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A Relatively Efficient, Relatively Small Antenna for the 160m Band




 
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Author Topic: A Relatively Efficient, Relatively Small Antenna for the 160m Band  (Read 944 times)
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R. Fry SWL
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« on: August 08, 2021, 06:47:15 AM »

For consideration and comment...

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W1ITT
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« Reply #1 on: August 08, 2021, 05:09:57 PM »

Looking at the physical layout sketch, I am unable to discern what's going on under the vertical portion.  If it's a ground, it had better be a good one. I'm not seeing enough detail to tell what's there, but the sketch looks rather sketchy.  The prediction seems to show about 10 ohms for the radiator itself, with an L network to transform things to 50 ohms.  Back in the '70s, Sevick, W2FMI (sk) did some interesting work with short radiators fed through unun transformers of various ratios.  The trick was that he had a nice flat yard with either 60 or 120 radials.  It's in Ham Radio Magazine.   I worked him on 75m a few times with his very short antennas and he had a fine signal.  Those who tried to duplicate his signal strength without a nearly perfect ground were disappointed.
Unless there's some magic in the blue stuff under the antenna, I'd go with a cross or a tee of chicken wire, or welded fence wire laid right on the ground.  Rob Sherwood did some work with this sort of ground in Ham Radio Magazine decades ago.  It's archived on his site sherweng.com with actual data.  A friend, in a suburban neighborhood in the midwest put up an inverted L in the trees with wire fencing hidden under the bark mulch in his wife's posy garden.  With a kilowatt on 160m he worked the Howland Island dxpedition a few years back, first call, on the first night he had it set up.  I don't promise instant magic, but a tall vertical wire or an inverted L over a wire fence laid on the ground will work well and it's much easier than burying radials in the back yard or getting tangled up in trick counterpoise arrangements that seem to want to sag down to neck level.  My 160m Tee is set up this way.  A simple matching network will facilitate tuning it up and down the band.  The fresh air does us good.
73 de Norm W1ITT
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K9MB
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« Reply #2 on: August 08, 2021, 06:37:36 PM »

I agree that it is difficult to see what is there, but it looks like a 1/8 monopole vertical and a single horizontal 1/8 wave radial.
There has been some work by K2AV on folded radials and some have made that a single one.
DJ0IP published an article with one elevated radial for 40 meters and he said it might work at 160meters
https://www.dj0ip.de/begin-here/
The bottom line is that the elevated radials must not be directly connected to an earth ground to prevent this thing from becoming a dummy load because you will expend most of the energy exciting the earthworms under your vertical. The radials are also resonant- while traditional radial fields are not.

Read K2AV articles online for details or get them from people copying his work. It requires a special matching- isolation transformer and high power might be difficult.

Bottom line- anything possible in simulation, but reality is less kind and that means that the best approach is 16 or more radials of at least 1/8 wavelength and fir bet performance, make that 40-60 at over 0.2 wavelength.
Ironically, fewer radials will give higher input impedances because ground losses provide a kind of perverse and wasteful matching network.
Sevick found out that a short vertical also performed best if it had a large top hat capacitor and top loading plus a good ground radial field. He was successful because he used these principles. His work is required reading for anyone building any kind of vertical low band wire monopole or inverted L antenna.

Here is one of those articles. Jerry also published an entire book on the subject.

http://www.arrl.org/files/file/History/History%20of%20QST%20Volume%201%20-%20Technology/QS03-73-Sevick_opt.pdf

Another prolific writer on this subject is Rudy Severns. Here is one of his articles.

https://www.okdxf.eu/files/short-160m-verticals.pdf

See Rudy’s references at the end of the article for more work in this field.
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WU2D
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« Reply #3 on: August 11, 2021, 07:46:34 AM »

I have a vertical groundplane for 160. Its not pretty, but it is full size and I managed 4 elevated radials. The top of the vertical wire is bent 90 degrees 3/4 of the way up over to another tree.
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steve_qix
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« Reply #4 on: August 12, 2021, 01:30:33 PM »

How small is small ?  I have a 160 meter antenna up here at Mirror Lake that requires only 140 feet of horizontal space, and it is center fed with coax (and a choke balun at the center).  The ends hang straight down approximately 50 feet each.  The antenna is VERY effective (amazingly so).  Obviously, the ends are more than 50 feet off the ground (in trees), as is the center.
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