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Author Topic: buried ground radial length?  (Read 2978 times)
W5WSS
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Posts: 1652




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« Reply #15 on: February 15, 2013, 07:53:44 AM »

This is all music to my ears! If at all possible Place the insulated radials used with a ground mounted system on top the surface of your ground medium.

Let the grass grow over them at most. Remember though that there exists plenty of chemical actions that reduce the longevity of yes even copper buried in various materials, insulation helps forstall this fact provided nicks are not allowing intrusion.

Periodically Inspect them for corrosion at the antenna base ring and use crimped and soldered ring terminals designed for this outdoor service.

Do not Earth the radial ends rather leave them isolated there.

Ideally Use a short length tinned copper strip from the ring to the actual antenna base shield driving point.
Or a flexible braid for if you plan to pivot the vertical.
Ideally the tinned copper strip avoids wicking as a braided strap draws in the water etc. and corrodes.
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K9MRD
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Posts: 331




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« Reply #16 on: February 15, 2013, 10:33:12 AM »

Just to add a little more confusion....

From the November/December, 2012 QEX, Tall Vertical Arrays by Al Christman, K3LC.

It was noted that both radial length and monopole height effect take-off angle and gain.

1/4 radials (60) and 1/4 monopole = 24.9 take-off and .39 gain
5/8 radials (60) and 1/4 monopole = 26.3 take-off and 1.0 gain
1/4 radials (60) and 5/8 monopole = 15.2 take-off and .73 gain
5/8 radials (60) and 5/8 monopole = 16.5 take-off and 1.0 gain

This was an EZNEC analysis and there is much more info regarding phased arrays, 4-square, etc.

Interesting....

Wayne
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RFRY
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« Reply #17 on: February 15, 2013, 11:28:25 AM »

Most likely this QEX study considered only the NEC far-field pattern at an infinite distance over an imperfectly conducting, flat ground plane.  Of course, this is not reality.

The radiation actually launched by all monopoles of 1/2 wavelength and less always is maximum in the horizontal plane, and less than that at all elevation angles above the horizontal plane -- regardless of the number and length of buried radials and the conductivity of the earth in which they are buried.

The link below leads to a more detailed analysis incorporating the NEC4.2 near-field calculation, which shows the elevation plane radiated fields in uV/m at a horizontal distance of 0.1 km from a 1/4-wave monopole, for three different path conductivities (other things equal).

If "takeoff angle" refers to the angle where the peak field launched by a monopole is highest, then for monopoles of 5/8 wavelength and less the takeoff angle always is zero degrees.

http://i62.photobucket.com/albums/h85/rfry-100/Monopole_Low_Angle_Radiation.jpg
« Last Edit: February 15, 2013, 11:31:12 AM by RFRY » Logged
RFRY
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« Reply #18 on: February 15, 2013, 03:45:14 PM »

CORRECTION

Earlier in this thread I posted: "The link below leads to a more detailed analysis incorporating the NEC4.2 near-field calculation, which shows the elevation plane radiated fields in uV/m..."

The fields shown in the link are stated in units of V/m, not uV/m. Sorry for the error.

Also note that the groundwave field shown for sea water conductivity is very close to the maximum groundwave field possible for 100W radiated by this system at that distance when driven against a perfect ground plane.

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KK5J
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« Reply #19 on: February 15, 2013, 04:54:20 PM »

Mr. Fry:

I would like to take this opportunity to thank you for taking the time to post your comments. I find them interesting and insightful and have often found myself going back to Brown etal, Wait, and others to look for answers to even more questions after reading your posts.

My regards and respect
Bob
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RFRY
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« Reply #20 on: February 15, 2013, 05:25:42 PM »

I would like to take this opportunity to thank you for taking the time to post your comments. etc

Many thanks for saying so, in public.
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W5XJ
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« Reply #21 on: February 26, 2013, 08:04:52 PM »

I like 1/4 wave length of lowest band and i'd do at least 32 if possible, bend them if needed but don't overlap them. there is a great article on this in the ARRL archives that is worth a read.

73 & good DX de W5XJ
EM12ou
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