effect of close property fence on dipole arrays and solution?

Bill Peters

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Feb 2, 2012, 8:14:41 PM2/2/12

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I put up a 20 meter dipole array a while back and I don't seem to be
receiving the greatest signals. The height is fairly low, about 20', and
one part of the array is fairly close to an aluminum fence I have
surrounding the property. I'd say the southern leg of the array is about
25' away from the fence. So I ask the question: what effect can fences have
on antennas and what are some possible solutions? I can rule out moving the
antenna because I lack the room to move it any further away from the fence.

Thanks,
Bill

Sal

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Feb 2, 2012, 8:35:46 PM2/2/12

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"Bill Peters" <peterswi...@excite.net> wrote in message
news:jgfce4$rr5$1...@dont-email.me...

Since length formulas treat your installation as being close to ideal,
consider that the antenna, being physically lower than ideal, may be too
long for 20m work. I made that mistake with a 40m dipole some years ago and
had to cut it shorter to reach resonance, due to the effect of the
too-nearby earth. (A tuner makes your radio happy but won't solve inherent
problems.

If the the antenna sees that chunk of metal fence as an upward extension of
ground, that just adds to your problem, although 25' away doesn't sound like
a big deal.

Have you access to an antenna analyzer that will clue you to the resonant
frequency of your dipole?

Know also: being too close to ground will cause the pickup pattern to favor
upward, vice off toward the horizon

Wayne

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Feb 2, 2012, 9:22:07 PM2/2/12

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"Bill Peters" wrote in message news:jgfce4$rr5$1...@dont-email.me...

-
For clarification, is the "dipole array" more than one dipole?

You could configure a reference antenna by mounting a nominal 16 foot
vertical element on the fence, using the fence as a ground. (If you don't
have spare tubing, a wire taped to a bamboo pole would do) Then you could
get an idea of whether or not the dipole is working OK, or the band being
dead.

--Wayne
W5GIE

tom

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Feb 2, 2012, 10:03:35 PM2/2/12

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This works fairly well in my experience.

I have a Gap vertical with the feed 8 feet above ground and a 40 or so
foot dipole at about 27 feet. Both connect to an LDG AT100Pro. The Gap
needs virtually no matching and on 20 the dipole doesn't need a large
amount. RG213 at about 70 feet on each.

It is interesting to compare the two. It also makes one understand why
diversity receive antennas are really nice.

By and large the receive signal strength is close, which makes sense
given how the new wonder digital S meters work. Transmitting to ground
wave stations is as expected (again obviously roughly) when considering
horizontal v vertical.

tom
K0TAR

J. C. Mc Laughlin

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Feb 2, 2012, 11:15:35 PM2/2/12

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The holy goal of resonance has little to do with the performance of a plain
0.5 WL dipole. Almost all of the power delivered will be radiated. What is
sure to reduce performance of a 20 meter dipole is being only 6 meters above
ground. 6 meters is about 0.28 WL and thus most of the radiation is towards
the zenith. This will be true even if you prune the length of the antenna
to have no reactance (to be resonant) at your operating frequency.

The old tale of resonance equating to enhanced performance is, to say the
least, an exaggeration.

We do not know how high the fence is nor what sort of grid it has. Since
the antenna can not be moved horizontally, not much point in speculating
about the effect of the fence.

What will improve performance is to move the dipole higher.

73, Mac N8TT

"Sal" wrote in message news:jgfdk6$196$1...@dont-email.me...

J. C. Mc Laughlin
Michigan U.S.A.
Home: J...@Power-Net.Net

dave

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Feb 3, 2012, 3:11:03 PM2/3/12

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At 20' AGL I'd be using a full wave horizontal loop for receiving. I
transmit with a vertical and use a loop plus the vertical (via MFJ-1026)
for receive. for receive

JIMMIE

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Feb 3, 2012, 6:04:08 PM2/3/12

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AT 20 ft above ground the antenna may be performing more like an NVIS
antenna radiating most of the signal upwards. Unfortunately 20M is not
a great NVIS band.

Jimmie
KD4RQE

Sal

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Feb 3, 2012, 7:32:55 PM2/3/12

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"J. C. Mc Laughlin" <j...@power-net.net> wrote in message
news:QqOdnVIGAqT1wrbS...@bright.net...

>
> The old tale of resonance equating to enhanced performance is, to say the
> least, an exaggeration.

The OP called it a 20m dipole. Pls take it up with him. ;-)

"Sal"
KD6VKW

Tuuk

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Feb 4, 2012, 8:03:02 AM2/4/12

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I agree with Tom and Dave

They seem to know what they are talking about.

"dave" <da...@dave.dave> wrote in message
news:TvqdnYzLi7jKorHS...@earthlink.com...