[Elecraft] Comparing continuously loaded and other short vertical ants

[Ron D'Eau Claire]

James wrote:
...they have a section on short continuously loaded verticals.  ... From
what they say performance could be comparable to full size antennas.
...They cite low radiation resistance and bandwidth....they show reduced
size dipoles using linear loading ... Again they say this can be
comparable to a full size antenna.  ... Again the radiation resistance
is low requiring matching....has anyone done any modeling of these short
antennas, or of shortened hat loaded antennas like the Force12 sigma's.
============================

I have a continuously-loaded vertical for 80 meters. It is home brew
consisting of about 120 feet of #12 copper wire in a helix about 3
inches in diameter. It's about 12 feet long. Resonates nicely at about
3.6 MHz. I have a tuned counterpoise for a ground system. 

I wanted to compare it with a simple "inverted L" I use on 80. That is a
wire that runs vertically from my shack window (where the ATU is
located) straight up 25 feet, then runs horizontally 25 feet. So it's
pretty short, being less than 1/4 wave long on 80. 

I use the same tuned counterpoise with it.

Tests over the past several months show the "inverted L" to have an
S-unit or two advantage. I will continue to run tests through the winter
months when 80 is more active, but I expect the same trend to hold. 

The difference, as you pointed out, is radiation resistance. The
"radiation resistance" drops very quickly as the antenna is made
shorter. That severely reduces antenna efficiency. The power consumed in
the "radiation resistance" is the only power radiated. All other power
is lost in wire resistance, ground resistance and any other
'resistances' in the system. All of these resistances are in series as
far as the r-f is concerned. So if the other resistances could be made
zero, you wouldn't care how low the radiation resistance got.
Unfortunately, even keeping the other resistances under 100 ohms can be
a formidable task. 

For example, suppose you have a short vertical with a 'radiation
resistance' of 1 ohm made of copper wire of 2 ohms and connected to a
'typical' Ham ground having a resistance of 100 ohms. The total
resistance in the circuit is 103 ohms, 1ohm of which is radiation
resistance. That not an unusual situation for a short vertical antenna
where one does not have a salt water lake to drop the ground wire into
or a big radial farm or monstrous ground mats on the earth. If you apply
50 watts of r-f to this system, there will be just about 0.7 amperes of
r-f current flowing. 

Now:
0.97  watts will be consumed in the 2 ohms of wire resistance, 

48.5 watts will be consumed in the 100 ohm 'ground' connection, and

0.48 watts will be radiated  to be heard by the other station. 

Okay, lets get that vertical up to a full1/4 wave long. The radiation
resistance will rise to about 35 ohms. In that case you'll have 0.6 amps
of r-f flowing at 50 watts and:

0.72 watts is consumed in the 2 ohms of wire resistance.

36.5 watts is consumed in the 100 ohm "ground" connection, and

12.7 watts gets radiated to be heard by the other station.

While you can 'work the world' on 1/2 watt, it's a LOT easier with 13
watts!  What gets tougher in my case is getting 60 feet of vertical
conductor up for the radiator. 

A doublet (such as a dipole) has no "ground connection", being a
balanced antenna. So while it's efficiency is still compromised by being
short, it is not compromised nearly so much as a "grounded" antenna like
the short vertical. That's why makers of short vertical dipole antennas
(like the GAP) claim that their antenna is much more efficient. 

BTW, the Force 12 antennas appear to be very good, but notice that their
use of the for DX-peditions it to place them on the beach by the ocean.
That's the best possible 'real-world' environment for a vertical.  All
verticals suffer from more far-field ground losses than horizontal
antennas at an effective height (approx. 1/2 wavelength above the
ground). Putting a vertical by the ocean where the salt water will
minimize those losses produces the best results. 

Ron AC7AC
K2 # 1289