My TH-II was almost 800 Hz off frequency. I would imagine that frequency could vary quite a bit from one unit to the next.
I added a "netting" capacitor to fine tune the PLL reference oscillator bringing my carrier within a few cycles of center.
The point? At night when the DX rolls in there is a station in Washington D.C. on the same frequency as my station. My and their frequency is now within a few cycles of each other. Both audio programs are very easy to hear as the beat note is sub-audible. Yes, there is a bit of whoop-whoop as the beat note goes but the program material mostly hides it.
Crystal controlled PLL oscillators can be very stable with proper design. If you could "net" your transmitters they might remain close enough on frequency to allow slightly overlapped coverage areas.
If you placed the transmitters such that only automobile radios pick up the overlap area, the whoopy wouldn't last long enough to be a problem. However, if you are also targeting in home radios it would be a problem for those in the overlap zone. Of course, your transmitters would probably have to be within 200 to 300 feet of each other for home radios.
What the heck, it would be fun to try it out. Hmmmm, Maybe you could market directional loop antennas...
My TH-II was almost 800 Hz off frequency. I would imagine that frequency could vary quite a bit from one unit to the next.
I added a "netting" capacitor to fine tune the PLL reference oscillator bringing my carrier within a few cycles of center.
The point? At night when the DX rolls in there is a station in Washington D.C. on the same frequency as my station. My and their frequency is now within a few cycles of each other. Both audio programs are very easy to hear as the beat note is sub-audible. Yes, there is a bit of whoop-whoop as the beat note goes but the program material mostly hides it.
Crystal controlled PLL oscillators can be very stable with proper design. If you could "net" your transmitters they might remain close enough on frequency to allow slightly overlapped coverage areas.
If you placed the transmitters such that only automobile radios pick up the overlap area, the whoopy wouldn't last long enough to be a problem. However, if you are also targeting in home radios it would be a problem for those in the overlap zone. Of course, your transmitters would probably have to be within 200 to 300 feet of each other for home radios.
What the heck, it would be fun to try it out. Hmmmm, Maybe you could market directional loop antennas...
The SStran 3000, of which I have two, seem to be very much "on frequency" when comparing them to far away stations sharing the same channel. There is rarely even a hint of "beating" that I have noticed. However, when both SStrans are run on the same channel the lack of co-ordination is there with pulsing in-out bad sync. There is no fine-adjustment in the circuit.
The Ramsey AM25 does have a fine tuning trimmer, but it does not zero. It tunes from pretty far out of sync with an underlying station, to not as far out of sync. Maybe changing the part will be something to try.
The SStran 3000, of which I have two, seem to be very much "on frequency" when comparing them to far away stations sharing the same channel. There is rarely even a hint of "beating" that I have noticed. However, when both SStrans are run on the same channel the lack of co-ordination is there with pulsing in-out bad sync. There is no fine-adjustment in the circuit.
The Ramsey AM25 does have a fine tuning trimmer, but it does not zero. It tunes from pretty far out of sync with an underlying station, to not as far out of sync. Maybe changing the part will be something to try.