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## Re: [TowerTalk] Minimum Elevated Vertical Height ?

 To: , Re: [TowerTalk] Minimum Elevated Vertical Height ? "Tom Rauch" Tom Rauch Wed, 11 Aug 2004 15:04:00 -0400
 ```> So, what is your experience in how high is needed before the loss reaches > low level? Doug, There are two loss mechanisms with elevated radials. The first is a "push against" loss. For every ampere flowing up into the antenna an exactly equal current must flow into the ground system. (something about this is at http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm and other places) The second loss relates to the antenna's fields moving charges in the lossy media around the antenna. In the Bobtail with the current up high, field density is lower than normal. You have a strong electric field between the high voltage hanging ends and earth below and around the ends. With a high impedance feed, current is fairly low at the feedpoint. This means "push against" or conduction losses will be low even with a poor ground. The major loss (other than Fresnel zone losses) would be from the HV antenna ends near earth. A small ground screen or a large number of short radials on or just above the earth below the wire ends (even those not fed) would do the most to reduce these losses. The wires simply serve to provide a low resistance path for displacement currents at the most concentrated points. If you have four elevated radials, the answer in the case of a normal Marconi antenna is 1/8th to 1/2 wl above earth. Eight elevated radials at 1/4 wl are about equal to a full system on the earth, four would be a little worse. It's even a good idea with dipoles (inverted Vee dipoles) to keep HV wire ends away from earth. By the way, my mobile antenna beats many home-station vertical antennas. I am happy with it and make many DX contacts including come one call SSB contacts into Europe and the Middle East, and it is about 10-20% efficient on 80 meters. 73 Tom _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread [TowerTalk] Minimum Elevated Vertical Height ?, douglas . snowden Re: [TowerTalk] Minimum Elevated Vertical Height ?, Paul Playford Re: [TowerTalk] Minimum Elevated Vertical Height ?, Tom Rauch <= RE: [TowerTalk] Minimum Elevated Vertical Height ?, Chuck Dietz Re: [TowerTalk] Minimum Elevated Vertical Height ?, Paul Playford