[VHFcontesting] Rover antennas for use in motion

[James Duffey]

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