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Author Topic: antenna help2  (Read 1083 times)
VK4TJF
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Posts: 94




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« on: January 29, 2013, 03:31:09 AM »

o.k. here is what I would like a vertical radiator made of wire that is 10 meters long and resonate on the 40 meter band
and only needs say 2 radials to be effective. it will be feed at the bottom. and be ground mounted.  any suggestions how
basically how to make a 1/4 wave ground mounted vertical effective with only 2 radials?
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K5LXP
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« Reply #1 on: January 29, 2013, 05:37:06 AM »

Define "effective".


Mark K5LXP
Albuquerque, NM
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W5DXP
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« Reply #2 on: January 29, 2013, 05:39:09 AM »

I guess it depends on your definition of "effective". IMO on the average, for effective performance, one has to choose between two relatively high elevated radials or maybe 8 ground mounted radials. Of course, a horizontal dipole doesn't require any radials.Smiley
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
AA4PB
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« Reply #3 on: January 29, 2013, 05:54:48 AM »

There is nothing you can do to a 1/4 wave vertical to make it more efficient using two radials. Given your soil conditions, the efficiency of two radials is what it is. The way you reduce ground losses is to add more radials. Basically every time you double the number of radials you cut the loss in half.
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VA3GUY
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« Reply #4 on: January 29, 2013, 02:11:49 PM »

My question is why only 2 radials?  I had a ground mounted vertical and because of my circumstances, I could only lay down radials in a 180 degree pattern.  I did what I could in the space I had and ended up with 13 radials, all of assorted lengths...ranging from 1.5' (.45m) to 27' (8.23m) and it worked great.  Generally, if I could hear them, I could work them...even with the convoluted radial pattern!
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WB6BYU
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« Reply #5 on: January 29, 2013, 09:07:32 PM »

Quote from: VK4TJF

...any suggestions how basically how to make a 1/4 wave ground mounted vertical effective with only 2 radials?



I ran a number of models.  My program works best when the wires are elevated, so
I placed them about 8cm above ground.  I'm not sure just how much difference there
will be between that and laying on the ground:  putting them up about 30cm high
improved the signal by nearly 1dB.  Soil conditions were "good".

I tried a center loading coil, and two off-center coils, equal or not, as well as a 2-wire
capacity hat, all of which resulted in a high feedpoint impedance.  About the best I was
able to get was a 1dB improvement in radiated power at a 15 degree vertical angle, and
I was being perhaps a bit optimistic about the coil losses to do that.  It's not clear that
you would actually get much improvement over the quarter wave wire once you include
possible losses in the matching network:  certainly the quarter wave wire is easier to
feed.

My recommendation is that you start with a quarter wave wire and see how it works with
the best radial system you can manage (which might mean elevating them along a fence
if you can.)  Then you can experiment with other loading options to see how much
difference it actually makes:  find a nearby ham to take measurements, or build a simple
Field Strength Meter and mount it in a fixed location, then check to see how much it
changes when you try different antennas.

If you want to try the different designs, I got about the best results using a 35uH loading
coil placed 3/4 of the way up the wire.  Using 20uH there and another 20uH at the 1/4 point
gave similar results.  These all have a high input impedance, so will need a tuner at the
feedpoint.  I'd suggest about 6uH and 80pf as an "L" network, adjusted as necessary
for best SWR.
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N4JTE
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« Reply #6 on: January 31, 2013, 09:07:21 AM »

If your stuck with ground mounting, I would raise the radials by tying them off a few feet up and away frome the vertical wire with a piece of string, like a gullwing, and pull out straight from there. Make them 180 degrees, opposite, from each other.
The radials can be lengthened to achieve around a 1.4 to one swr by careful measurement. Be advised the radials are gonna be high voltage at the ends, so tie off with insulator to non conductive support.
Lastly, put a 1 to 1 current balun at the feedpoint and try to keep feedline from running parallel under either radial.
Good luck,
Bob
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