I thought I would share my recent experiences with rover antennas to
use while in motion.
My first 6 roves, through the June 08 VHF contest, I did not operate
while in motion. I went to good spot, stopped erected good antennas
and operated. A point and shoot rover if you will. When the June
contest was over, I realized that I needed to operate while in motion.
I had spent as much time traveling from one site to another as I had
operating. I had missed out on openings while traveling. I had to pull
up stakes while the band was still open at one site to make it to
another site. It almost made me cry. I figure that I could have made
at least 50% more contacts if I had operated in motion.
So I clearly needed to build antennas for use while in motion. I
intended to operate the July CQ VHF WW contest on the way back from
vacationing in the midwest, so that formed the impetus to finally
build and design antennas for use in motion.
For 6M I chose a half wave horizontal loop. Anything else is pretty
large to travel with. I used this design from the web:
< http://neasmn.org/squalo/squalo.htm >
using 1/2 inch aluminum angle instead of the 1/2 inch copper tubing.
This made the antenna much lighter. I fastened the corners together
with #8 self tapping screws. The antenna was a bit floppy, so I
supported it with a small fiberglass rod in the middle from one side
to another across the mast support. I use a driveway marker for the
rod. The antenna is somewhat touchy to tune, due in large part to its
small bandwidth, but it did get a good match slightly below the
calling frequency. I adjusted the stub position to bring the whole
antenna to resonance and then the feedpoint position to get a
reasonable match to 50 Ohms. The two positions interact and there may
be a better way to tune this.
This loop is about 20% bigger than typical gamma matched 6M square
loops like the lawn chair loop. The ends are farther apart as there is
no longer any need to provide capacitive loading to bring the loop up
to resonance. The feedpoint impedance is higher and better behaved,
and as a result, it is less sensitive to SWR variations caused by rain
or proximity to other metallic objects.
On 2M, antennas are smaller so I thought I should get a bit of gain
out the antenna I chose. Horizontal loops are down a couple of dB on a
dipole and stacking them gets you only a dB or so over a dipole. I
finally settled on a 3 element Yagi, pointed forward. I figure that I
am usually going somewhere, there are hams there, and so if I point
the antenna forward I can work them. If I hear someone weak, I can
always pull off and rotate the antenna to peak their signal. Or point
the car toward them. I used WA5VJB's Cheap Yagi 3 element design for
this antenna:
< http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf >
with 6 gauge copper wire for the elements. I mounted it from the rear
so that it sticks out in front of the mast.
The 6M squalo went on top of the EMT mast at 11 to 12 ft above ground.
The 2 M antenna was 2 ft down from that.
How did this all work? I had the pleasure of driving through Smith
Center, Kansas on my way back to NM to work N0LL. I took the straight
road west out of Smith Center and copied Larry's beacons along the
way. The 6M squalo could copy the beacons consistently out to 50 or 60
miles. I heard it when parked on a small hill about 90 miles away, but
not after that. I worked K0HA and N0UNL at distances of 120 miles or
so. N0KE and I had a nearly complete QSO in DM76, a distance of 180
miles or so. I am sure that we could have completed this QSO with my 2
element Yagi. So my conclusions are that the squalo worked fine for
line of sight and diffraction paths out to 50 or 60 miles. It is poor
on troposcatter paths unless the station on the other end is well
equipped and does all of the heavy lifting and there are path
enhancements. There was sporadic E on 6M off and on during the contest
and the squalo did a good job on working most stations I heard. I also
could run stations when the band was open steadily. So I pronounce the
6M square loop experiment a success, althoguh I will see if there is
something I can do to improve troposcatter performance while in motion.
On 2M, the 3 element Yagi could consistently copy the N0LL beacon out
to 120 miles and I had no trouble making contacts out to this range. I
worked W3DHJ at a distance of 90 miles on 2M, while on 6M I had to be
much closer. Unfortunately, there wasn't much 2M activity on during
the rove, so I didn't get a chance to get a good feel for the antenna.
It performed well enough that I am thinking of putting up 2 back to
back and switching between the two. Or maybe going to a 4 element
version, although that is considerably longer.
The mast is 1 1/4" EMT telescoped into 1 1/2" EMT mounted to a trailer
hitch. It is braced with a 2" x 6" mounted to the luggage rack. I can
raise the antennas to 16 feet or so when stopped. The antennas
traveled fine at interstate speeds of 75 MpH when stowed at 12 ft. I
can lower them to 8 ft or so to clear obstructions, like the phone
wires in my fahter-in-law's drive.
For the recent UHF contest, I built a symmetrical double rectangular
(SDR) loop for 432 MHz. It consists of two loops, each 1 1/2
wavelengths in circumference and 1/2 wavelength high mounted
vertically so that they share a common short side. It is fed in the
middle. This is much like the old Skeleton slot antenna drivers and
also similar to the magnetic double slot used horizontally on 160M and
80M. Here is my poor ASCII art rendition of the antenna.
1/4 wavelength
---------
[ ]
[ ]
[ ]
[ ] 1/2 wavelength
[ ]
[ Feed ]
---- ----
[ ]
[ ]
[ ]
[ ] 1/2 wavelength
[ ]
[ ]
---------
I built this from the 1/2 inch aluminum angle using the self drilling
sheet metal screws. This is essentially 3 stacked half waves in phase
and a rectangular rendition of the double diamond or double delta. I
mounted this on a 3/4" by 3/4"inch wood boom running vertically. This
broke after 2 hours at 75 MpH and I had to splint it back together
with electrical tape and a piece of scrap aluminum I had along. I will
mount this on an insulated aluminum U channel in the future. I
fastened this to the mast with hose clamps. With that single exception
of the mast breaking, it traveled fine with the middle of the antenna
at about 12 feet above ground. You can see photos of this antenna in
action on the UHF Contest Soapbox page.
This worked very well in motion on 432 MHz. I made contacts out to 120
miles while in motion from the tops of hills. I also carried an 11
element Cheap Yagi for use while fixed. An unscientific comparison
between the two antennas by K7ICW on a 70 mile path showed that the
SDR was down 4 to 5 dB on the Yagi that was 3 ft below it. Putting the
Yagi up is worth it as it probably adds another 50 or so miles of
range. It is a bit long to travel with though. Preliminary modeling
shows that this SDR loop is better to or equal than the gain from a
halo loop over about 270 degrees of azimuth, so you don't give away
much. It is much better in the forward 90 degrees. And it is much
simpler to feed and build than a loop. I am happy with the performance
of this antenna. One could stack two more loops on top and bottom, but
that starts to get pretty tall. I may make a 2M version, although that
is getting pretty tall as well. There is a bit more gain to be had by
making the verticals a bit taller and the horizontals a bit shorter.
Two band versions are also possible, so I will explore those.
So there you have it. Three different solutions for three different
bands. I am happy with the performance of the 2M and 432 MHz antennas,
although they could be tweaked a bit. I would like to improve on the
6M antenna, but nearly anything else is pretty big to put on a vehicle
while in motion. I am considering a reversible Moxon, which may be
manageable with some additional support while driving. Perhaps in the
September contest. But that starts to violate my KISS philosophy.
I have covered what I do for power in the rover while in motion in a
previous post. That has continued to work well.
I hope this helps someone. If it does, let me know. - Duffey
--
KK6MC
James Duffey
Cedar Crest NM
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