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#31
April 8th 14, 12:24 PM
posted to rec.radio.amateur.antenna
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Radials
On 4/7/2014 8:54 AM, Wimpie wrote:
El 05-04-14 9:46, Ian Jackson escribió: In message , writes On Friday, April 4, 2014 9:16:01 PM UTC-5, Fred McKenzie wrote: 3. Ground is relatively flat. Drooping radials to approximate a sleeve dipole is stretching the definition of a ground plane! The best modeled version of the 5/8 with 5/8 radials scheme, I consider more of a dual 5/8 collinear than a 5/8 ground plane. You don't get the full 5.1 dbi free space gain of the straight collinear, but you get fairly close. But.. I don't really care about the name.. Just as long as they work.  I understand that my 2m 5/8 mobile antenna (on a magmount) is essentially electrically a 6/8 (ie a 3/4 wave - hence a good match). The actual whip is around 5/8, and the other 1/8 is the 3-turn spring steel 'loading' coil at the bottom end. Again my understanding is that a 5/8 gives the maximum broadside gain (a tiddly bit more oomph than a 1/2 wave), and if you make the antenna longer, the predominant broadside lobe collapses, and most of radiation moves to the higher angle lobe. The "theoretical" more gain for a 5/8 lambda radiator over a half wave dipole is only valid when towards the direction of reception, you have a fully constructive image antenna in the ground. To fullfill this: 1# The elevation angle needs to be well above the (pseudo) brewster angle, to make sure that the ground reflection is strong and more or less in phase. 2# The ground plane needs to cover at least about the first Fresnel zone as seen from the negative image antenna towards the reciever. When you look to real far field patterns of 5/8 lambda antennas as used in AM broadcast over land, the pattern only matches the theoretical pattern for elevation angle say above 5..10 degr (depending on soil type). For lower elevation, you are below the brewster angle and then the effect of the negative image becomes more destructive with decreasing elevation, hence decreasing gain. For HF, VHF and higher, the ground plane will never meet 2#. To meet this, the ground plane (metallic plane) needs to extend from the transmitter towards the receiver. You can't count mother earth as constructive ground as that serves as a destructive image as the elevation angle is well below the (pseudo) Brewster angle. For mobile LOS use, the advantage of the 5/8 lambda is not in the gain, but in its heigth, as propagation in a mobile path is roughly spoken proportional to h^2. So when the current center goes from 2.5 m (quarter wave on a car, 2 m band) to 3.3 m (5/8 lambda on same car), you win 2.4 dB. For a base station where the ground provision (for example 4 quarter wave radials) is already many lambda above mother earth, the additional gain due to the increased length is mininmal. Excellent information, Wim. Thanks. John, KD5YI |
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