I converted a UHF Mitrek for full duplex packet operation once. Don't
recommend it because the Mitrek rx and
tx are on a single board which results in transmit chain spurs and harmonics
getting into the rx. The task required
a spectrum analyzer and several hours experimenting with various shielding and
filtering techniques to get the
rx desense down to a reasonable level.
73 de Jack - N7OO
----- Original Message -----
From: Tony KT9AC
To: Repeater-Builder@yahoogroups.com
Sent: Monday, April 26, 2010 8:09 AM
Subject: Re: [Repeater-Builder] Mitreks as UHF Repeaters?
Good question Paul. Remember the Mitrek RF decks are almost the same used in
the MSR2000 repeater, and with their tuned helical front end make great
receivers.
For the transmitter however, I would not run it on its own. Also reducing the
power output below 50-66% of rating might cause spurious issues. The best is to
get a manual and bypass the PA, running driver power only. Then use an external
amp to get your power up to where you need it. This is how every Micor, MSR,
MSF repeater is designed. Motorola typically spec'd transmitters at 70.7% to
get the best balance from the parts, and typically used multiple stages to get
there (i.e. 200mW-2W-15W-50W).
I recently picked up two 50W Mitrek Plus models that I will be using for
linking duty, and will be bypassing the PA on those as well. At only 10 miles
driver power only should work, but I have yet to complete that phase.
Good luck on your project! You'll never know what duty cycle is needed during
an event, so better to overprepare than worry.
Tony
On 04/26/2010 01:47 AM, Paul Plack wrote:
After a few years on the sidelines, it looks as if I'm going to be jumping
back into repeater ownership. I have a few nice pieces left from my last
adventure, including a TX/RX duplexer and a loaded S-COM 7K, but I'm pondering
choices in RF decks. This will be a local UHF machine designated as an asset
for a emergency net in a suburban area, at a modest height, and the only RF
device at its site.
I have two Mitrek 30w UHF mobile radios, and am aware of their duty-cycle
limitations, but would like to consider using them. They have channel elements,
and I'm not averse to spending the money to have the elements redone properly.
Looks like the pair will be 447.xx transmit / 442.xx receive. I have a couple
specific questions about these radios...
(1) If Mitreks are converted for full duplex, how well do they work? I'd
like to have a complete, swappable RF setup, so trips to the site are short,
and repairs can be done on my bench at home. (As opposed to requiring the two
radios as separate receiver and transmitter.)
(2) Would it be a reasonable pursuit to adapt a larger heatsink, and would
that safely allow 100% duty cycle at 25-30 watts? (I'm philosophically opposed
to fans which introduce the opportunity for a bearing to take the machine out
of service.)
(3) Any comments on the front ends?
(4) I know these lack the sophistication of the Micor mobiles. Is the lack
of a circulator a big deal in this application, at an isolated site with no
other transmitters?
(5) Was Motorola's quality in the Mitrek era still good enough to make a
Mitrek preferred over, say, synthesized commercial transmitter boards from
lesser manufacturers, but of more current vintage?
All comments welcome.
73,
Paul, AE4KR
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