Some duplexers pass the desired frequency, lose
something on each side, but then pass stuff from DC to
over 1GHz as if it's not even in the circuit. My
Celwave 526 operates that way. The bandpass tuning is
rather broad and serves to match the impedance more
than effect the bandpass of the signal. The notch
tuning is very deep (40dB per section). It will keep
the transmit signal out of the receiver very nicely.
If you take an isolated bandpass can and sweep it,
you'll probably see low loss at one frequency and high
loss everywhere else. Duplexers don't always respond
this way. Your mileage will vary depending on the
design.
The duplexers on UHF are typically set for 5 MHz
spacing, although closer spacing is often available.
The purpose of the duplexer is to allow the receiver
and transmitter to share one antenna. They offer very
little filtering of other nearby signals. You need a
good, selective receiver front end to handle that. The
transmitter can often be cleaned up by adding an
isolator to the output; some units may already have
that feature, especially commercial base stations.
You always tune your duplexer to your two frequencies;
other nearby repeaters are not a concern when tuning
the duplexer. It's just there to allow your TX and RX
signals to share the coax/antenna. If you have nearby
stations that you need to filter out, you must add
your own very narrow filters external to the duplexer.
Usually these will be notch filters or even shorted
stubs. A bandpass filter is rarely narrow enough to
reduce interference from signals less than several
hundred kiloHertz away. It will have no effect on the
signals from a repeater 16kHz away. The tuning of the
duplexer just isn't that tight, and in fact even the
best duplexer will allow a lot more leeway in the TX
and RX frequencies without requiring retuning.
Bob M.
======
--- 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 >>>
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