RadioBanter - View Single Post

[email protected] · November 6th 14, 08:56 PM posted to rec.radio.amateur.homebrew

wrote:
I'm planning on building a 10 meter ground plane antenna out of PVC pipe
and #12 wire radials drooping about 40 degrees. This will be mounted
on my roof on a tripod and mast, with the radials also serving as
guy wires. The ARRL antenna book mentions that a ground plane antenna
should be mounted at least one half wavelength above "ground". I know
that this sounds like a stupid question but I have to ask: for this
example do I consider the roof, which is more than 5 meters above
ground level as "ground" or do I need to be at least 5 meters above
the roof. I would like to secure the radials, (guy wires) to my roof,
but with the antenna at 5 meters high my droop angle will be off and
then I can't guarantee a 50 ohm match. The house is wood frame, no
foil backed insulation and a non metallic roof. Thanks for any advice. Lenny

You can likely neglect the wooden house for the most part.

The gain and elevation angle of the main lobe of a 10M ground plane at
various heights (to the bottom of the radials) is shown below:

1M 1.23 dbi @ 20
2M 1.60 dBi @ 15
3M 1.60 dBi @ 15
4M 1.49 dBi @ 15
5M 2.07 dBi @ 40
6M 2.56 dBi @ 35
7M 2.98 dBi @ 35
8M 3.18 dBi @ 30
9M 3.00 dBi @ 10
10M 3.22 dBi @ 10

BTW, you can do this yourself with the free demo version of EZNEC.

The height will effect the impedance which can be adjusted with
the radial angle and can also be shown by EZNEC.

--
Jim Pennino