[Towertalk] Re: [Towertalk] 20M Yagi ?

[i4jmy@iol.it]

Mike,

What is important to keep in mind is that the antenna near field has 
little to do with pattern that's instead a far field, the combination 
of fields but at a certain (noticeable) distance from the antenna.

At your club station, the first ground, that one below the antenna is 
close and it's effect will be primary to modify the dipole impedance 
(it will be low) and to enforce radiation straight up being the ground 
reflection in phase with that of the antenna itself.

Actually, beeing the dipole centere only at 45', the energy lost in 
losses won't be too small although a noticeable energy will be still 
shooted vertically and the out-of-phase reflection at low angles that 
would cancel the low angle radiation won't occurr in the far field.

In summary, one can wait the dipole will behave like a low dipole with 
a peak radiation straight up (minus losses) but will also show a lower 
angle lobe, with a plus or minus 45 deg aperture, with a noticeable 
gain toward the cliff edge.

I've quickly made a rough model with AO6 and two grounds, one 
cilindrical but extending to infinite 500 ft below, and another of 250 
ft radius 45 ft below the antenna, but the antenna is not in the 
diameter beeing instead shifted ahead to be closer the cliff edge.
Imagine the antenna installed over something like a mini Mount 
Rushmore, the dipole beeing shifted toward the cliff edge.

Results seems to confirm what one waits for with a low and quite narrow 
lobe (in the E plane) that peaks its gain at about 15 degrees elevation.

73,
Mauri I4JMY

Inverted V dip 
over ground
2 zones
5 2 0 250
5 2 -500
1.84 MHz
2 copper wires, feet

h = 45
l =123

1     200    0     h    200    l   10      #18
1     200    0     h    200   -l   10      #18
   
1 source
wire 1, end1

Peak gain at 0° azimut @ 14° elevation
Peak gain at 90° & 180°azimut 90° elevation 


> Mauri,
> 
> How about the case where the ground immediately under the antenna
> is very close, but there is also a  nearby cliff? Ray tracing to the p
oint
> of
> distant reflection suggests this antenna will produce a very low angle
> of radiation. On the other hand, the electrical field induced in the
> ground immediately under the antenna will be out-of-phase with the
> electrical field from the antenna suggesting that the antenna will
> have a null towards the horizon.
> 
> A concrete example of this sort of antenna is the local club station.
> They have a 160 meter inverted-vee mounted on a 40' telephone pole
> close to the edge of a ~500' mesa. Looking in the direction of the 500
'
> dropoff, what is the effective height of this antenna? Is it 40' or 50
0'?
> 
> Anyone know how to properly simulate the pattern of a dipole in
> this configuration?
> 
> 73 de Mike, W4EF....................................