Back to back mobile whips have long been used to create
rotatable portable dipoles. You may have
heard varying reports on how well they work.
Some folks even claim that they are nearly unusable because of high
SWR. My experience working contests
while portable QRP with them has been good.
My setup has good SWR and I don’t seem to have problems making contacts
with my FT-817 on SSB. I was actually
please with the ability to turn the dipole in null out strong signals. The one disadvantage I found was that my
setup only covered about 100k on the 20 meter band at less than 2:1 SWR. I expected that because it is the nature of
wound antennas to have a narrow bandwidth.
I began to wonder why others had such bad experience with
this type of antenna. I believe I
discovered the reason when I began to consider buying a commercial mount design
for back to back mobile whips. I noticed
that the commercial version simply grounded one whip to the mount while
insolating the other one as in a standard mobile installation. My home brew setup uses insolated mounts on
both whips. I connect the feed line,
coax, as any other dipole. Shield to one
side and center conductor to the other side.
I use a coaxial wound choke to control RF on the outside of the coax.
That got me to
thinking. I decided to experiment with
grounding one whip, the one connected to the coax shield as the commercial mounts
are designed. I used a short jumper for
the connection. I usually use about 18
inches of PVC as an interface between top of my mast and the antenna
mount. I went ahead and set up the
antenna as usual with the PVC as usual. I
didn’t see any difference.
I next decided to try doing away with the PVC and connect
directly to the mast still leaving the antenna and coax shield connected to the
mount and thus now connected to the entire mast. The mast I was using is 4 foot sections of
the military mast I bought off the surplus market. I have both aluminum and fiberglass
sections. I had use the aluminum in the
previous tests and did so this time.
This time the SWR exceeded 5:1 which is well out of the
range of many automatic tuners. I
grabbed my MFJ Travel Tuner Model MFJ-902 and was able to tune the antenna to
an acceptable SWR. However, tuning the
FT-817 more than a few kHz required retuning the tuner. I decided I was onto something but just to
satisfy my curiosity I replaced the top two section of the mast with fiberglass
sections. The SWR dropped down to where
it was when I still had the PVC between the mount and the mast. I decided to confirm my result. I put the PVC back in place and went back to
the aluminum mast section. The SWR
remained low as expected.
So if you are
considering back to back mobile whip, I would avoid commercial mounts that
simply ground one whip to the mount unless you plan to use non-conducting mast
material or at least 18 inches of PVC between the metal mast and the
mount. I also recommend that you make
provisions for keeping the RF off the outside of the coax. A 1:1 balun, ferrite beads, coax wound on a
ferrite core or a coiled coaxial choke are all solutions for this. The coiled coax is the least expensive but if
it is not constructed correctly it will not be as effective as one would
like. This will be the topic of a future
post.
73
K7PDW
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