[FADCA] Separation of UHF beams at FPAC sites

[bud thompson]

NOTE:  I've started a new FADCA thread here with Doug's response to Evans on
the Polk County tower and antennas.  This is an important topic as we are
now moving into two-, three-, and maybe four- UHF backbone ports at an FPAC
site!  No doubt-each site presents its own opportunities!

----- Original Message ----- 
From: "Welcker" <wb4kgy@bellsouth.net>
To: <fadca@mailman.qth.net>
Sent: Wednesday, March 10, 2004 08:57
Subject: Re: [FADCA] Antenna recommendations for the SNCP, BARTOW, 200 foot
tower


> You might want to think through #3 "the UHF Beams".  With this
> configuration you will encounter severe desence to the non transmitting
> radios plus IM products
> if two radios are transmitting simultaneously.  UNLESS you provide a
> minimum of 60db of quality bandpass filter per radio (very expensive).
> The alternative is to
> stagger the beams up and down the tower depending on the path loss
> caluclations with a minimum separation of thirty feet..  We (the Palm
> Beach Packet Group) have aready corrected two sites (Lake Placid & Vero)
> in the past six months that had their UHF beams collocated.
>
> Doug (WB4KGY)
>

Doug- I don't mean to put you on the spot - anyone can answer this... When
we may only have 30 ft of tower space or less to use, what are we going to
do with the other two antennas on a four UHF port switch? Even at that, on a
roof-top site 30 ft of vertical separation may not be possible at all.  What
are the practical aspects other than cut/try?

PO and receiver front ends need to be addressed.  Where desense is observed,
can the PO on one or more co-located transmitters be reduced to minimize the
desense w/o degradation of the intended service?  My  limited experience
with these Motorola Mitreks is that with 30-35w  and just 1 MHz of
separation, and the 6-el beams at the same elevation on the tower (but
back-to-back - one horizontal, one vertical), we've experienced little
degradation by either port to the other. (However, after chatting with Doug
at W. Palm last month, I now plan on running some more definitive tests!).
At that, I believe our general experience with Mitreks indicates they offer
the best bang for the buck so far as minimizing off-channel receiver
desense. Commercial radios by other manufactures may do as well or better.

Can we make the following caution at this time for those contemplating more
than one UHF backbone at a site?

"It is recommended that typical commercial 'mobile' UHF radios (i.e.
Kenwood, Icom, Yaesu, Alinco, etc, etc,) not be used for this service."

Or is this premature?

bud