W9UCW's post brought back a lot of memories. I could write a book about
topband mobile'ing around the south side of Chicago in the early
60's! Instead, I'll offer a few things I've learned in the last 40 years
of building and using mobile antennas.
First of all, there are two kinds of mobile setups; "mobile-in-motion" and
"contest mobile". Contest mobiles do not need to withstand the hazards of
daily driving and the antenna requirements are much different than those
needed for the "mobile-in-motion". Been there, done that, no fun any more!
Mobiles-in-motion must survive trees, carports, drive-ins, and vandals. My
present mobile has seen 65,000 road miles in the last two years without a
failure.
With that in mind, here are my design criteria in order of importance:
1) Survivability - Even the most efficient antenna is worthless when broken
2) Low cost / easy replacement - Sooner or later it will get broken or
vandalized (or you will want to "improve" it)
3) Efficiency
In the early 60's, AM mobile was king, everybody operated on 1810
kilocycles, (what's a "hertz"?) high Q coils won the shootouts, and life
was good. With a 20 foot tall antenna and a huge coil, I could usually win
the Chiburban Radio Mobileers shootouts, however my mobile career was cut
short buy the all expense paid vacation offered by my Uncle Sam. Returning
home in the late 60's was a real culture shock! All my old buddies were
running SSB, VFO's, and "Minooka Special" antennas! The SSB and VFO part
was easy to accept, but it took a while to become convinced that buggy whip
coil was worth anything. Eventually I became a believer and I still have
two of those antennas in my pile of old antennas.
I tell this story because it played an important part of the learning curve
I experienced designing, building, and using loading coils for both mobile
and fixed antennas.
One of the lessons learned was that a coil with a L/D of 20 or 30 to 1 is
not the same as a coil with a more traditional L/D and the same "Q" and
inductance. Many field measurements have proved this, despite theoretical
arguments to the contrary. You can't model the coil in EZNEC, it won't
allow wire lengths small enough (or enough segments). Sticking a
mathamatical load at a point in a wire is NOT the same as a coil with
physical attributes!
Another lesson learned was that, overall, common PVC is the best coil form
material. During my career in the aerospace industry, I had access to many
exotic materials and a machine shop. I've lost count of how many coils I've
created using acrylics, polycarbonates, teflons, YN25, G10, and other
uncommon materials. None of them, not even one, ever out performed a same
sized coil made with a PVC form.
Contrary to Barry's experience, I've had no trouble with high voltage
breakdown. The PVC failures I've seen resulted from corona discharge
igniting the PVC. This will happen with every form material I've tried
except ceramic! Stop the corona and stop form failures.
As far as Q, PVC is better than need be. For instance, my present 40 meter
coil is wound on common PVC pipe and encapsulated with many coats of PVC
pipe cement. It is essentially WX proof but sill measures a Q of 480 on the
bench! Even if you double that Q by using an exotic coil construction, you
won't see any improvement in signal at the other guys RX.
I'm all for technical debates. It helps everyone understand why stuff is
the way it is. But let's not let theory get in the way of getting on the
air cheaply and quickly. After all this is a hobby, let's have fun!
73,
Larry - W7IUV
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