50-60 feet is a good average height to shoot for. (Wish I could get my 2x5el
stack that high.) However, sporadic-E can want take-off angles all the way
from flat (0 degrees) up to around 16 degrees for very short skip. The
length of the skip depends on the intensity of the cloud's ionization (the
Es MUF). When the Es MUF is very high and the skip very short, you're much
better off with a lower antenna (as low as 20 feet), assuming your
surrounding ground is relatively clear so you can make best possible use of
the ground bounce gain.
Apart from a low take-off angle (not always what you want, but often) -- The
other thing that greater antenna height affords you, and a lot of people
overlook this fact, is that the higher the antenna, the further it is from
man-made noise sources on the ground. A quieter band is a band where you can
hear (and work) weaker signals! Noise is also discrimminated against in the
azimuth plane, so a longer yagi with a narrower beamwidth not only has more
gain on transmit, but picks up less noise (unless it's pointed right at the
noise source). That is assuming the long yagi has been computer-modeled
properly and has minimal side- and back-lobes.
Putting a 6-meter yagi REALLY HIGH (like, over 100 feet) will give you the
maximum possible signal at the horizon for tropo and bleeding-edge Es and F2
DX -- but it will also give you lots of ugly NULLS in the elevation pattern
where your signal can drop by as much as 15-20 dB. So you might be pinning
somebody's S-meter at 1,450 miles away, but the guy who is 900 miles away
isn't going to hear you anywhere near that loud, all other things being
equal. Then you need to go to a lower antenna to cover that nulled take-off
angle.
As all the 6m Big Guns will tell you, the ideal solution for 6 meters is a
stacked set of long-boom yagis at various heights from "real low" to "real
high" that you can switch around either for single use or for use in phased
combinations. Since most "normal people" can neither afford nor find space
for an antenna system like that, it all comes down to compromises and
trade-offs.
Bill W5WVO
--------------------------------------------------
From: "Steve Kavanagh" <sjkavanagh1@yahoo.ca>
Sent: Tuesday, June 29, 2010 4:34 AM
To: <dave@egh.com>; <vhfcontesting@contesting.com>
Subject: [VHFcontesting] Optimum 6 meter yagi height
> Dave
>
> I did look briefly at this subject a few years ago....no measurements,
> just some simplistic analysis along the lines of what W2PV did much
> earlier for HF. This work was published in the 2004 Central States VHF
> Society Conference Proceedings. I could send you an electronic copy of
> that article if you are interested.
>
> For sporadic E, assuming flat land, it appeared that the best compromise
> height was about 50-60 feet. For tropo, on the other hand, the higher the
> better. There's a strong argument for having antennas at different
> heights for different purposes.
>
> Urban development around the station, sloping ground, etc. will change
> things, of course.
>
> Meanwhile, I do most of my contesting as a rover, with a dipole at 12
> feet, which is definitely the wrong height !
>
> 73,
> Steve VE3SMA
>
>
>
>
> _______________________________________________
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> VHFcontesting@contesting.com
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>
>
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