I was looking at cheap HT's last night on eBay and I found several vendors
selling duplexers for repeater use. They sell for about $60 each.

They are 6 cavity (three transmit, three receive) and the closest they can
go on VHF is 3.5mHz.

Doing a web search I find they are not very good (what did you expect?)
but with a good front end they will work ok.

My problem is not only one of money (don't tell me to not waste my money
and buy a good, one, I can't afford it), but my 2m band is only 144-146
mHz, and the repeater portion is really just 145-146. I'm also limited
to 20w output.

So my question is anyone familar with them? Is it possible to take one
and convert it to a 6 cavity input filter? For example, I could put the
input on one side my yard and the output on the other, about 30 feet apart.

For example, if the input of the repeater was 145.600, could I have them
both tuned to 145.600 and connect the input of the receiver to one
set of cavities, the antenna to the other and leave the antenna connection
untouched, effectively placing them in series?

Thanks in advance,

Geoff.






--
Geoffrey S. Mendelson N3OWJ/4X1GM
Making your enemy reliant on software you support is the best revenge.

Geoffrey S. Mendelson wrote:
So my question is anyone familar with them? Is it possible to take one
and convert it to a 6 cavity input filter? For example, I could put the
input on one side my yard and the output on the other, about 30 feet apart.

No. There are two sides, one of them for the high frequency and the
other for the low frequency. The filter response is such that it passes
on one frequency and notches on the other.

Rob wrote:

No. There are two sides, one of them for the high frequency and the
other for the low frequency. The filter response is such that it passes
on one frequency and notches on the other.

Ok thanks. Note that they are tunable anywhere in the 136-17? range.

Assuming the pass side is set to pass 145.600 (the input) and the notch
side is set to notch out 145.000 (the output) and they are connected in
series, would that work?

Geoff.


--
Geoffrey S. Mendelson N3OWJ/4X1GM
Making your enemy reliant on software you support is the best revenge.

Geoffrey S. Mendelson wrote:
Rob wrote:

No. There are two sides, one of them for the high frequency and the
other for the low frequency. The filter response is such that it passes
on one frequency and notches on the other.

Ok thanks. Note that they are tunable anywhere in the 136-17? range.

Assuming the pass side is set to pass 145.600 (the input) and the notch
side is set to notch out 145.000 (the output) and they are connected in
series, would that work?

It is not a pass side and a notch side. Both sides are pass/notch.

But on one side the pass freq is above the notch freq, and on the
other side the pass freq is below the notch freq.

On the Chinese duplexers you have found, the Q is probably not high
enough to tune the pass and notch to frequencies 600 kHz apart.

Rob wrote:

On the Chinese duplexers you have found, the Q is probably not high
enough to tune the pass and notch to frequencies 600 kHz apart.

Ok, thanks.

Geoff.

--
Geoffrey S. Mendelson N3OWJ/4X1GM
Making your enemy reliant on software you support is the best revenge.

On 30 Aug 2011 13:01:01 GMT, Rob wrote:
Geoffrey S. Mendelson wrote:
Rob wrote:

No. There are two sides, one of them for the high frequency and the
other for the low frequency. The filter response is such that it passes
on one frequency and notches on the other.

Ok thanks. Note that they are tunable anywhere in the 136-17? range.

Assuming the pass side is set to pass 145.600 (the input) and the notch
side is set to notch out 145.000 (the output) and they are connected in
series, would that work?

It is not a pass side and a notch side. Both sides are pass/notch.

But on one side the pass freq is above the notch freq, and on the
other side the pass freq is below the notch freq.

On the Chinese duplexers you have found, the Q is probably not high
enough to tune the pass and notch to frequencies 600 kHz apart.

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

Allodoxaphobia wrote:

On 30 Aug 2011 13:01:01 GMT, Rob wrote:
Geoffrey S. Mendelson wrote:
Rob wrote:

No. There are two sides, one of them for the high frequency and the
other for the low frequency. The filter response is such that it passes
on one frequency and notches on the other.

Ok thanks. Note that they are tunable anywhere in the 136-17? range.

Assuming the pass side is set to pass 145.600 (the input) and the notch
side is set to notch out 145.000 (the output) and they are connected in
series, would that work?

It is not a pass side and a notch side. Both sides are pass/notch.

But on one side the pass freq is above the notch freq, and on the
other side the pass freq is below the notch freq.

On the Chinese duplexers you have found, the Q is probably not high
enough to tune the pass and notch to frequencies 600 kHz apart.

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

He is wondering if something can be modified. He wants a half-duplex
radio with a 600 kHz split, on a common antenna. The duplexer he is
looking at has a 5 MHz offset. He wants to know if it can be
re-configured for ham with a much smaller offset.

http://www.repeater-builder.com/rbtip/2mduplexer.html

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).

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.

--
Geoffrey S. Mendelson N3OWJ/4X1GM
Making your enemy reliant on software you support is the best revenge.

On Tue, 30 Aug 2011 11:44:03 +0000 (UTC), "Geoffrey S. Mendelson"
wrote:

I was looking at cheap HT's last night on eBay and I found several vendors
selling duplexers for repeater use. They sell for about $60 each.

They are 6 cavity (three transmit, three receive) and the closest they can
go on VHF is 3.5mHz.

Doing a web search I find they are not very good (what did you expect?)
but with a good front end they will work ok.

My problem is not only one of money (don't tell me to not waste my money
and buy a good, one, I can't afford it), but my 2m band is only 144-146
mHz, and the repeater portion is really just 145-146. I'm also limited
to 20w output.

So my question is anyone familar with them? Is it possible to take one
and convert it to a 6 cavity input filter? For example, I could put the
input on one side my yard and the output on the other, about 30 feet apart.

There are calculators that will predict the isolation. At 20 watts
(+43dBm), with a broadband synthesizer noise level at about -30dBm,
and an RX sensitivity of about -106dBm, you'll need at least 76dB of
TX-RX isolation. The little cavities have too wide a notch (i.e. low
Q) and will attentuate the RX signal and smoke the TX signal at 0.6Mhz
spacing. Seperating the antennas won't solve that problem. If you
try to get sufficient isolation using vertically isolated antennas,
you might get sufficient isolation, but the Q of the notch filters
will still kill the signal both ways.

Vertical antenna isolation:
http://awapps.commscope.com/products/bsa/_calculators/qvisolation.asp
Horizontal antenna isolation:
http://awapps.commscope.com/products/bsa/_calculators/qhisolation.asp

For example, if the input of the repeater was 145.600, could I have them
both tuned to 145.600 and connect the input of the receiver to one
set of cavities, the antenna to the other and leave the antenna connection
untouched, effectively placing them in series?

Nope. Not enough Q (too wide a notch).
Bigger cavities = higher Q.
More cavities = deeper notch and therefore more isolation.

Quiz: The Q of given cavity is 1000. If I critically couple 2 such
cavities in series, what is the resultant Q? 10 such cavities? etc?

Answer: 1000 in all cases. The 3dB bandwidth does not change when
tuned circuits are critically coupled.

Read the specs on the eBay web page. For example:
http://www.ebay.com/itm/110460354111
"Minimum Tx and Rx frequency difference: VHF 3.5Mhz"
That's not going to work with 0.6MHz spacing on the 2m ham band.

This is what a proper (Phelps-Dodge) 2m duplexer looks like:
http://www.LearnByDestroying.com/k6bj/K6BJ%20Repeater/slides/2m%20duplexer2.html
Note the much large size cavities.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Tue, 30 Aug 2011 15:29:03 +0000 (UTC), "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.

Retch.

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).

Plugging into:
http://awapps.commscope.com/products/bsa/_calculators/qhisolation.asp
I get about 30dB isolation (assuming 2dBi antenna gains).
The synthesizer noise belching from your xmitter is maybe -60dB down
from the +43dBm xmit carrier. That puts the noise level at -17dBm at
the receive antenna. Your receiver sensitivity is probably -106dBm,
which means you need about 90dB of isolation. The antenna spacing
will provide 30dB of that. You cavities are suppose to provide the
remaining 60dB of isolation. That's not going to happen with tiny
mobile duplexer cavities.

Hint1: You'll be close with vertical antenna isolation.
Hint2: Some radios have quite a bit of synthesizer noise, much of
which will be on your repeater receive frequency. This is why some
repeaters still use crystal oscillators instead of synthesizers.

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.

Somewhere? Duplexers are usually bench tuned to some specific
frequency. It's not a trivial exercise and requires some expenditures
in time and equipment. You can't easily move in frequency.

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?

You can't improve things by simply adding more cavities. All you'll
do is add more loss:
Bigger cavities = higher Q and therefore closer frequency spacing.
More cavities = deeper notch and therefore more isolation.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558