[NLRS] 6m rover antennas

[John P. Toscano]

Over 2 months ago, Jon Platt (W0ZQ) wrote:
> 
> Just playing around with EZNEC before the Vikings game and following
> up on a previous thread.  For the following analysis I used the ...
> 3-ele NBS yagi...
> ...
> 6m 3-ele NBS yagi with boom at 10' above ground :
>   Boom length = 7.8'
>   Max Gain = 13.25 dBi at 23 degrees take off
>   Gain @ 3 degree takeoff angle = 0.09 dBi.

Jon:

Do you (or anyone else on the mailing list) have any experience with
"optimizing" yagi designs?

After giving careful thought to the discussion on this thread, I felt
it might be useful to design a lightweight 4-element beam of about
12 feet in boom length, and mount it about 20 feet above the ground.
I figured I would use fiberglass tube for the boom, two pieces of
around 6-8 feet length, to break down the antenna into two parts for
travel.  Each part could lay on the roof of my van sideways (because
the elements are too wide for the highway, just about 10 feet for the
reflector, if mounted in the direction that the vehicle is traveling).

When I get to a site to operate from, I would have a mast extension
with a 4-foot fiberglass rod on its end at a 90 degree angle, so that
it would extend 2 feet forwards and backwards when the extension is
slipped into the existing mast.  The solid rod would be just the right
OD to fit nicely into the open ends of the two tubular boom halves.
They would slip over the ends and get "pinned" into place.  Then the
whole assembly would get inserted into the top of my existing mast, so
that the 6M antenna would be at least 8 feet above the top of the
existing assembly.  Since the existing assembly's top is 12 feet above
the ground, the 6M antenna would end up at 20 feet or so.  In order for
me to erect it myself, it needs to be a lot lighter than the 6M5X
monster I brought out last time.  (I did mount that on the mast all
by myself last time, but it would be way too heavy and too awkward to
raise and lower once per grid stop for the whole contest.)

I just started playing around with EZNEC, starting with a design in
the "ARRL Yagi Antenna Classics" book.  I designed a 4-element yagi,
on a 12-foot boom, using thin rods (0.188") for elements to minimize
weight and wind load, figuring that would mainly affect bandwidth, and
for a rover in a contest, 50.125 - 50.250 MHz would be plenty of
bandwidth.  My first attempts seemed to have acceptable patterns but
horrible SWR.  After awhile, I had a design that looked pretty good to
me:  14.11 dBi at 13 degrees takeoff, 12.58 dB F/B, and SWR of 1.01 to
1.23 over the range of 50.0 - 51.0 MHz.  Actually, SWR was pretty flat
and low even beyond those frequencies.  Again, this was done with the
antenna at 20 feet over ground.

Then I compared my results to yours for the 3-element NBS antenna, and
found it wasn't as good.  At 10 feet above ground, the Max Gain fell
from 13.25 dBi to 12.83 dBi (at 23-24 degrees of takeoff), and fell
from 0.09 to -0.72 dBi at a 3 degree takeoff angle.  Slightly LESS gain
than the NBS design in spite of being about 50% LONGER.  Seems like
there is room for significant improvement here.

I am assuming that for modeling purposes, I model the antenna above the
earth, rather than above the roof of my van.  Maybe that's a bad
assumption on my part, and if so, then I'd only be at 14 feet above the
roof, and in even worse shape.

I did use an Excel spreadsheet to simplify the calculation of the
(x1, y1, z1), (x2, y2, z2) coordinates needed for each element's ends,
but even so it was pretty tedious to try out various little changes to
try to optimize this thing.

Any thoughts would be appreciated.

John (W0JT)
(still undecided about rover vs. home station in January contest)