[TowerTalk] Top hats on mobile HF antennas

[Mark .]

Jim asked:

<<...snip... Would a top hat attached to a mobile screwdriver antenna be a 
beneficial addition even if used only when stationary?  Just wondering if it 
would make a significant difference down on 80 or even 40 meters, or make it 
possible to work 160 on an antenna that can only get down to 80 without it? 
Are there any thoughts or guidelines on size?  Do they need to be "tuned" to
a frequency?   TIA, Jim KG0KP >>

Hi Jim,

Homebrew mobile HF antennas have been the focus of much of my experimenting 
in the past few years, and I can tell you that a capacity hat placed at the 
top or free end of a shortened antenna *will definitely* increase 
transmitting efficiency and your signal reports.
  The idea, in a shortened radiator, is to draw as much current as possible 
over as much length of the antenna as possible. You will find that you use 
much less of your coil to resonate the antenna, reducing losses in the coil, 
and passing more current up higher on the radiator. This is why cap hats 
work best when they are at the top of a vertical, or the free end of the 
radiating element.

Tom has provided us with good theory to back this up.

To answer your question about tuning: you will have a different (lesser) 
amount of coil to resonate your antenna on 20, 40, and 80. If you just tune 
the screwdriver from inside while watching for an SWR dip, then you can 
continue the same procedure. If you have marked your coil for tuning 
positions to allow you to visually inspect it via your rear view mirrors, 
these positions will change with the hat.
I'm not sure, but I think some screwdriver designs have an impedance 
matching circuit that kicks in at the base for 40 and 80, and may be 
triggered by a certain point on the coil. These trigger points will change 
with a hat in place. Those in the know, please jump in with comments.
I use a homebrew version of the Lakeview Inducti-match on my homebrew 
bugcatcher syle antennas, and manually change the tap for 40 and 80 when I 
change the tap on the resonating coil.
  The problem with mobile antennas is that the proven wire hat with 
perimeter wire, which is quite effective, is not particularly practical 
because it catches on tree limbs and, if made from sheet metal, catches too 
much wind.
If you are stationary, this is not a problem. You can make your own cap hat 
assembly that will clip onto your whip.
See a plan I developed a few years ago at: 
http://www.qsl.net/n1lo/mobilhf.pdf

In this case, I used stainless wire to construct a mobile hat that cut 
through the wind fairly enough to be useable at highway speed, but still 
subject to low tree limbs.
Some real numbers from this experiment (roughly 9.5' vertical mast, 32" dia 
hat w/perimeter wire):
  1) Resonance without top hat: around 21-22 MHz
  2) Resonance with top hat: around 18 MHz

It's doubtful that with a similar size hat, that you could take an antenna 
already resonated at 80m down more than 1-2 MHz. Definitely not all the way 
to 160m without a taller whip and more coil.

Look around at some of the ads in QST for 'Hot Rodz' and you'll see that 
commercial suppliers are catching on to the advantages. To make them more 
practical for mobile use, they use just the radial spokes without the 
perimeter wires. Passes through limbs better with some loss of capacity 
without the perimeter wire.

Again, for stationary use, you could buy a ready made Hot Rodz set and use 
it as is. Or you can add your own perimeter wire, for the most extra 
capacity for the least amount of work. However (there's always a compromise 
somewhere!) you may have to go to a stiffer whip to support the cap hat.

I made a new design of top loaded mobile HF antenna this spring that does 
not catch at all on trees, and so far, has been self supporting at highway 
speeds. I would like to eventually post a plan, but it involves homebrewing 
the *entire* antenna. The efficiency boost on 40 and 80 is significant.

Happy experimenting! Almost anything you clip on the top will work 
stationary, as long as you have a tight connection.

--...MARK_N1LO...--