Batboard

Does anyone know if you could run 2 identical duplexers, tuned the same, together in order to increase the wattagee that 1 would handle?

I would run coax off the receive, transmit and antenna connections. Then I would utilize a "T" coax connector RX to RX, TX to TX and ANT to ANT.

Then I would connect the receiver to the RX "T", the transmitter to the RX "T" and the antenna to the ANT "T".


I am probably out of my mind but I can't afford a 100 watt duplexer.

Mark

Well, the first problem you'd have is that each port on the duplexer either expects a 50-ohm load or presents a 50-ohm load. When you connect two in parallel, suddenly everything is now 25 ohm impedance. The 50-ohm antenna will no longer be matched efficiently. The 50-ohm transmitter will bitterly complain about the 25-ohm load and may lower power and/or shut down. The receiver probably will become less sensitive. Your 100 watts of transmitter power will probably end up under 50 watts going to the antenna port(s), you'll have spent twice as much money for two duplexers, and still not be happy. Plus, the cable lengths are likely not going to be exactly equal, so you'll have all sorts of interactions and tuning difficulties. To put it simply, this is NOT a good idea.

It might be possible with suitable isolation circuitry, preamps, splitters, etc, but by the time it's all done, you'll lose a lot of power and sensitivity and have spent a lot of money.

For all the effort and expense, it's simpler to just set the transmitter to 50 watts and use the single duplexer. You'll even save a little on your electricity bill because of it, and lengthen the life expectancy of the equipment. Now, in a perfect world, it's highly likely that the duplexer WILL operate with more than 50 watts applied to it - how much more depends on the materials that it was constructed from. Most 50 watt duplexers are really intended for the mobile radio market and may just consist of several notch filters. These do an adequate job in mobile installations but for a base station you really want a duplexer that has both notch and bandpass filters, and three sections on both the receive and transmit side will give you all the isolation you'll ever need or want. But as you've discovered, once you go to a base-station duplexer, you also get 250-350 watt capability and pay dearly for it.

Duplexers can be found on eBay for 20% of the manufacturer's list price. Sometimes the cables need to be replaced. If they've been treated well, haven't been dropped or opened up, they're fairly reliable pieces of equipment. Just make sure you obtain one that was manufactured for the frequency range you need, as one model rarely fits all uses. Cable sets may be necessary to convert one band-split to another, and this can be costly if you're not prepared for it. If you need to go with new equipment, new 6-cavity VHF duplexers can cost from $1500 to $2500; new UHF 6-cavity duplexers can cost up to $1500.

Bob M.

All is true as stated above, it is not that the mobile duplexers can't handle more than 50 watts, but the fact that you only have 65 to 70db of isolation, this is OK at 50 watts or less, but as you raise the power you need more isolation, a 100w duplexer has between 90 and 110db of isolation, so that the higher power does not get back into the receiver.

Butch

Depending on your particular situation (topography, site, coverage you have vs. coverage needed, mobile antennas used, frequency band, etc)...

I am not all that sure you would notice anything different if you doubled your power. I agree with the previous posts, you are now adding more variables (like lower impedance) which the system will not like.

Wondering if you just want to upgrade the system or correct a specific coverage problem...

Is the talk back that much better than the talk out?

Can you improve the mobile end of the system before getting into higher station power? I assume you already did that and the station antenna is working up to spec.

That being said, there MUST be an inexpensive 100w duplexer around Batwing or (heavens) on eBay.

If you havent, check the other stuff first...

Report back... we'll all come up with a bunch of answers!!!

Good luck and of course

73!

mike in CT
KM1R

Most mobile duplexers are small in comparison with base station ones. All other things being equal, each section will provide about 30-40 dB of notch or rejection. A four-cavity notch-only mobile duplexer will give, as said above, 60-70 dB of rejection of the transmitter's signal at the receiver; a six-cavity bandpass/reject base station unit gives over 100 dB. But there are some mobile 6-cavity duplexers that should give enough rejection, and these still seem to be rated at 50 watts. I get the feeling that the size of the cavities has something to do with, at least, the narrowness of the notch and/or bandpass, but it could also decide to arc at higher power levels, unlike a larger base station unit (at least on UHF). I can't say I've played with enough VHF duplexers, mobile or base, to come up with the same conclusions.

However, there's always something to learn here on the BBS. No one is too old or too young to do so (at least I'm not at one of those extremes yet). That's what makes this forum so great !

Bob M.