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Author Topic: Possible Dipole configuration?  (Read 296 times)
KL7AIR
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Posts: 7


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« on: September 18, 2008, 11:58:40 AM »

If I took a basic half wave dipole on say 40 meters and instead of teminating it as a single wire I Tee off the ends.   Idea being to gain some length on an A-Frame roof.  I live in an antenna controlled area (HOA) and while the G5RV I have "hidden" on the roof works ok, if I can improve it......

Right now there is a stealthy 102 foot G5RV (300 ohm twin lead fed) lying flat on a non-conductive roof.
A-Frame style roofline.  By running a tee off the ends I lengthen a dipole.  Would that actually do anything for me or would it simple be better to run the single wire off at a 90 degree down the roof line on each end?
Wondering if the Tee end would act like a tophat of sorts.

I would gain possbily another 30 feet of length on each end as a single wire and say 45 feet if tee ended.

So in theory I go from a wire thats 102 feet long to one that is 180-190 feet long.  Or if a single wire run 165-168 feet long with the ends run off at a 90 degree angle.  Not looking to add to the G5RV will build a new dipole using a balun and ladder line etc..as needed.

The single wire would get close to being tunable on 160m (expect a narrow portion anyway) and the Tee version if it tunes would be an interesting radiation pattern I know.  But considering that I currently use a G5RV lying on the roof, odd radiation pattern is how I live already...hi hi.

Ron, KL7YK
Anchorage

(yes I know I am logged on as the Club Call, no worries)
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WB6BYU
Member

Posts: 13331




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« Reply #1 on: September 18, 2008, 12:41:58 PM »

It depends what you want the dipole to do.

If you can get a full 40m dipole up, you won't see a lot
of improvement on that band by making it longer.  If you
want to work 80m or 160m where you don't have enough
room, adding the extra wire may be a good idea.

Regarding the question of whether to use a single wire
at each end (basically a bent dipole) or two (a capacity
hat) that also depends on your desired radiation pattern
and what bands you want to use.  With the capacity hat
version (wires running in separate directions) the extensions
don't radiate (at least not on a first order approximation
anyway).  So if you started with a 40m dipole, you'd still
have, in effect, a 65' length radiating on 80 or 160m.
The pattern on each band would depend on the current
distribution along that center section.  If you use the
single wire extensions on each end then those wires also
radiate (since there isn't a wire in the opposite direction
to cancel the radiation) but it doesn't lower the resonant
frequency as much as it does with two wires.

This is one of those cases where you really can't know
for sure without trying it.  I'd suggest experimenting
with it in one of the modeling programs to get a feel
for how the performance changes on different bands.

(Meanwhile, if you are feeding in the center with twinlead
there are some interesting multi-band options you can
try using wires of different lengths on each end.)
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W7ETA
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Posts: 2527




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« Reply #2 on: September 18, 2008, 06:12:21 PM »

W4RNL's site has a lot of good info on top hats.
73
Bob
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W7ETA
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Posts: 2527




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« Reply #3 on: September 18, 2008, 06:12:32 PM »

W4RNL's site has a lot of good info on top hats.
73
Bob
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N1LO
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Posts: 1039


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« Reply #4 on: September 18, 2008, 06:50:17 PM »

Hi Ron,

Dale makes a good suggestion. You'll just have to model it to find out. Download EZNEC, and go ahead and buy the basic version, as the free version doesn't allow you enough segments for a proper model of an antenna this size.

When I was first licensed, I lived in a townhouse and built what I called an 'H pole' inside the attic. It was an end-loaded dipole. Each side was 15 ft long, with a 20' cross-piece at the end, all made from 1/2" copper water pipe. I fed it with ladder line, and tuned it multiband.

Another alternative would be to make the largest loop you can, ringing your roofline.

If you use tiny, stealthy standoffs, even just 1" long, I think you'd see an improvement over having the wire just laying on the shingles.

GL,

Mark
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WW5AA
Member

Posts: 2086




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« Reply #5 on: September 19, 2008, 06:45:46 AM »

I think that the only difference you will see is improved receive. Even on 20 meters, the height of most home roofs is not high enough for low angles of radiation or even any pattern varying much from omni directional. I always wonder why folks think that they have to orient a G5RV at 20'. Have fun.

73 de Lindy
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KB4QAA
Member

Posts: 2405




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« Reply #6 on: September 19, 2008, 10:12:08 PM »

Adding the T would be equivalent to adding the length of both legs.  It would be equal to just the single leg length.

There have been antennas that had flared ends... The advantage to their use is to achieve a somewhat wider bandwidth, not a longer equivalent length/lower resonant frequency.  

The closest example I can think of would be an HF Caged Dipole, but instead of running all the wires parallel, having them flare from center outward.

Bill

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