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