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Author Topic: Multiband antenna for limited space!!  (Read 2033 times)

KA4VNM

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Re: Multiband antenna for limited space!!
« Reply #15 on: February 28, 2022, 07:57:03 AM »

It is interesting to note that many HOA's have not updated the OTA rules changes the FCC rewrote years ago. That is a door that in many situations might open up discussions with the HOA's as to accommodations which mimic or emulate TV/FM antennas. Most 2-440 and even 6 meter antennas look like TV aerials.  Something being better than nothing. My HOA is in VA and they actually incorporated the new statute into the HOA rules.  Sometimes just talking it through works.
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KK4GMU

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Re: Multiband antenna for limited space!!
« Reply #16 on: July 22, 2022, 11:23:51 AM »

I ended up installing a G5RV Junior flat on my asphalt shingle roof.  The roof pitch faces the back yard, so not seen from the street.  The dipole connection is around 5 feet higher than the ends of the dipole, so it is roughly in the shape of an inverted V.

The SWRs are within non-tuner range on parts of 10 and 20 meters.

I haven't yet broadcasted on it.
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IC-7100, RSPdx, AT-D878UVII-Plus HT, TGIF Spot

WB6BYU

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    • Practical Antennas
Re: Multiband antenna for limited space!!
« Reply #17 on: July 22, 2022, 08:21:11 PM »

Adding an extra 5’ or 10’ coax extension to your
cute t feedline may permit a match - you’ll have to
experiment with the length.

It also depends on your tuner, but there are a number
of ways around the problem.

KE0VT

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Re: Multiband antenna for limited space!!
« Reply #18 on: July 22, 2022, 10:14:53 PM »

I have a broad-banded terminated dipole in the attic. 100W in and about 25 out. SWR is 1.6:1 or better from 10-160, 2.36:1 on 6 meters. You generally don't get high signal reports
but I have worked Spain, Italy, Slovenia and near Sao Paulo, Brazil. Inexpensive to build. Has very good ears but can't always work those you hear. It can be zig zagged around, just
keep each leg very close to the same length. Mine is 55' per leg. I am in Colorado. Longest contact so far is 5700 miles to Japan. I use a 1K ohm resistor. I also have Isotron
antennas on 6, 10, 15 and 20M. Despite what some think of the Isotrons they do work.   73
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KK4GMU

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Re: Multiband antenna for limited space!!
« Reply #19 on: July 24, 2022, 08:24:44 AM »

... but asphalt shingles are much more lossy than the old
wooden ones. 
Can you elaborate a bit on asphalt shingles being lossy when the wire is layed down directly on the top of it?  Is there a typical amount of lossiness (measurable how?) compared to the wire being 1" or 1' or 5 feet above the shingles?

If lossiness is significant when the wire is layed down on the asphalt shingle, my plan B would be to mount the center of my G5RV Junior on my retractable pole, a foot (or more????) below my 2 meter/70cm vertical.
« Last Edit: July 24, 2022, 08:42:32 AM by KK4GMU »
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WB6BYU

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Re: Multiband antenna for limited space!!
« Reply #20 on: July 25, 2022, 01:52:54 PM »

Quote from: KK4GMU
Quote from: WB6BYU
... but asphalt shingles are much more lossy than the old
wooden ones. 

Can you elaborate a bit on asphalt shingles being lossy when the wire is layed down directly on the top of it?  Is there a typical amount of lossiness (measurable how?) compared to the wire being 1" or 1' or 5 feet above the shingles?...




I don't have accurate numbers.

There are two effects:  the dielectric loading, where the antenna
resonates at a lower frequency when close to the roof, and
the losses in the material itself.  The first one is easier to measure.

I've installed a 40m loop around the roof at several houses, often
finding the resulting antenna resonated significantly lower in
frequency than I had expected.  The loop at this house was about
5.5 MHz, with most of the wire laying on the shingles.   Then I
tried propping it up on a dozen plastic pint refrigerator containers,
perhaps 4" or so (although the wire still touched the roof in places
in between them).  That raised the frequency to 6 MHz or so.

It still didn't seem work very well, but I was plagued with noise
so I didn't get to try it out much.  Now my noise has (mostly)
disappeared, and I have a 40m doublet fed with 300 ohm twinlead,
about 2 - 3 feet above the roof.  Still doesn't work very well, but
part of that problem is due to losses in the tuner (the impedance
at the end of the feedline on 20m is about 5 ohms, which is not
good for tuner efficiency).

By contrast, when I've done that with wood shingles, they were
relatively transparent to RF (at least when dry), and I've had
good results just tucking the wires under the corners of the shingles.


It would, however, be an interesting study.  I think erecting a loop or
dipole on the roof and measuring the impedance over frequency,
possibly at different heights above the roof, might give us some
more details to work with.

Some day...


Of course, there are a lot of details that will affect the results, like
the use of foil-backed insulation beneath the shingles, the actual
composition of the roofing, other wiring in the attic, etc.
My guess is that a foot or so off the roof would make a significant
difference.
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