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Author Topic: Long wire with counterpoise??  (Read 2079 times)
KJ6TSX
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Posts: 116




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« on: March 06, 2012, 08:01:49 PM »

I just installed a long wire antenna on the top of my fence, just laid the wire on top of the fence around my backyard in a "U" shape around the property approximately 140 feet long about 6 feet high. It receives well but doesn't get out very well. I realize if it was a 1/4 wave high it would work much better <GRIN> anyway my question is would adding a parallel wire below the long wire maybe 5 feet below connected to the ground on the balun improve my transmission?
Thanks
George
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WB6BYU
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Posts: 13244




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« Reply #1 on: March 06, 2012, 09:18:14 PM »

You certainly want some sort of ground system connected to the other terminal of your
feed system, otherwise it is like only plugging one side of the AC cord into an outlet -
it doesn't work very well.

But running it closely parallel to the antenna itself is not a good approach, as the
radiation from the two wires tends to cancel.  If you made the wire the same length as
the antenna you'd just have a long open transmission line that wouldn't radiate much
at all.

Better to run one (or more) wires off in other directions.


Quote from: George Salet

... connected to the ground on the balun...



"Ground on the balun"Huh??

How are you trying to feed it? 

You certainly don't want a balun since it isn't a balanced antenna.
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KJ6TSX
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Posts: 116




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« Reply #2 on: March 06, 2012, 09:38:38 PM »

The Positive side of the balun is connected to the long wire and the negative is to a ground rod, the balun is a 1:1  the coax runs about 50 feet into my shack. I didn't want a transmission line at ground level in the front yard kids might chew on it??
I was also thinking of center feeding the wire, truth is I had more copper wire than coax so I went with what I had Roll Eyes
always open to suggestions the more dumb questions I ask the more I learn Grin
George
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W0BTU
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« Reply #3 on: March 07, 2012, 07:24:02 AM »

Even a few short radials stapled to the earth will be a lot better than your single ground rod! Here's a couple of reasons (there are more):

1. The RF does not travel down the ground rod very far
2. The top portion of a ground rod does not make good contact with the earth until after it has been installed for a long time.

Put some ground radials down (at least a half-dozen), and connect them to the ground rod. You'll be glad you did. Insulated copper wire works just fine. When the grass grows this spring, they'll disappear.

You mention a counterpoise, which are elevated, resonant radials. You can use fewer of those than ground radials. I've used both methods.

http://www.w0btu.com/Optimum_number_of_ground_radials_vs_radial_length.html
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K4SAV
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Posts: 1840




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« Reply #4 on: March 07, 2012, 08:46:12 AM »

A 140 ft wire 6 ft off the ground isn't going to resonate on any ham bands, and the feedpoint impedance will be high on the ham bands.  A 4 to 1 balun might actually reduce the feedline loss some.  Balun loss might be high however.  A one to one balun won't make any difference other than reduce the feedline common mode currents, assuming it's a current mode balun (a choke).  If the antenna was resonant on the bands, radials might help.  That single ground rod probably adds about 50 ohms loss right at the feedpoint.  However since the feedpoint impedance is very high, that ground rod probably won't be the major source of loss.  The low height will be the major source of loss.  The only solution to that is to increase the height of the antenna.

If you add another wire below the top wire and connect that to ground, that will serve to cancel most of the fields from the top wire.  The antenna will then function mostly as a vertical.  That vertical is the little 5 ft wire at the feed point. 

Jerry, K4SAV
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WB6BYU
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« Reply #5 on: March 07, 2012, 10:33:29 AM »

If you need an impedance step-up, use an Un-Un rather than a Bal-Un.  That's because
both the feedline and the antenna are Un-balanced.

If you use a common 4 : 1 voltage balun in this application it forces half the RF
voltage to appear on the outside of the coax.  You really don't want that.

The step-up transformer (Un-Un) might or might not improve the feedline losses,
depending on the band, coax losses, etc.  Even with it, feedline losses may still
be rather high on some bands.  The best method is to put a tuner at the antenna
feedpoint rather than in the shack:  my typical approach would be to build a box
with a switch (or relays) to select different networks that are preset to match
each band.  Takes a bit more time to set up, but in the end is cheaper than an
auto-tuner.
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KJ6TSX
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Posts: 116




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« Reply #6 on: March 07, 2012, 09:47:45 PM »

I was thinkin about this last night and i can extend the ground side up to the roof and probably pick up a 100 feet, this would give me a center-fed antenna with 100 feet on the roof and 140 feet on the fence. I would also remove the ground rod, do you think it would be wise to wrap 40 feet of wire around a piece of plastic pipe to make it balanced or is that mental masturbation  Roll Eyes
Thanks
George
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WB6BYU
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Posts: 13244




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« Reply #7 on: March 07, 2012, 09:56:03 PM »

Winding up wire isn't the same as running it out straight.

You could try bending the wire around the house to add some length, or shortening
the wire along the fence to make them roughly equal.  But try it first and see how
it works.  If the wire over the house has enough height above ground it may work
better than the wire strung around the fence.
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