[Elecraft] Comparing continuously loaded and other short vertical ants

[Charles Greene]

Ron and All,

Good explanation.  Some of the manufacturers have, with careful design, 
raised the radiation resistance by placing the inductive loading near the 
top.  My hustler mobile antenna and 6 BTV vertical antenna which is 
inductive loaded near the top for 80 meters come to mind.  I have done some 
modelling of a linear loaded antenna with a capacitance top hat for 160 
through 30 meters.  It would require a matching network at the base.  A 
switcheable L network or a remote antenna tuner would do it.  I'll probably 
never build it.  I'll send my EZNEC file to anyone who wants it.

Any At 08:00 PM 11/3/2002 -0800, Ron D'Eau Claire wrote:
>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


73, Chas, W1CG
K2 #462