Vern,
Typically a BP/BR has a wide pass and narrow notch. On mobile
duplexers the pass is usually very broad. Base type often have
pretty good attenuation a few MHz away. Adding a bandpass will
improve this.
No cavity is going to illimate something 16 kHz awaywell and let
the desired sig thru. On many duplexers a notch might be 50-100 kHz
wide, but the pass is MHz wide. For 16 kHz one would have to rely on
the receiver selective IF filters, but for typical FM NB repeaters
this is a bit much. For SSB no problem assuming the signal is not
very strong.
73, ron, n9ee/r
--- In Repeater-Builder@yahoogroups.com, w6nct [EMAIL PROTECTED] wrote:
I'm tuning up a cavity-type duplexer for a 70cm repeater, and in the
process, I showed my results to a fellow ham. He asked a couple
questions that cause me to think; so I decided to ask the experts...
FIRST QUESTION TOPIC:
For reference, my primary cavities were made by Tx-Rx Systems, and
have both bandpass and band-reject tuning controls on each cavity.
When tuning them (either singularly or as pairs in-series), I see
and
can adjust the band-pass and notch for the desired Tx and Rx
frequencies.
However, in the broad-band sweep, I can also see a bunch of other
signals passing through the cavities; all of which are well away
from
my Tx/Rx frequencies. I suspect that these are normal, and are a
side
effect of how the can-type resonant cavities work. Am I correct in
this assumption?
As a sanity check, I combined this set of cavities with another
(Phelps-Dodge) set I have, and tuned these supplemental cavities for
band-pass only (one cavity for my Tx frequency, and one cavity for
my
Rx frequency). Sure enough when I put these in series with my
primary
set, I can eliminate nearly all of the other signals from the
broad-band sweep. This observation seems to reinforce my initial
assumption about the Tx-Rx cavities. Do you agree?
SECOND QUESTION TOPIC:
The other ham thought that I should end up with a band-pass that is
narrow enough to eliminate adjacent repeaters (at 16kHz spacing, as
per the current SCRRBA band-plan separation). I tried but I cannot
get either set of cavities to have that narrow of a band-pass; at
least not without sacraficing most of the signal in the process. I
suspect that the receiver and transmitter need to actually inforce
these much narrower bandwidth requirements within the broader
protection provided by the cavity-duplexer. I suspect that I should
focus my duplexer tuning on passing the desired frequency, notching
the alternate repeater frequency, and trying to do so with the least
amount of signal attenuation. Am I correct in these understandings?
For both of these question topics, feel free to point out anything
that I might be missing or misunderstanding. I'm by no means
sensative about this stuff, and still consider myself on the
learning
curve about duplexers and repeaters in general.
Thank-you (in advance) for your time, thought, and opinions.
vern
