>"Richard Sharp, KQ4KX" <[EMAIL PROTECTED]> wrote:
> Well, I don't necessarily disagree with using
> an isolator/harmonic filter. However, the
> isolator will not keep 800MHz signals from
> getting to the transmitter of the 2m repeater.
Most circulators are followed by a low pass filter
network. I should have mentioned that.
There will always be unwanted energy from nearby
transmitters. Reduction of that unwanted energy
is the key. A dual port circulator will not keep
all the 800Mhz energy, but it will provide a
much welcome reduction and measure of isolation.
If followed by the typical trailing Low Pass
Filter, energy reaching the Power Amplifer is
much less. It's a very good practice and required
by many serious site owners.
> I'm sure the harmonic filter may reduce the
> 800 levels some but the isolator isn't going
> to stop it.
Most of the UHF Low Pass filers are 5 pole (or
better) circuits. Very nice to have in line. Nothing
will stop all the energy. You have to manage
the numbers.
> Of course, an isolator will keep other VHF
> signals from getting back through it but I
> don't have any VHF signals that are a problem.
There are no other VHF Transmitters at the site?
Is there an FM or VHF TV broadcast station nearby?
> The duplexer is a new TX/RX 4 can vari-notch.
> I've added bandpass cavities on the rx & tx
> side between the duplexer & repeater.
What are the cavity diameters? Did adding the
additional BP cavities help?
> I'd don't know exactly how old it is but it
> is in "good shape" according to the tower
> crew. It was previously used by a law
> enforcement agency before they switched to
> the 800 system.
The real story is a personal inspection or
testing. If the system was your bread and butter,
you'd be pressed to have it play properly out
of the starting gate. You'd test and inspect
the cable regardless of what someone else said.
> I purchased the PD220-2 antenna new late
> last year. It was drop shipped directly
> from Celwave.
I would test swap out the Celwave Antenna, I've
had over 20 right out of the box that were self
made IMD generators. I'm not making this up...
We had to stop using certain models.
Based on my experience, specific models of
Celwave antennas are problem generators.
Temp Swap the antenna and you'll see for
yourself. I know it's probably not easy to
get done.
> I installed a new polyphaser at the entry
> port but I haven't tried taking it out of
> the circuit to see if it's contributing to
> the problem. Has anyone seen a polyphaser
> do bad things?
No, only good things...
> > You can measure the system desense of the terminated
> > feed, which will provide additional clues.
> Good idea.
If the terminated line tests ok, the antenna is
suspect.
> The closest antenna to the PD220 is about
> 4 to 5 feet away.
That should be enough, if the antenna is the
right type.
cheers
skipp
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