A line often heard around here: More antennas are better.
Having a pair of tri-banders on the same tower is a fairly common
strategy. Although combination gain is a consideration, it is not the
only one. Particularly in contesting, this setup can have several
advantages:
1) transmit high or low antenna. Many report that one antenna is
better than the other or both together at a given time of day, or
certain conditions. This is related to the different vertical patterns
of A, B, or Both.
2) typical 2.5 db additional gain at mid takeoff angles, phased
together.
3) able to point one antenna one direction, one another, both fed at
the same time when conditions have more than one major path open.
The spacing is always a compromise, but for tribanders, thirty
something feet works, even as little as 25 will work. Practically,
however, this distance is usually dictated by the sets of guys on the
tower. For purely mechanical reasons the lower tribander is almost
always placed just above the second set of guys from the top.
The tribanders need to be identical. Phasing dissimilar antennas is a
very dicey piece of work requiring professional grade equipment and a
group effort.
A matching box to switch the two feedlines is required, as in the
transformer based WX0B stackmatch. You cannot use a 75 ohm phasing
harness as they are single frequency devices. Instead use electrical
3/2 wave at 21.2 MHz, 50 ohm coax to feed each antenna from the
matching box.
Top and second set guys need to be non-metallic, or broken up and
insulated from the tower in ~11 foot sections, for the first 33 feet
away from the tower.
The lower antenna is frequently not rotatable, and pointed in a
popular direction. DX contesters on the east coast using this setup
will fix the lower antenna at Europe, and rotate the top antenna.
Good luck and 73,
Guy.
----- Original Message -----
From: "Sid Proctor" <sproctor1@triad.rr.com>
To: <towertalk@contesting.com>
Sent: Friday, October 04, 2002 9:29 PM
Subject: [Towertalk] Fw: Phase tri bander
Subject: Phase tri bander
let me try this again...I am interested in phasing two tri band yagi
for 20-15-and 10 meters. Not sure where to start or even if this is
possible. I need to know what parts and equipment that will be
needed. And would like to know if the gain is worth the effort.
Thanks
Sid Proctor KE4RIG
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