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Re: Pre amp use with arrow..



At 02:00 AM 4/27/2005 -0400, NCC-1701E wrote:
>Scott:
>Since I have a few days of real world use with the arrow, I do not yet
know the exact isolation level between ports, and as such, I wanted to be
sure there would be no extraneous RF from the transmitted signal on the UHF
receive port that might be strong enough to take out the GaAs FET and ruin
his QSO count.
>
>I'd expect to 'assume' a >45dB isolation between ports, but I have yet to
test this, and as such, would rather have erred on being safe over sorry.
>And if the RF switched UHF pre amp did actuate on VHF transmit, then the
amp would be okay, and would prove the isolation is poor and will cause too
much desense to hear the downlink without a notch filter inline as well.
>
>Thank you for the reply Scott!
>I appreciate differing viewpoints and opinions.
>
>73!
>
>Ernest A. Erickson, ka9uce, EN55
>3960 Moen lake Road
>Rhinelander, WI. 54501
>715.617.9823(cell)
>ka9uce@netscape.net

You guys are on the right path.  For duplex operation, mode V/U, you need
either a radio with separate VHF and UHF antenna connectors such as the
FT-827, FT-736R, TS-2000, etc.   ....or your radio needs a diplexer to
split the VHF and UHF to separate antennas (i.e the Arrow has one).

So the question is if you run 5w in Tx on VHF how much isolation do you
need on the UHF port?  Well, simply done you have +37dBm in Tx and most FM
receivers are about -118 to 122 dBm signal threshold.  I guess you need a
min155 dB isolation to avoid desense.  Now the good news is your UHF
receiver has tuned circuits that pass UHF and not VHF.  I do not know what
one should expect here, but the diplexer makes up whatever rejection the
receiver lacks.

I know that my FT-817 works fine with my Arrow on UO-14/AO-27 so there is
adequate isolation for its receive threshold.  When you add a preamp say
with 0.5 dBNF and 20-dB gain, your threshold will drop another 25 dB to
-145 dBm or so.  So using a preamp may require 25 dB more isolation.  In
any case, I suspect the signal level is not sufficent to blow up the preamp
if it does OK with a dual-band radio.  Desense is your problem, if any.

I once added a U-310 preamp to a Micor VHF repeater and ended up with about
7-dB desense.  Talk about self-defeating.

Hope this helps,
73's Ed - AL7EB

PS: I have used the Par Electronics notch filters for commercial VHF use to
cut down internmods from off frequency paging transmitters.  Nice units!  I
run a Par-Angle horizontal loop (triangular halo) for 2m-ssb mobile.
73's,
Ed - KL7UW  <new call>
=================================
http://www.qsl.net/al7eb
144-MHz EME - BP40iq:
FT-847, mgf-1801/1402, 4xM2-xpol-20, 170w
=================================
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