Saturday, May 12, 2012

Back To Back Mobile Whips For Portable Stations

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.


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