> "Dave VanHorn" <[EMAIL PROTECTED]> wrote:
> I have a choice on where I apply a bandpass filter
> in the system, and I don't see a clear winner..
There isn't one unless you need the filter. Kevin replied
to ask what the filter bandwidth was, which is one part
of the bigger picture we might need to know before a
reply post.
> On a 2m repeater, I have a preamp after the cans.
A common layout...
> .. I'd like to add a filter in front of it.
Do you need it?
> It occurs to me that I could put the filter between
> the cans and the antenna, which would give me the
> same filter action for the amp as if the filter were
> behind the cans, but in this position the filter also
> acts on the transmit signal.
You know the old saying... "if it works don't mess
with it"..? If you really don't need the bp filter,
don't bother. The extra loss and potential for problems
from coax - connector joints, junctions and extra
hardware just might shoot your repeater in the foot
type thing. A generic rule of thumb might assume each
coax connector is about .2 dB of loss added to the
system. Add the connector loss to the filter loss
and your system performance can go down.
> Assume that I can tune the filter so that both
> frequencies are well in the passband, and that
> I don't mind the additional loss in the transmit
> path.
>
> What other effects should I consider?
The "tinker too much" effect.
If you have an extra band pass bottle, which is more
of a narrow band filter, you could add it to the rx path
in a number of possible places. In front of the preamp
from the duplexer, the filter proper set up would help
protect the preamp from adjacent signal overload.
The above is helpful when you duplexers are not so
great or the pass-reject type.
If you put a narrow band bp filter (properly set up)
on the transmit side, it would help clean up the
transmitters sideband noise and extra unwanted (relative)
low level generated products.
****
If you have a wide band filter (ie the DCI type...)
there might not be a great place to run it in the system
unless adding it into the system at a very busy repeater
site can help protect it from other equipment in operation.
I've used a large number of DCI wideband filters for
frequency agile and rx pre-selection applications.
I have mixed bag opinions about running a smaller (standard
size) DCI filter in a high powered repeater tx application.
****
What you do with the filter also works with where your
repeater is, the quality/type of equipment you're using and
how busy the site/location is.
We'd need a bit more information about the filter...
cheers,
skipp
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