ground plan antenna design and height.

[captainvi...@gmail.com]

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 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. Thanks for any advice. Lenny

[Jeff Liebermann]

On Wed, 5 Nov 2014 20:57:12 -0800 (PST), captainvi...@gmail.com
wrote:

I'll pretend that this has something to do with "repair".

>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 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. Thanks for any advice. Lenny

Don't worry about the exact angle of the ground radial drop. If the
fell against the coax cable, the antenna would be about 70 ohms. If
they stuck straight out and were perpendicular to the coax, about 35
ohms. If at a 45 downward droop, 50 ohms. The VSWR and mismatch loss
for these are:

70 ohms = 1.4:1 = 0.12 dB
50 ohms = 1:1 = 0.0 dB
35 ohms = 1.4:1 = 0.12 dB

I think you can handle 0.12 dB of loss quite easily and 1.4:1 VSWR is
not going to blow up your 10 meter transmitter.

There also will be little effect to the vertical antenna pattern as
the vertical beamwidth is a rather large 80 degrees.

If you add a 5 meter pole to the base of the ground plane, then your
radials will be almost covering the coax cable feed and you'll have
something closer to a 70 ohm antenna. Instead of trying to fix that,
just call it a "coaxial antenna" instead of a "ground plane". End of
that problem.

I'm not sure what part of your house to consider as ground level. It
can be anywhere between the roof and the ground, depending on how much
metal you have in the roof, walls, ceiling, slab, etc. If you have
foil backed insulation in the attic, use that ceiling as the ground.
If you're house is miserably insulated and effectively RF transparent,
use the earth ground level. I can't be more helpful without a
description of the building and ground.

--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

[Pat]

On Wed, 05 Nov 2014 22:39:41 -0800, Jeff Liebermann <je...@cruzio.com>
wrote:

Are these radials cut to length (perhaps with insulators) or are they
many wavelengths long? Coaxial antennas usually have the portion of
the shield that goes back over the feed line cut to a quarter
wavelength. And radials on typical VHF/UHF ground planes are usually
cut to around a quarter wavelength. I would think adding length to
make them double as guy wires would have a significant effect. I am
not an expert in this area so I am not critisizing ...just curious.
Pat

[captainvi...@gmail.com]

On Wednesday, November 5, 2014 11:57:15 PM UTC-5, captainvi...@gmail.com 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 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. Thanks for any advice. Lenny

Typically when building a ground plane antenna (according to the ARRL books), you would make the "ground" radials, (usually there are at least 4 of them) five percent longer than .25 wavelength. I don't know why that is but it's mentioned in the books. And yes at the end of those radials you would put insulators. The other end of the insulator,( I use a piece of Lexan with two holes drilled in it) becomes the "guy wire". It would be tied off if possible at the 40 degree or so angle I previously mentioned thereby making the entire length double as a radial and a guy wire.

I already have another ground plane antenna I built several years ago which is mounted on the roof for my business band base station. That operates in the the 40 MHZ range and although naturally shorter is still nowhere near .50 wavelength above the roof but it has always seemed to have decent coverage.
This all just got me wondering though.

To answer your question about the house Jeff, there is no foil insulation, non metal roof, and unless you count the copper plumbing and electrical wiring as having a significant effect on this scenario there's not much else that I could see that could be factored in.

I do have an SWR meter, however except by extrapolating the information I would need by looking at the SWR at the operating frequency I don't have an easy way to measure feed line or antenna impedance. Lenny

[Jeff Liebermann]

On Thu, 06 Nov 2014 08:40:30 -0500, Pat <p...@nospam.us> wrote:

>Are these radials cut to length (perhaps with insulators) or are they
>many wavelengths long?

Cut to length for about 1/4 wavelength at 10 meters or about 2.5
meters long.

>Coaxial antennas usually have the portion of
>the shield that goes back over the feed line cut to a quarter
>wavelength.

If you actually test such a coaxial antenna, you'll probably find that
it doesn't quite work. If the braid is too close to the coax cable,
bad things happen. That's why they usually have a small spacer.
Typical 2.4GHz coaxial antenna.
<http://802.11junk.com/jeffl/antennas/coaxial/coax-ant.jpg>
If you look carefully, you'll notice that the length of the driven
element (on the left) is longer coax "sleeve" element. It's not
really symmetrical.

>And radials on typical VHF/UHF ground planes are usually
>cut to around a quarter wavelength. I would think adding length to
>make them double as guy wires would have a significant effect.

Correct. I didn't notice that problem in the original question. I
assume that the ground radial wires would be cut to the proper length
and extended to the roof line with insulating nylon rope. I was only
concerned about their droop angle.