[NLRS] Rover antenna spacing

[jcplatt1 at mmm.com]

Hello all.

I think Ken's comments about trying to stay out of the near field with
respect to detuning is right on.    Yet another consideration is
interaction with the car' roof AND interaction with the ground.

I did some very quick playing in EZNEC with a simple four element yagi on
432.   I placed this antenna over average ground (high accuracy model) at
heights that varied from one foot to 10 feet.    The peak gain did not
start to significantly degrade until I was within about two feet of the
ground when it dropped about -1 dB.  At one foot above ground the peak gain
had dropped -3 dB.   HOWEVER, what was more significant is the change to
the peak gain's take off angle.   Even at 432, ground effect with respect
to take off angle, that same monster rovers fight on 6m, is a factor.   I
looked at the gain at a fixed two degree take off angle for this same
model.   Using the antenna at 10' above ground as the reference point (and
looking at the gain at 2 degrees), this same antenna was down -5 dB at 5',
-9 dB at 3', -13 dB at 2', and -18 dB at 1' above ground.    So although
you can get pretty close to "ground" without significant detuning, the low
angle gain drops dramatically due to ground interaction.   Rovers with
antennas close or very close to the ground suffer more from loss of gain at
low take off angles then they may due to detuning.

However, rovers mount their antennas on the roof of their car.    My second
pass through the model used the same antenna, but I added a metal radial
roof 5' up from the average ground with a 5' radius (this was a simple,
quick model ..... few cars are shaped like a flying saucer !).     I then
reran the model with the same heights where those heights are now above the
car roof ..... the advantage is the extra 5' that the car roof adds to the
overall height above true ground.   However, the question is how does the
metal roof of the car interact.      What a difference !    With respect to
peak gain, there was no difference with the 432 yagi 10' above the roof or
one foot above the roof.    The gain at the two degree take off angle was
proportional to the antennas true height above ground, not the height above
the roof.   As such, the extra height added by the roof is a big help at
the low take off angles.

Some conclusions may be:
1.   At a minimum, stay away from the roof per Ken's note to avoid near
field detuning.
2.   After that, the roof does not act like the real ground for low take
off angles.   The roof is not ground ..... ground is ground.
3.   The height added by placing antennas on the roof is complementary
..... just like adding height to a tower or mast on a non-rover setup.

On the "low bands", height above ground is important.   Parking that rover
on the slight hill top where the land drops away is a good idea.   For the
"low bands", getting enough height (for a rover) may be more important than
actual antenna gain .... a small well designed yagi at 10' may perform
better than a long boom yagi at 5'.    On the higher bands, the
UHF/microwave bands, this may change ..... it would be interesting to see
where the cross over frequency may be - perhaps 1296 or 2304 ?    Stuff for
future fun !

73, Jon
W0ZQ