[TowerTalk] RE: Improving 6-mtr setup.....

[Avila, Edward]

John, good advice and thanks for taking the time to detail your experiences
here.......they are very very helpful!

73...../k6sdw

-----Original Message-----
From: John Godwin [mailto:kb5iua@tgn.net]
Sent: Thursday, November 09, 2000 1:33 PM
To: EAvila@caiso.com
Subject: Re:Improving 6-mtr setup.....


Hello Edward

There is a limitation in the amount of gain that you will receive with
either a stacked system or a single longer boom yagi. Stacked the
theoretical is 3db with a longer boom you will see about 2.2db every
time you double the boom length. At least on VHF antennas where you are
working with boom lengths longer than 1 wavelength.

Now to the real world challenges. Stacking while on the surface can
realize more gain in the real world the additional gain will be less
than 3db. From my location 6 meters is fairly noisy and I live in the
country with no loud noise sources nearby. I live 50 miles from downtown
Houston and with an 11 element M2 I can hear a noise increase when
pointing at Houston (small though it is). With a fair amount of modeling
you should be able to keep the F/B ratio of a stack close to that of a
single antenna in the stack. You will also need to either use a T or a
power divider to feed the stack. On 6 meters the T is probably the way
to go because you will not have to buy or build a power divider. In
either case both antennas will need the feed point impedance's as close
as possible or you will degrade your theoretical gain. You will also
have to use as low a loss coax from the T/power divider out or you
decrease your theoretical gain. The coaxes from the T/power divider will
also need to be the same electrical length or again you will degrade the
pattern. Another thing to consider is the feed system of the antennas.
You can get some divergent views on which is better but I am under the
impression that a T-match is more efficient than a gamma match. I would
guess that the 5 element yagis you are considering for the stack will
have a gamma match and longer boom antenna will probably use a T-match.
While you may have the mast space to stack a pair of 6 meter yagis you
will severely limit yourself if you decide to add anything in the
future. The wind load of the stack and long boom yagi might be close but
there will more moment weight on the long boom antenna, so you will need
to look at the capabilities of your rotator. Another advantage that by
using a longer boom antenna is that you will have a 5 element antenna
that you can side mount on your tower, point in the most often used or 
desired  direction so that when you are using your long one in another
direction you can instantly change with a simple A/B switch.

Sure are a lot of variables but my choice was and is a single long boom
antenna. I have had a pair of 5 element antennas and a single long boom
antenna up at the same height and did some A/B testing, after about 6
months I took the stack down to make room for other antennas. In my case
the single was quieter than the stack if nothing else so the stack came
down.


John K5IUA

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