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Author Topic: vertical antenna counterpoise  (Read 3356 times)
KC5YNP
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Posts: 1




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« on: June 25, 2001, 08:58:41 PM »

I'm going on vacation soon and want to take along my 10 meter mobile to use as an inexpensive low power base station.  I plan to use a retuned mag mount CB antenna.  For a counterpoise I'm thinking of using a piece of plywood with 1/4" wire mesh stapled to it.  My question is, how small can this counterpoise be and still be effective?  Would a 2'x 2' counterpoise be effective?  Tnx and 73!

Brian
KC5YNP
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N6AJR
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« Reply #1 on: June 25, 2001, 09:53:21 PM »

A counter poise, in opposition to ground radial, can just be a piece of wire around a quarter wave length long...for 10 meters, a piece of hookup wire in the 8 foot to 8 foot 6 inch range would be fine. Hook it to the ground on the radio (prefered) or the shield side of the coax on the antenna.  If you make it out of a piece of lamp cord, cut one long and one short( say 8 ' and 8' 6") and it will see it as a big piece of counterpoise, or use a piece of 3 wire rotor cable at 8', 8' 3" and 8' 6" and it wil be an exceptional counterpoise...I love counterpoise's , they are the best way to make an antenna more functional.. Just lay it out reasonably straight to use, or go around the edge of a room with longer ones on lower frequencies (i.e. 33' to 33' 6" or so for fourty meters) just so it doesn't double back on it self to closely.  Try cliping a 19 1/2 inch piece of flexible wire to the ground side of the antenna on your 2 meter HT and see if you don't talk and hear better..but keep it a secret, this is magic stuff..

Bye the way, I know these are not the exact, prefered, figured out with a slide rule, lengths but they work. especially when you do the short and long trick. Also, very important, TAPE THE FAR END OF THE WIRE. There can be some awsome voltages present there, so tape up the wire's end good. Don't get an rf burn from the far end.

If any of you can come up with the appropriate formula and tell this gentle person I'm sure it will be appreciated, I just know the old rule of thumb stuff, 19 1/2" for two meters, 9 3/4" for 220, 5 7/8" for 440 mhz, 8' 3" for ten, 8' 6" for 11 meters ( used to be a ham band), 12' 5" for fifteen, 16' 6" for twenty, 33' 4" for fourty, 67' for 75/80 and 134' for 160 meters. If you use fat wire, you can shorten them a tad, if you use skinny wire then lenghten them a bit.  How's that for technical....

The story goes that the counterpoise works in a fashion similar to a ground radial in the fact that it presents a "return" for the other half of the antenna.  This makes a quarter wave rubber duck on the hand-held into one half of a dipole sort of.. thats how you have to look at the ole' counterpoise...I hope this is of some help.  


tom N6AJR
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N2ICZ
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« Reply #2 on: June 26, 2001, 10:11:26 AM »

I think Tom is right, Brian...  Try using the counterpoise instead of the 2 foot square wire mesh.  You'll be better off.

Here's another thought.  I once used a pizza pie plate (the largest I could find in the supermarket)(non-teflon) I drilled four holes in it, inserted wing nuts and attached four radials for 20 meters.  I placed the 20 meter hamstick in the mag-mount, and placed it in the center of the pizza pie tin.   I placed the whole thing on the ground and worked DX with 30-50 watts.  I tried it with the 40 meter whip later that night, not bothering to increase the size of the radials.  I managed to get out at least 1500 miles.  Nothing to brag about but had some fun...  There was no science involved.  I just thought it would work...  I guess in a sense, I had some ground coupling effect similar to the tripod extension offerred by Alpha Delta and that new ground coupling device offered by MFJ.  My cost though was about $5.95 including the wire!

If the wire radials are a pain for you, them Tom's counterpoise at the station feed point is the way to go...

Good luck and have fun!

73,

Larry-N2ICZ
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N2ICZ
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« Reply #3 on: June 26, 2001, 10:19:00 AM »

Quick question for Tom....

Do you find you have better success with a counterpoise when it is attached to the rig's ground?  Or at the antenna tuner ground?  I ask because I have connected in series, my rig, tuner and artifical ground...  Where to put the counterpoise has always stirred up a discussion Smiley

73,

Larry N2ICZ
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WB6BYU
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« Reply #4 on: June 26, 2001, 02:52:48 PM »

If you are using the counterpoise to replace the ground plane for
a mobile antenna, it should be connected to the shield of the coax
at the antenna end.  (The pizza pan approach does this.)

If you are using it to reduce problems with RF in the shack, then
I would put it on the tuner.  If I still have problems (maybe the cable
from the rig to the tuner is long) I would add another to the rig.

Usually I have only needed a counterpoise when I am using an
end-fed longwire, though I think there were one or two instances
where I was using some weird feed system (coax spliced to twinlead
perhaps) where I used one for one or more bands.  In each case,
I have just approximated a quarter wave of wire and run it along
the base of the wall and it worked fine.  (You can buy a lot of wire
for the cost of a "ground wire tuner"!)
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RobertKoernerExAE7G
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« Reply #5 on: June 28, 2001, 01:53:48 AM »

The length for a single "radial" should be managable for you.  Easier to make than a 2x2 of wire stapled to a board.

At one time, I ran a 10-20ft coaxial cable to the mag mount on my car, and brought the mobile rig into the house on a power supply.

I've made a temp indoor dipole antenna out of zip cord, tied a knot in it and pulled it a part for the dipole portion, and then soldered a PL259 onto the other end. Plugged it into the rig. taped it to the ceiling and adjusted for low VSWR. 100 watts on 20 meters.  An old QST said zip cord was about 75 ohms when used as a feed line.
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KA2QFX
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« Reply #6 on: June 28, 2001, 05:41:32 PM »

Most vertical antennae are simply a 1/4 wave radiator, or odd multiple thereof.  As such they're missing their other half to make a complete antenna. On an automobile, the car body, while not resonant, serves this purpose somewhat poorly when the greatest distance from the antenna to the farthest part of the vehicle is less than a 1/4 wavelength.

Since you are removing the vertical from the vehicle you must provide the other half of the antenna in the form of a radial. Otherwise, the RF will attempt to use the coax braid as its radial element and you'll get burned on the other end.

You could use a counterpoise to alleviate that problem but that still produces a lot of RF at the wrong end of the system. So attach a few 1/4 wave wires to the ground side of the antenna and keep the RF where is should be, at the antenna.  

Remember, all antennas function on the principle of a dipole. That is, two resonant elements, out of phase working against each other. Even rubber duckies on two meters or 440 will see a vast performance increase when a 1/4 wave element is attached to their ground side.  

The radials must be resonant on the desired frequency to be effective. They can be shorter if they're loaded but they must be resonant!  So a 2' x 2' square would be pretty much useless. If you use the pizza plate method you'll need to add shorter radials since you must include the radius of the plate in the measurment. But the wire is all you really need.

You'll probably still get RF back at your operating position unless you do two things.1) Add cable to the feedline and get it as far away as possible and 2) wind a few turns in the coax or slip a ferrite choke on the cable to choke off any RF from coupling to the feedline.

For more info on grounding, counterpoises, etc. check out my web site. http://www.bridgeport.edu/~msedutto/amateur.html

Before you start cutting radials for an inefficient base loaded antenna why don't you just take those two radials and make a dipole. It'll will be far more efficient, and much easier to deal with in general. On ten meters even strung 10 feet off the ground it will be pretty effective. Just an idea.

Best of luck,

Mark
KA2QFX
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