On 8/30/2011 8:29 AM, Geoffrey S. Mendelson wrote:
Allodoxaphobia wrote:

We still don't have a clear description of what the OP is trying to
accomplish.

Take two radios, two j-poles and a lot of coax and make a repeater.
Obviously I will need some sort of controller in between but that's
irrelevant.

I want to place one J-pole (or similar antenna) at one corner of my garden,
and another at the other one (10 meters apart).


This will actually help a bit

The free space attenuation for 10 meters at 144MHz is
32.4+20*log10(0.01)+20*log10(144) 32.4-40 +43.2 = about 36dB.

If your Tx is 20W (+33dBm), then the receiver is going to see about
-3dBm at the Tx frequency. If you want to get the Tx signal down to
around -40dBm or so, you only need 30-40 dB of attenuation at the Tx freq

Then there's the Tx phase noise to worry about. Figure it's 60dB down,
so +33-60-36 = about -63dBm.. probably enough to completely mask any
received signals (what's your sensitivity.. -130dBM?) so you need a
notch with 70dB rejection on the output of your Tx.

You're 600kHz away, implying you need a Q better than a few hundred.
If you just tune up some of those cans for that, and the Q is too low,
then you'll be attenuating your transmitter.


Any reason why you're using a split of 600kHz? Why not go to a
non-standard split to make life easier. Pick two frequencies 3 MHz apart
(assuming you can get them coordinated, which is more a political than a
technical issue)

One will be used to receive a signal, the other to relay it. They both
will be somewhere on the 144-146mHz band, with the output being a few watts
with a max of 20.

Besides the distance, I was looking for a cheap way of not having
the transmitted signal block the receiver. The duplexer in question
is rated at 75dB isolation with a 3.5mHz split, I can only have .6 mHz.

What I was wondering is that since at a .6mHz split, the isolation will
be a lot less, can I somehow combine the two sides to make a better
filter?

Thanks,

Geoff.