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Author Topic: Vertical HF Antenna Radial Field  (Read 496 times)
VE6JJO
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Posts: 26




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« on: October 27, 2009, 11:30:28 AM »

Any thoughts on the effect that having only half of a 360 degree radial field would have on the effectiveness/directionality of a vertical HF antenna?  For example having the antenna close up against my property line, I can put as many radials as I want on my side obviously but not on the neighbours...so a 180 degree radial field.  What effect would this have?
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WA3SKN
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Posts: 5453




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« Reply #1 on: October 27, 2009, 12:10:15 PM »

The pattern would be best in the direction of the radial field. It will load.
Sometimes you have to compromise.
73s.
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N1OU
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Posts: 70




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« Reply #2 on: October 27, 2009, 01:29:44 PM »

WA3SKN is right, the transmit pattern will favor the radial field.  However, if you want to avoid this AND have a wee bit of room, you can bend radials up to 90 degrees (make them L-shaped).  Thus, with some creativity, you might manage 360 degrees of radials.  I did this once on a small lot and never noticed any directional effect.

73
Gordon, N1OU
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KB4QAA
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Posts: 2327




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« Reply #3 on: October 27, 2009, 03:56:55 PM »

Any radials are better than NO radials.

Do what you can.

b.
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W6OP
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Posts: 338




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« Reply #4 on: October 27, 2009, 04:47:46 PM »

Look at the articles at http://www.antennasbyn6lf.com/design_of_radial_ground_systems/

N6LF has done a lot of actual testing of different radial systems. Once you have a decent number of radials the difference in directivity with 180 degree coverage is in fractions of a db.

Pete W6OP
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AB7E
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Posts: 117




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« Reply #5 on: October 27, 2009, 05:56:09 PM »

W6OP:  "N6LF has done a lot of actual testing of different radial systems. Once you have a decent number of radials the difference in directivity with 180 degree coverage is in fractions of a db. "

That's not totally true.  N6LF recently performed extensive tests with radials trying to simulate the case where it might only be possible to have radials with 180 degree coverage, such as if a vertical was located near a building or edge of a lot.  He found several db of reduced signal strength in the direction of the missing raidals, and if I remember correctly even a db or so reduction in the direction with the radials.  I think his article is due to be published in the next edition of QEX or something like that.

Of course, suffering a few db disadvantage in any direction is preferable to not putting up the antenna at all.

73,
Dave   AB7E
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K0ZN
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Posts: 1543




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« Reply #6 on: October 27, 2009, 06:48:16 PM »

Hi...

This is a HOBBY...  Unless you can go out an purchase a large piece of open real estate for your antenna farm, you do the best with what you have. Such being the case, you put down as many radials in whatever direction you can (up to some reasonable number).

If you can run even a few radials OR tied into some large metal in some manner in the other direction opposite your radial field, it will always help.

I was in the U.S.Army Signal Corp "back in the day" and in our Signal Company, we had a long time, seasoned, old First Sergeant who had "been there and done that" with various military radio systems and equipment... he had a great quote I have never forgotten: "You CAN'T have too much ground!!"  Good advise...you just do the best you can given the situation and real estate.

73,  K0ZN
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W6OP
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Posts: 338




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« Reply #7 on: October 27, 2009, 07:41:57 PM »

AB7E - You may be more correct than I was. I was going by memory of some of his articles. I do see in Download antenna_ground_system_experiment_3.pdf on his web site that for elevated radials he shows (if I read it right) about 3 db loss over 4 radials but does not indicate any directivity. I was sure I read somewhere where a good 180 degree radial field showed very little difference. If I can find it I'll reply here with the reference.

Though as you and K0ZN said, it's better than no antenna and while I would love to add 3 db gain to my verticals I live with what I have.

Looking forward to Rudy's next article in QEX.

Pete W6OP
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KA3DNR
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Posts: 74




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« Reply #8 on: October 27, 2009, 08:23:26 PM »

Look at the articles at http://www.antennasbyn6lf.com/design_of_radial_ground_systems/

N6LF has done a lot of actual testing of different radial systems. Once you have a decent number of radials the difference in directivity with 180 degree coverage is in fractions of a db.

Pete W6OP

Thank you so much Pete, the radial presentation was excellent, and aligns with my work on ground vs elevated radial systems... It is so much easier to elevate your vertical, even up 2-4' and use 4-8 radials, than it is to use 64+ radials at ground level for a ground mounted vertical.

Great stuff! Love it.'

Marc
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W8JI
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« Reply #9 on: October 28, 2009, 05:02:01 AM »

Why would you need 64 radials?

When we measured FS here it flattened off at about 15-20 buried radials on 40 meters.

Somewhere around ten buried radials were equal to about 4 elevated radials, which were all within a dB of a large system.

People make too much about nothing. Use as many radials as you reasonably can and get them out of the way so you and the radials live a long happy life.

If you use a small ground system be prepared for common mode current on the feedline.

Tom
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W9OY
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« Reply #10 on: October 29, 2009, 03:39:21 AM »

I totally agree with Tom

I don't know about Rudy's report, but I don't see how radials over 180 deg vs 360 deg are going to change the pattern of a vertical much, and certainly not by many dB.  Radiation occurs due to acceleration of electrons UP THE POLE.  The radials are there to provide an efficient means of collecting returning current to the other side of the coax.  That is their job.  The pattern of the antenna is established many wavelengths (like ten) away from the antenna.  

On 40 that means the pattern is finally formed 1/4 mile from the antenna and by then the radials look like a little tiny spec on the horizon

If you have an element like an inv-L I can see how that might change things a little as far as pattern but a vertical pole is a vertical pole, and that is the thing doing the radiating.  The radials merely improve the efficiency.

Tom's point about common mode is a good one.
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WX7G
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Posts: 5955




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« Reply #11 on: October 29, 2009, 04:53:05 AM »

Radials, as with the vertical wire we call the vertical, can radiate. Acceleration of charge thru a wire, as W9OY says.

When opposing radials have the same current magnitude and phase, the radial magnetic fields largely cancel and there is little radiation. Use but one radial and that radial radiates just fine. Install radials only in a 180 degree arc and the antenna exhibits an asymmetrical radiation pattern.

A NEC-2 simulation of a 7 MHz 1/4 wavelength vertical over 'average GND' and having 6 radials (1' above GND) in a 180 deg arc shows a 2 dB pattern asymmetry at a take-off-angle of 10 degrees.

With half of the GND return current taking a path through the earth the radiation efficiency suffers. Calculations show approximately a 1 dB decrease in signal.

For this antenna the far field can be considered to be 1 wavelength away.
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W9OY
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« Reply #12 on: October 29, 2009, 08:23:09 PM »

My guess in reality the asymetry is even less if you use more like 20 on the ground or buried radials in a 180 deg arc.  Once you are well coupled into the ground you're not going to get much radiation from the radials.  The lack of common mode on the coax is a reasonable indication of how well you are coupled into the ground.  

It is probably parts of a dB not several dB

The usual NEC is not really a good tool to model something like this.  So I would take any pattern variations from that program with a grain of salt.  I think there is a special version of NEC that handles something like this better  Even making comparative field strength measurements would be tedious.  You would almost have to set out the 360 deg radial field in 3 parts one side with say 10 radials and the other with 20 that is split into 10 and 10 then measure a 20 radial 360 deg pattern and then a 20 radial 180 deg pattern

73 W9OY
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