Gary:
Quite a few guys on this reflector have used the situation that you describe
and faced the same choices.
I personally ran a two element 40 meter yagi (The Mosely S402) which was on
a (maybe, my memory is fading) approx. 20 foot boom. About 20 years ago I
ran it above a TH6-DXX 20-10 yagi, and did this for many years. During most
of that time, I had the Mosely rotated 90 degrees to the orientation of the
TH-6DXX and the system ran very well on all bands. Under my best set up in
this regard, I had fabricated a tilting platform that was welded to the top
of the mast used to hold both antennas. The 40 meter yagi was mounted to the
tilting apparatus on top of the mast. The tilting mast had a piece of 1 or 1
1/2inch black pipe (galvanized) also welded to the tilt plate and ran
parallel to the mast when the 40M yagi was horizontal and in position. I had
insulated "tag lines" to each end of the 40M boom, so I could pull the end
of the boom down to me at the top of the tower then I unlatched the tilt
mechanism.
The reason that I had the tilt plate fabricated was this. I had it
engineered so that I could tilt down each of the two elements of the 40
meter yagi so that each end of the boom would tilt right down to just above
the TH6. This way, I could work on each of the two 40 meter elements (or put
them on or take them off (Hurricanes)) without needing to climb the top ~10'
feet of mast above the TH6. Since this was a two element shorty forty, with
feedline attached to the driven element at one end of the boom, this let me
bring the feedpoint down to the top of the tower. My experience was that I
needed to periodically work with the feedline attachment point of the 40
meter yagi and this let me do it easily.
Now, here is the biggest problem (to me) of the 40 meter yagi 10' above the
tribander concept that you are working on. It is a bear to do anything to
that arrangement as the mast is very "top heavy" with the 40 yagi above the
top of the tower and the 10' of heavy wall mast up above the tower. My
arrangement had the rotor at the bottom of the top section of Rohn 45. I
used a Rohn thrust bearing at the top of the tower, and the rotor (Hygain
Ham - IV ) at the bottom of the top section mounted on an accessory plate.
These two yagis were probably an overload for the Ham-IV, but I ran one
none-the less. To begin with I had no intermediate accessory shelf with an
additional thrust bearing above the rotor. My set up (and the overload on
the Ham - IV ) meant that I was probably needing to have the rotor and break
reworked every two or three years, and I needed to swap out the rotor.
However, when I did this, and before an intermediate second accessory shelf
and thrust bearing, when you remove the rotor from its position, the mast
and antenna won't stay vertical and tilt off to one side. It can only angle
so far because the mast catches inside the Rohn 45 but the whole thing is a
problem to handle and not a lot of fun. Except for this problem, I really
liked this arrangement.
A second accessory shelf just above the rotor with an additional thrust
bearing will hold the mast vertical even if you have to swap rotors or do
other repairs.
By the way, I had no interaction between the 40 meter yagi and the
tribanders when I had them rotated 90 degrees from each other.
As an after-thought, I would now simply rent a crane to put the whole thing
up and over the tower and drop it down into position. At the time, I had a
very tall custom made gin pole with a custom made bracket to handle the
whole thing going up and going down.
Good luck with it. 73s
Gary W5FI
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Gary Slagel
Sent: Saturday, February 02, 2008 9:10 AM
To: towertalk@contesting.com
Subject: [TowerTalk] Interaction between yagis on the same mast
I want to have gain antennas for 10/15/20/40 on top of
my tower and am trying to decide between a 4 band
antenna like the Force 12 C4XSL or a tribander and a
separate 40m yagi on the same mast. Struggling with
which is worse, interaction from having the 40M
elements interlaced on the same yagi or interaction
from having the two antennas stacked vertically.
I'm leaning toward two separate antennas separated by
about 10 feet and one oriented perpendicular to the
other.
Does anyone have any opinions?
Thanks,
Gary
Gary Slagel/N0SXX
Lakewood, CO
http://marina.fortunecity.com/sanpedro/351
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