[TowerTalk] optimum vertical length

[Dan Zimmerman N3OX]

>
> If one uses the old CB antenna rules of 12-15 ft max height above
> structure as a constraint, are you better off with some sort of short
> vertical dipole, up 30 ft on the roof, or with a 45 ft vertical, on the
> ground?


 When you start talking about 12 foot high vertical dipoles, you really need
to revisit your assumption of a lossless matching network.

With big, fantastic loading coils at the center with Q=500 , a 12 foot
dipole will have around 5dB losses on 40m.  With heavy end loading you can
do better, of course, maybe only 3dB loss?  But that ends up being quite a
bit less stealthy.

Bandwidth will be razor sharp, so you're going to need continuously variable
tuning, and an ordinary autotuner isn't going to cut it without much more
significant (and possibly damaging?) losses.  The assumption of "lossless
tuner" really breaks down when you have major capacitive reactance to tune
out, so from a matching standpoint, the much taller vertical has a lot to
offer.

As far as good multiband operation without launching too much signal
skyward,* let's not forget that hamdom has been brainwashed against traps by
antenna manufacturers' marketing departments.  With proper design, traps in
operation can dissipate an insignificant fraction of a dB each.  Why isn't
anyone offering a 43 foot tall vertical with really beefy traps ?  Maybe
it's because "no lossy traps" has become a ubiquitous marketing mantra.
Same goes for "no lossy coils."

Shove all the loss into a black box autotuner at the base or into the coax
and a UNUN, and you can blame poor performance on the installer's mistakes
rather than the antenna's inherent properties :-)

I think "what is the best height for a single monopole assuming a lossless
matching network" is maybe an interesting question to toss around
theoretically, but practically speaking, I think some other unobtrusive
approaches could work well.

I was considering the old approach of a ladder line fed doublet up the side
of a Spiderbeam fiberglass pole fed as a T vertical on the lower bands and
as a doublet on the higher.  I guarantee that on the highest HF bands over
most soil conditions, that approach would work better than any ground
mounted vertical, and on the low bands you have an advantage of significant
top loading for your vertical.

There are  mechanical issues compared to aluminum tubing, of course.  I
think that's part of the appeal of the single tube... low visual impact and
strength.

I wonder what the losses would be if you  ran  a 1/4 inch soft aluminum tube
up the center of your vertical with a handful of dielectric spacers to keep
it centered, bonding the outer conductor really well, and using the
resulting tapered impedance hardline as a feed for a doublet?

The doublet would provide some top loading on the low bands and would
probably work great on the high, up high and in the clear.

Anyway, this is straying off the original topic, but I think that the best
all-HF-bands approach even in a situation where unobtrusiveness is very
important may be more than one type of antenna...

73
Dan