Hello John,
For the higher bands of 144 and up, I used to think that it all depended on
your horizon angle (The angle from your antenna to the horizon) If there was
any obstrruction in front of your antenna, it made sense to increase height
to minimize that horizon angle so as to make it closer to zero degrees or
even a negative angle. I also figured that if the view looked the same from
your tower when you added the extra height, you would not gain much from the
increase. In short the horizon angle looked the same for both spots.
Years ago, Steve Powlishen, WA1FFO/ K1FO, wrote an article that covered
tropo scatter as it pertained to the low microwave bands. He was comparing
the then new 900 MHz band and 1296. That was a good primer on horizon
angles. He got much information from military studies done before satellites
were so prevalent. Tropo scatter was very important to them in the 60s. They
used bands around 900 MHz, 1.7-2.3 GHz and some higher bands as well. I also
have some good data from those sources. To be brief, if you can get a
negative horizon angle, you will pick up many dB on over the horizon paths.
A 0.5 degree negative angle was good for about 5-10 dB of extra margin at 2
GHz over the same setup with a zero degree angle!! That must be why VHF hams
always seem to go to mountaintops. I should compile some of that data for
the web.
I even ran an experiment on 144 MHz in 1985. I had an early 4218 CC
prototype long yagi mounted at 40 ft, and fed with 7/8" coax. I rigged up a
coax switch to a second tower with a revised production version of the same
antenna that was almost 1/2 dB better. It was at located nearby at 95 ft and
also fed with a longer run of 7/8" coax. Both antennas were on a hilltop
with essentially the same view at either height. I concluded that height
made no difference worth worrying about for signals arriving from beyond the
horizon. I used this arrangement for a few weeks, seeing no real difference
at any time. The horizon angle was essentially the same for both yagis.
Now I am not so sure. Since then, I have tried many different antennas
on many different bands, with the average height increasing over time. I
believe that local signals are not really affected that much assuming the
horizon angle remains unchanged, but I am beginning to think that added
height can make a big difference when tropo DX is coming in. The effect is
slight to be sure, but I believe that marginal tropo can be enhanced by a
much higher antenna. This conclusion was formed after a few years of using
antennas all at 100 ft plus heights on the bands 144 thru 10 GHz. (3456 is
at 137 ft above ground) There have been times when I work stuff that comes
bombing in for a short period, and other stations nearby hear nothing. It
seems to happen more often with the higher antennas. You won't see this
effect with propagation programs such as Radiomobile!! It only seems to
apply to anomalous propagation!
I would not be surprised if this effect was not evident in other areas.
It may be a condition of being on a hilltop and surrounded by other
hilltops. Things might be entirely different on flat ground as seen in the
midwest, or coastal Florida. It would be interesting to hear more anecdotal
evidence from those areas!! I know that hills can really affect tropo. It
may be linked to where the stable air is sitting! It may not ever get stable
near the ground in hilly areas. Just a thought.
So then, my not so scientific results may not mean much. Of course we
are talking about higher bands where the antenna is already many wavelengths
above ground and an elevated pattern is not a concern. On six meter Es,
there is no miracle height. To cover all your bases, you need multiple
antennas to provide lobes at many angles. Many times as an opening develops,
I have observed the best reception to begin at a very low angle, and then
increase as the opening progresses. I recall one double hop opening in 2006
that started best with all four yagis in phase on the horizon. (very low
angle!) It then progressed to where a phase delayed 4 yagi pattern at 9
degrees was best. It finally progressed to where a 2 yagi stack at 18 &42 ft
was better than that! The point being that at any given time, the arrival
angle can be all over the place. Every opening can be different. If I had to
choose, I would have two antennas arranged to cover many angles.( One high,
and one low!)
As far as coax losses go, you must keep that as low as possible. On
higher bands, tower mounted preamps or even entire stations must be topside
to take advantage of things with tall towers. I guess the simple answer is
that if your horizon does not change when adding extra height, there is no
point in going higher unless you are willing to cut your coax losses. Don't
go up 130 ft on 432 and run LMR-400 and expect to see improvements unless
you are going to clear green treetops or a hilltop nearby!! My previous
comments on high yagis vs lower yagis were formed after using them with very
low loss cables (1 5/8") and preamps on 432 and above. My losses were low or
non existent!
Always a fun topic.
73
Dave K1WHS
located in rural and hilly Maine.
----- Original Message -----
From: "John Geiger" <johngeig@yahoo.com>
To: <vhfcontesting@contesting.com>; <vhf@w6yx.stanford.edu>
Sent: Wednesday, November 29, 2006 6:43 PM
Subject: [VHFcontesting] Optium antenna height
> Another local VHFer and myself were having a
> discussion as I was driving to work today about
> antenna height for VHF. One thing we wondered about
> is if there is an "optium" antenna height for VHF, or
> is "higher always better." Is there a point where the
> extra height isn't worth the cost of tower, coax, etc,
> and the extra loss in the coax-in terms of the
> increased distance you will work? Or does extra
> height always outweight these other factors?
>
> 73s John W5TD
>
>
>
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