well, the tradditional wisdom says to get your antenna up as high as
possible to get the best results.
However, for 40, and especially 80/75 meters, you can have an antenna fairly
close to the ground and still get good results. There is a principle called
NVIS, or near vertical incidence skywave.
Certain frequencies, specifically 1.8 to about 15MHZ can propigate as either
a ground wave, or a sky wave. 80/75 meters, and 40 meters, typically use a
ground wave type of propigation
This means, you can use a horrizontal wire, relatively close to the ground,
and the earth itself will act as a reflecter or as a simpethetic radiater.
This is not meant to be a good dx antenna, and your range on the lower
frequencies will be limited to ground wave propigation, or out to maybe
650KM.
However, if you put another wire, or radiator, on the surface of the ground
running underneath the primary wire, and have it slightly longer then your
main wire, then you can increase your gain by a bit and thus increase your
range slightly.
wikipedia describes NVIS much better then I can, check out this link for a
better run down:
http://en.wikipedia.org/wiki/Near_Vertical_Incidence_Skywave
Now, some anicdotal evidence of this theory working.
At my previous QTH, i had a near ground antenna, approximately 5 feet off
the ground along the top of a wooden fence.
It was 14 gage enamaled copper wire and about 63 feet long.
It ran in a U shape around the back yard fence line. I used regular RG8
coax to connect to it, with the ground braid of the coax connected to a
convenient water main access pipe.
I was able to obtain a 1 to 1 swr on 3.755MHZ by trimming the original 75
feet of wire until it was around the 63 foot mark.
I did not use a sympathetic radiater along the ground under the wire. The
results were good. I had the expected 650 kilometer propigation every
night after sun down. I also often got bette rpropigation with consistent
contacts out to 2000 kilometers. Signals within the NVIS standard range or
600 to 700K were nearly always S9 and above.
The interesting thing was that I quite often was using skywave propigation
because i was hearing and talking to stations far on the other side of the
rockies to the west. Also, stations in ontario, about 2500 kilometers east
of me could be heard and talked to on a regular basis. I suspect my
extremely good ground had something to do with it.
anyway, the NVIS antenna worked great on 80 and 40, though not as well on
20M of course where it is mostly dx more then 2000K away.
So, if you are primarily interested in working 3 and 7MHZ, an antenna either
5 feet, or 20 feet off the ground will work about the same.
You will not get better results necesarily by having the antenna 15 feet
higher on those bands.
You would have to surpass the minimum quarter wave above ground on 40 which
is 33 feet, before seeing anything better on that band, and at least a
quarter wave on 80, 66 feet, before seeing better results on that band
compared to an antenna 5 feet off the ground.
73
Colin, V A6BKX
----- Original Message -----
From: "Shaun Oliver" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, March 25, 2008 12:14 AM
Subject: antenna hight
> Hi all, I've read on all kinds of sites that for hf, hight is important
> when raising an antenna. my question is, how important is it really?
> I've a random length wire atop my fence which is roughly 5 and a half
> feet high. the fence is wood. would raising this wire provide me better
> results on receive? if so, given that I want to work predominently 40
> and 80 meters, what is a safe hight without upsetting the xyl or the
> neighbours or local government?
> actually, I should ask, what is the reccomended hight and what is a
> reasonable hight.
>
> --
> Shaun
> web sites:
> http://www.myspace.com/blindmanshaunoliver
> http://blindman.homelinux.org/~blindman/
> skype: brailledude
>
>
>
> --
> No virus found in this incoming message.
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6:50 PM
>
>
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