connector before going into the phasing/tuning cabinet. I'm almost positive it would fry something...but I just want to make sure. ;D

Thanks, Kyle

 

[littlejohn]

There's more to it that just hooking both of them to the same antenna, although - if they're pretty much identical transmitters, it can be done. You ought to be able to buy a 40 Watt amplifier, or a complete 40 - 50 Watt transmitter for the price of combining them.

 

[w9wi]

I have two 20 watt AM transmitters, both on our frequency, 630khz. I was wondering if it would work if I connected the output of both transmitters and connected them into a 'T' connector before going into the phasing/tuning cabinet. I'm almost positive it would fry something...but I just want to make sure. ;D

One problem you'd have with that arrangement is that chances are, both transmitters aren't precisely on 630. One might be on 629.95 and the other on 630.02, something like that. In which case, if you combined their outputs they would "heterodyne" at 70Hz -- you would hear a 70Hz (very low-pitched) tone along with the program.

When full-power stations combine multiple transmitters to make higher power, they lock all the transmitters to a common frequency reference to ensure they're on precisely the same frequency.

 

[speakerman]

If you are worried about frequency lock you can feed them both from the same frequency source through a DA. But why bother in the first place?

The Harris MW-1 essentially did that with a rejection combiner section fed by a number of modules.

Rant: Rather than the digital AM crap they are pedaling theses days I would have preferred the FCC demand all AM night-timers to have their transmitters locked to a GPS standard like cell sites do. The main co-channel interference problem would then be propagation delay differences. Selecting one sideband with synchronous detection would greatly minimize the audible annoyances of adjacent channel interference also.

 

[w9wi]

Rant: Rather than the digital AM crap they are pedaling theses days I would have preferred the FCC demand all AM night-timers to have their transmitters locked to a GPS standard like cell sites do. The main co-channel interference problem would then be propagation delay differences. Selecting one sideband with synchronous detection would greatly minimize the audible annoyances of adjacent channel interference also.

I don't think that would make any difference. The vast majority of AM stations are plenty close enough to the right frequency. (there are occasional exceptions, quickly noticed by DXers!)

The problem is the cacophony that results from dozens of stations on the same frequency running different programs. If you could get everyone on (say) 1330 to run the same program overnight, (and synchronize their audio delays) the channel would sound a lot better! (I wonder how much difference propagation delays would make?)

 

[rfry]

I was wondering if it would work if I connected the output of both transmitters and connected them into a 'T' connector before going into the phasing/tuning cabinet.

Unfortunately even if the output of each transmitter was identical in frequency, modulated power, r-f and modulation phase -- connecting their outputs together using a tee would provide no isolation between the transmitters. Some of the power from one transmitter would appear in the output circuits of the other one, which could cause major damage.

Some circuit element or network will be needed to combine the transmitters while giving ~ 30 dB isolation between its input ports.

RF