[Elecraft] Comparing continuously loaded and other short vertical ants

[Dave Hottell]

Ron and all,

A very good explanation of ground effects.  The only point I would add is
that my measurements indicate that 100 ohms of ground resistance is not
typical.  With only 16 radials that are approximately 1/2 the length of the
vertical radiating element, ground losses are about 13 ohms, i.e. the input
Z of a 1/4 wl vertical is nearly exactly 50 ohms.  The Z with a perfect
ground would be about 37 ohms, so the difference of 13 ohms is ground
losses.  This is only about 1.3 dB of loss due to near field ground.

I also measured with less than 16 radials.  Started with 2 and doubled
until I got to 16.  The highest input Z was about 100 ohms total, meaning
that the ground resistance was about 60 - 65 ohms (4 db of loss) with 2
radials.  Plenty bad, but it was reduced to about 30 ohms (3 db) by going
to 4 radials from 2, and was reduced further to about 20 ohms (2 db) by
going to 8 radials.  I quit at 16 radials.

This is over poor, sandy, dry ground here in south Alabama.  Losses would
be lower over better conducting ground.

Now it is true that if you have only 1 ohm of radiation resistance in the
shortened antenna, the radiated signal will still not be much to brag about
-- with 16 radials the ground losses would be 11 dB or so.

As someone has already mentioned (I think) the matching losses for such a
low radiation resistance is apt to be rather high.  Maybe another 5 to 10
dB.  Shortened antennas do have some real-world problems.  It is best to
avoid them if possible, but it is not always possible.

73 es gl,
Dave
AB9CA



At 08:00 PM 11/3/02 -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. 
>

<big snip>