Hi Folks,
Anybody heard of one of these?
It was taken out of service being used as a duplexer in the 154 range.
It has 8 cavities, 4 of which have an additional 'T' at each cavity that
goes to a 15 1/2" stub made of RG-9 that hangs down. (it is shorted
at the end.. braid soldered to center conductor).
The opposite 4 cans do not have this stub.
Any idea on how to tune it up? It's going into service @ 147.1
Thanks,
Tim
DB Prod SP-1894 Combiner Network
Moderator: Queue Moderator
Re: DB Prod SP-1894 Combiner Network
My two bits worth:
I have attempted to find some specs on this unit, but all I find is links to this item.
I SUSPECT that this is a notch type duplexer. The stubs may be a harmonic filter.
An analyzer with tracking generator would reveal what you have.
I have tuned several similar duplexers years ago (over 30).
The important consideration is that one side is LOW reject, the other side HIGH reject.
The loops are different between the two sides, it will not tune properly if you reverse the connections.
Tune up:
Tune 1 cavity at a time.
Terminate the antenna terminal with a 50 ohm load.
Generate into one side, monitor the other.
Disconnect all but one cavity by disconnecting the "T" from all but one cavity.
Tune the cavity for MINIMUM signal.(You are rejecting the OTHER frequency.)
Disconnect this cavity, connect another until you have tuned all cavities.
Measure isolation, as well as loss.
A signal source, and some way of measuring signal level such as a spectrum analyzer, or a radio.
Connect to your repeater , test power, receiver sensitivity, and desensitization.
There is no material difference between a transmitter combiner and a duplexer.
Filters with precision cables.
I have attempted to find some specs on this unit, but all I find is links to this item.
I SUSPECT that this is a notch type duplexer. The stubs may be a harmonic filter.
An analyzer with tracking generator would reveal what you have.
I have tuned several similar duplexers years ago (over 30).
The important consideration is that one side is LOW reject, the other side HIGH reject.
The loops are different between the two sides, it will not tune properly if you reverse the connections.
Tune up:
Tune 1 cavity at a time.
Terminate the antenna terminal with a 50 ohm load.
Generate into one side, monitor the other.
Disconnect all but one cavity by disconnecting the "T" from all but one cavity.
Tune the cavity for MINIMUM signal.(You are rejecting the OTHER frequency.)
Disconnect this cavity, connect another until you have tuned all cavities.
Measure isolation, as well as loss.
A signal source, and some way of measuring signal level such as a spectrum analyzer, or a radio.
Connect to your repeater , test power, receiver sensitivity, and desensitization.
There is no material difference between a transmitter combiner and a duplexer.
Filters with precision cables.
Aloha, Bernie
Re: DB Prod SP-1894 Combiner Network
Hi Bernie,
Thanks for the reply.
That's what I found out. They are indeed notch filters.
The stubs however were the baffling part.
When I put them back to the original 166 mhz & swept them, there
was a notch at 166, but also a wider one at 225 mhz. The 225 was
there because of the stub.
Only one bank of the cavities had the stubs.
Here's what I found.
If you set one of them to notch at 147.70, the low side (147.10) had a relatively small
loss through each can. But 600khz above was pretty bad.
The stubs are used to pull up the high side response. (reducing the loss)
I had to make new stubs for a lower frequency. (Just proportional to the orig freq)
Now the cans for the lower frequency have a small loss also.
Think I lost more hair on this one!
Again, Thanks,
Tim W5FN
Thanks for the reply.
That's what I found out. They are indeed notch filters.
The stubs however were the baffling part.
When I put them back to the original 166 mhz & swept them, there
was a notch at 166, but also a wider one at 225 mhz. The 225 was
there because of the stub.
Only one bank of the cavities had the stubs.
Here's what I found.
If you set one of them to notch at 147.70, the low side (147.10) had a relatively small
loss through each can. But 600khz above was pretty bad.
The stubs are used to pull up the high side response. (reducing the loss)
I had to make new stubs for a lower frequency. (Just proportional to the orig freq)
Now the cans for the lower frequency have a small loss also.
Think I lost more hair on this one!
Again, Thanks,
Tim W5FN