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.
--
Wim
PA3DJS
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