Batboard

I need a duplexer for VHF with a 6.49 mhz spacing. I found one in the right sub band but for 6 mhz maximum spacing. Why does maximum spacing affect the operation if the frequency range is correct for the duplexer?

All passive cavities have a min and max range. There is only so far the screws will turn before they are fully in or fully out. 6M is pretty wide for VHF. Most of the time you're lucky to get a couple hundred kilohertz between input and output. 6M is luxury. I'd tune this puppy up and see how it performs. It might just work for you.

What is it - pass? reject? pass-reject?

The small ones about the size of a book are reject only, and you have to be dead nutz on with the tuning to keep your desense minimized. Being another half meg out in your separation might make a reject cavity a poor choice. It just won't keep xmit energy out of the rcvr.

Pass and pass-reject tend to be larger cylinders ganged together. Both will have a tuning rod to set the pass freq for each side. The pass-reject have an additional cap to set the reject (not to be confused with the insertion loss adjustment). Those are especially helpful if you have close pairing, or if you have a colocated adjacent channel user you need to keep out of your rcvr, or to keep your xmit out of their rx.

To add a few comments along with what Bill has said, duplexing a radio system is going to take some careful engineering. The type of transmitter will have a great
bearing on the duplexer selection. This means is the transmitter a tube or solid state unit? Is the transmitter able to be tuned or is it a wide band, no tuning type
of construction? What power level will the transmitter be used at? What kind of a receiver will you be using? Does the receiver have a pre selector in the front
end or is it one of these wide band jobbies that doesn't have any tuning?

You need a minimum 0f 70 db isolation to make a repeater work. If your going to run much TX power, then you probably need to look at having 100 db of isolation
between the TX and RX. This isolation can be had in a number of ways. You can get a good duplexer. You can get a real clean transmitter. You can use a
receiver that has a pre selector in the input of front end. You can run low power on the TX. The best thing to do is to combine all the above.

If your trying to buy a used duplexer, you need to walk carefully. It would be best to see if the duplexer will tune to your channels. Make sure the old frequencies
are close to your new frequencies. If not, the cables connecting the different cavities may need to be changed. These cable lengths are fairly critical in length.

If this is unit very old, you will probably want to take the duplexer apart and cleaen the surfaces where any tuning connections are made. The surfaces will tarnish
over time and cause very erratic adjustments. Taking the cavities apart will require some skill. I have had to do this to a number of DB cavities over the years.
In fact I have had to drill out some rivits in some fairly new cavities and replace them with some stainless steel screws. The center tuning rod assembly was
not making a good electrical connection and was causing major problems.

Just a few thoughts to think about. Plus you need some good test equipment to tune these cavities with. There have been a number of threads on here about
how to tune cavities, so I won't go into that again.

Jim

arlojanis wrote:I need a duplexer for VHF with a 6.49 mhz spacing. I found one in the right sub band but for 6 mhz maximum spacing. Why does maximum spacing affect the operation if the frequency range is correct for the duplexer?

Just a quick comment:

If your's is the only VHF repeater or radio on the site you're likely OK with a simple duplexer. But, if the site has other VHF equipment on it the issue is likely more complex and may require more than a simple duplexer.

Good Luck, Bob

Duplexers are a exacting science, if your looking for Isolation you can fudge some setups but it can cause problems,

Big problem is the matching harness connecting the cans, its setup for the Min and Max range for resonance and impeadance matching. Other is the tuning selectors if the manufactuer cut you short and wacked off more rod than some leaving you at a loss on extra tuning that can throw your ability to tune.

If you can run seperate antennas get a set of BpBr cans for Rx and a seperate set for Tx.

Day before Turkey day Instructions for setup:

Place cans accordingly, sprinkle Rf dust over creation and set antennas on an equal spacing table for maximum RF flare.

Enjoy well chilled or warmed slightly.

CoM

If your DPXR has a tuning range of 6 Mhz , then your max separation is 6 Mhz, but the min. separation is an important factor , find the tuning rang of the DPXR ,if it covers your RX and TX freq. both with couple of 100 Khz margin ,then you will be fine , when it is tuned properly. you would need more than 80dB
isolation between TX RX port ,and less than 3 dB loss from antenna port to TX and RX ports ,