On 10 Dec 2009 00:06:16 GMT, Ed wrote:

One reason I hate the thought of adding antenna relays to switch the Rx
around the isolator is that this is a busy packet node and I don't like
the reliability issues a couple of added relays might add to a presently
solid state switched radio.. . although I could do this if forced to.

We have T/R switches using the stock reed relays that come with Mitrek
radios on several 420Mhz links. My guess about 1,000 cycles per day.
The average lifetime for the reeds is about 2 years. I've lost count
how many reeds I've replaced. Fortunately, Digikey has a nice
assortment of reeds. I hear the Mitrek packet boxes are about the
same, although that's not my headache. If you're going to do the
external T/R switch, think solid state.

no calculations have been done...

Do them. I manage a local site stuffed full of assorted xmitters.
I've prevented disaster more than once with some simple calculations.

http://www3.telus.net/PassiveRF/
http://www.tcstx.com/software/Intermodulation.cfm

but the new owner
"requires" isolators.

That's just the beginning. The site I run is rather loose. Mostly
hams and junk. So far, there's been no need for a major cleanup,
although I'm tempted. From memory, some other sites requi
1. Circulators *AND* cavities on all xmitters.
2. Demonstrate that spurious tx rubbish is -60dB down.
3. Heliax everywhere.
4. No dissimilar metals on the coax connectors and adapters.
5. No nickel plating on anything carrying RF. (no Radio Shack junk)
6. All modems, routers, computers, and must be in a metal box with
RFI/EMI protection.
7. Remote on/off switching (to disable the xmitter if it goes
insane).
8. Posted current licenses (FCC requirement).
9. Lightning protection everywhere.
10. Shielded and grounded data lines (CAT5).

Here's what CDF (Cal Fire) requires to share their vault:
http://webmain02.fire.ca.gov/pubs/issuance/8000/rvsapp.pdf
I've seen worse.

Ah, I found one that's worse:
http://www.elsinorepeak.com/documents.html
Ummm... don't let the new site owner see these.

Incidentally, I spent several months trying to identify the source of
an intermod mix that was wrecking one of the ham repeaters. It would
come and go with no obvious pattern, making it difficult to find. I
finally got lucky by breaking protocol and unplugging customers
equipment while listening for changes. The last desperation act was
to unplug the 117VAC to 24VDC battery charger for the diesel
generator. That was it. The problem was that it was completely
shielded in both the case and the in/out wires. It was also directly
below the antenna structure, which is a low radiation location. The
moral is "That which is most obvious correct, beyond all need of
checking, is usually the problem". Do things the *RIGHT* way.

The best solution for this situaion might be to
gather a few respected technical persons who this non-technical site
owner is familiar with and perhaps convince him this isn't necessary.

Well, intimidation does work. Make sure your respected technical
people bring along some muscle. If that doesn't work, try beer or
wine.

OR.. we could easily add a pass cavity of two and hope that this
would passify him.

The cavity will work well enough if you don't have any other xmitters
on nearby frequencies in the building. However, as one of those
technical types, my never humble opinion is that the DR-135T is not
the best of radios and will need all the protection you can provide.

I'm still open to another other new or inovative solutions.

Plan A:
Hybrid Ring Duplexer.
http://www.repeater-builder.com/antenna/hybridring.html
It should give enough isolation so that you can run the receiver full
time without a T/R switch. The problem is that I'm not sure it will
work on a single frequency. I gotta do some RTFM first. You'll still
need to bring out the receiver input coax, but at least there's no
external T/R switch or relays. Also, put something on the receiver
input to keep from blowing it up if there's too much VSWR.

Plan B:
Build a clone of the internal DR-135T T/R antenna switch. Bring out
the TX and RX coaxes externally. Install a cavity in the antenna line
and an isolator in the TX line. Install the T/R switch near the
antenna junction.

Plan C: Use 2 radios, one for TX and one for RX.

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