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Author Topic: Radials  (Read 359 times)
N2CST
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« on: January 24, 2009, 06:14:04 AM »

After countless hours of reading articles about radials on ground mounted trapped verticals, I am still in the dark regarding their intended purpose. Some articles say they are to decouple the antenna from ground, and others say they are to increase ground conductivity. These seem to me opposite in intent, if I am interpreting the authors correctly. To add to the confusion even more, some say they should be cut to ¼ wave length, and others say any length is ok. If increasing ground conductivity is the goal, wouldn’t several ground rods be just as effective?  Any advice would be appreciated.  TU es 73 de N2CST
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AA4PB
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« Reply #1 on: January 24, 2009, 06:29:15 AM »

You are being confused by confusing terms. Radials on or in the ground are one thing, radials suspended above ground are another.

In/on the ground, radials are not resonant and serve to improve the ground conductivity. Suspended in the air, radials need to be resonant (1/4 wavelength long) in order to provide a low impedance termination at the antenna feed point.

In either case they also serve to decouple the feed line by reducing the amount of current on the outside of the coax shield. Think of parallel resistors. The current divides among the resistors in proportion to their resistances. The shield is in parallel with the radials. The lower the impedance of the radials, more of the current flows in the radials and less in the shield.
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KC8VWM
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« Reply #2 on: January 24, 2009, 06:43:01 AM »

I am still in the dark regarding their intended purpose.

-----------

Vertical antenas function on the basic priciples as outlined in the "Marconi Antenna" theory.

See:

http://www.hnsa.org/doc/radio/chap20.htm

73







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AA4PB
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« Reply #3 on: January 24, 2009, 07:12:16 AM »

If increasing ground conductivity is the goal, wouldn’t several ground rods be just as effective?
------------------------------------------------------
No. The idea is that dirt is not a good conductor therefore it is very lossy. Ground rods make a good connection to a poor conductor so you still end up with a lot of loss.

Covering the area below the antenna with wire radials permits RF currents to flow in the wire instead of the dirt, thereby improving the conductivity of the return path to the antenna. In general, the area covered by the radials (on or in the soil) needs to extend in all directions from the base of the antenna about as far out as the antenna is high. The more wire you put in this area, the better the conductivity and the less loss in the ground system.

As you add more and more radials you reach a point of diminishing returns for the effort, generally thought to be around 50-60 radials. If you have one radial and you add one more then you have doubled the number. If you have 59 and you add one more you haven't made that much of a difference.
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K3GM
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« Reply #4 on: January 24, 2009, 07:33:45 AM »

If you can picture return currents flowing back towards your antenna, think of radials as "gatherers" of those currents.  For ground mounted verticals, they act as helpers for the surrounding soil on or in which they sit.  Unlike ground rods, radials reach out into the near field of the antenna and provide a path for the return currents to flow.
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W7ETA
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« Reply #5 on: January 24, 2009, 09:23:03 AM »

W4RNL site has good info on not only radials but also top hats.

73
Bob
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W4VR
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« Reply #6 on: January 24, 2009, 01:58:03 PM »

Assuming you have the room and supporting structure(s) why don't you put up a horizontal dipole, then you won't to worry about how or why radials work!
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G0GQK
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« Reply #7 on: January 24, 2009, 02:55:26 PM »

Or better still, buy a vertical which doesn't require ground radials because the signals have "longer legs" than a dipole.

G0GQK
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W8JI
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« Reply #8 on: January 24, 2009, 03:06:39 PM »

<<Some articles say they are to decouple the antenna from ground, and others say they are to increase ground conductivity. These seem to me opposite in intent, if I am interpreting the authors correctly. >>

If by ground conductivity they mean the conductivity of the soil, it won't change the soil itself. The soil is what it is.

Radials provide a much lower resistance path for any earth currents and they also "shield" the earth from strong fields surrounding the antenna.

In an end-fed antenna like a base fed vertical, they give the antenna something to "push against". Otherwise we could never get current up into the antenna.

<<To add to the confusion even more, some say they should be cut to ¼ wave length, and others say any length is ok. If increasing ground conductivity is the goal, wouldn’t several ground rods be just as effective?>>

There is no minimum length where they suddenly are effective. The longer you make them the better, until they are close to 1/4 wave long. Beyond that you won't see much change. There is also a diminishing return on the number of radials. At about 25 to 50 when they are close to 1/4 wave long, you pretty much have all you will ever get.

Some of your confusion might come from elevated resonant radials, which generally are "tuned" as an odd quarter wave in order to make the feedpoint impedance reasonable. This is so the antenna pushes against a good "low impedance", and the system remains resonant. If the antenna only has a small number of elevated radials, the radial's length will seriously affect the antenna resonant frequency.

73 Tom
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N2CST
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Posts: 15




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« Reply #9 on: January 24, 2009, 05:22:26 PM »

Gentlemen,

Thank you all very much for your help. You were all very helpful and I appreciate it very much. What a great bunch of people. That's why this is a great hobby.

73   N2CST
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WB2WIK
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Posts: 20611




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« Reply #10 on: January 25, 2009, 11:50:45 AM »

Another great truth relating to radials and almost everything else:

Hams have been experimenting with this stuff for about 100 years now.  Many of them have been great scientists, engineers, Ph.Ds.

If "doing it another way" from the norm actually *worked,* we'd all know about it by now and we'd all be doing what worked better.

When you see everybody using lots of radials, it's not because they had a lot of wire laying around.

:-)

73 & welcome!

WB2WIK/6
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