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 8/30/2011 6:01 AM, Rob wrote:
Geoffrey S. 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.

one might be able to cascade two of them, but Rob's point about Q is
well taken.

It depends on whether the problem is the "depth of rejection" or the
"steepness of skirt". A single resonant unit has a narrow spike on top
of gentle shoulders. Say one section has Q of 1000 (i.e. the 3dB
bandwidth is 100kHz for 100MHz center frequency), but the rejection at 1
MHz away is only 20dB. I could cascade 3 sections, tuned exactly the
same, and I'd have 9dB loss at 100kHz, and 60dB rejection at 1 MHz away.
The 3dB bandwidth might be around 30kHz, or, you could stagger the
tuning slightly to get a little broader flat top, but keep the 60dB
(i.e. moving the center frequency of one cavity by 10kHz won't change
the attenuation at 101 MHz)

The design challenge is that you need to suppress the transmit signal
(at, say, 50W, +47dBm) low enough so that the front end of the receiver
can handle it without blocking (say, you want it down around -50dBm).
With our not so hot cavities described above, you'd need to stack up 5
sections at 20dB per section to get the 100dB suppression. Well, that's
not so great, because you probably now have a bunch of additional loss
in the receive path, AND you've got a real tuning chore on your hands to
make sure that all of them are tuned appropriately to get the required
bandwidth.

Enter the idea of a notch. Let's say our example is 101 MHz Tx and 100
MHz Rx. Rather than rely on the "far away" response of a resonator to
suppress the transmit, I can put a 101 MHz notch filter on the input to
my receiver. If I can get 30dB/section rejection, then 3 or 4 sections
will knock the Tx power down low enough to not block the receiver's
front end.

But wait, there's more... that Tx isn't a narrow spike. It has phase
noise sidebands that go out fairly far. Unfortunately, the run of the
mill transmitter might only suppress "off channel spurious and noise"
by, say, -50dBc (and a rig designed to operate half duplex is probably
worse). That means that your 50W (+47dBm) transmitter is putting out
-3dBm *at the receive frequency*, so no amount of filtering on the
receive path will help. Again, enter the notch filter.. you put a 100
MHz notch on the output of the transmitter.

Now.. as Rob points out, the Q might be kind of low. If the Q is, say,
100, then that 100MHz filter is 1MHz wide, and by the time you cascade
enough to get the BW down, the loss will be huge.


As you can see, this whole duplexer/diplexer/multicoupler design thing
can get pretty complex, and there's not usually a simple cookbook answer.

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