I made this post to the W6YX vhf list in 1998 went
this same topic came up......you might find it
useful.
----------------------
I have been reading the thread on 6 meter antennas
with interest. First I have to say that I come from
(for the most part) an HF contest background. I have
been very lucky in that I have operated from stations
with a lot of antennas at diffrent heights. You can
see what I mean by going to: http://www.kkn.net/~k5tr/
and "touring" some of these stations.
I have done the June vhf contest over the years from
WB0DRL (3 times) and now from W5KFT's station - where I
have gotten to setup the station the way I think it
should be set up.
I think that 6 meters is VERY much like 10 meters and the lessons
that are learned on 10 meters can be applied to the design
of a competitive 6 meter station.
Several of the previous posts talk about "low" antennas - and although
30 or 40 feet might be "low" it is not really that
low in terms of wavelengths at 6 meters where a wavelength
is about 19 feet. I think that you need to keep this
in mind as it is the height above ground in wavelengths at
the frequency of interest - not just the "low" physical height
that makes a diffrence.
Also I have not seen any comments on the type of terrain
that sorrounds the antenna installations being described.
It is very important to take into account the terrain
from the base of the tower out to a mile or so as this will
greatly effect the performance of the antnena system.
You can not assume that an antenna over flat ground at 30 feet
will perform the same as an antenna at 30 feet on a hill.
You can use programs like TA or YT (YTAD) to model antennas
over terrain.
The W5KFT station is setup the following way:
Cushcraft 617 boomer at 38' that rotates stacked over
another 617 boomer at 18' that is fixed towards EM79.
This tower is about 40 feet from the edge of a large
lake and has about 5 miles lake water in front of it
to the northeast. So in this direction - and in many others
from this location this antenna is over "flat" ground.
The other antenna is a 5 ele cushcraft (12' boom) at
140 feet - this tower is about 100 feet from the edge of the lake.
The land in the other directions is mostly flat. With a slight
rise here and there.
The stack is the main antenna and it works very well.
According to the antenna programs it has the following
peaks and relative gain.
1st lobe 1st null 2nd lobe 4db points 1st lobe
--------- -------- -------- -------------------
38' antenna - 6 12 20 3 - 9 deg
38'/18' 7 17 22(-14db) 3 - 12 deg
18' 13 27 >30 3 - 20 deg
What I find is that on single hop e-skip that any one of
these antennas can and does work very well. On double hop
e-skip either the stack or the bottom antenna is the best
antenna. Often to the stations (W1 and VE1) that are just beyond my
1 hop distance the bottom antenna is the best. My guess is
that the "short" two hop e-skip takes a higher angle
to make the two short hops.
BTW - CT3FT was a tad stronger on the stack than he
was on the low antenna - but not much.
The stack is the best all around preformer and has a bigger
arrival angle "footprint" than the top antenna and has more gain
than the low antenna. It also has the added advantage of haveing
one major lobe (the one centered at 7 deg.) and as a result
maybe less prone to multipath coming in at diffrent arrival angles.
The antenna at 140' is a small antenna and as a result of that
and poor design has less gain and less pattern in general - even so
I find that this antenna is a very good antenna at times and can even be
better than the stack - usually on some arrival angle that
one of the antennas in the stack does not cover well or at a
low angle. It should be pointed out that a 6m antenna up this high
has many peaks and nulls - as it gets closer and closer to a free
space pattern.
I see alot of diffrence in how selective the arrival angles
are on 6m eskip and I think that you need to have an antenna
system that can cover as wide a range as possible to take
advantage of this fact. As others have noted it is possible
to switch between antennas and have s-9 signals disappear or
even go away. This is one effect that seems to me to
be much more pronounced than on the bands below 50 mhz.
I think that it is well worth your while to have at
least two antennas at diffrent heights on 6m.
Also 6m eskip openings can/are very short in nature so it is
also a good idea to have antennas that you can point in
diffrent directions at the same time so that you can beam
your signal in more than one direction during an opening.
So what does all this mean?
If I were going to put up one 6m yagi for eskip - I think I would
put it up somewhere between 20'-40' feet if I could.
If we are talking about an all around antenna for 6m - eskip and
tropo etc. - then you might want to go a bit higher.
I would try to put up several antennas - even if they are
just 3 or 4 element yagis or even a vertical.
I have decided that the W5KFT station will have the
following 6m antenna setup:
1 - 38'/18' cushcraft stack to the NE with top rotary.
2 - 155' M2 30+ boom yagi - rotary.
3 - 30'/15' hygain 66dx stack fixed to southern W6 - in phase all the time.
(so I can call CQ in that direction more that I have been)
4 - A vertical.
The switching will be set up so that I can feed any of the three
antenna systems at one time and/or any combination
of them as well. (not including the vertical)
Anyway - I hope that I have added something to this
discussion and I also hope that the exchange of information
will help others build bigger and better stations.
--
George Fremin III
Johnson City, Texas "Experiment trumps theory."
K5TR (ex.WB5VZL) -- Dave Leeson W6NL
geoiii@kkn.net
830-868-2510
http://www.kkn.net/~k5tr
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