[Elecraft] HF Verticals?

[Charles Greene]

Hi All,

Anyone contemplating the use of elevated radials should read the article by 
Dick Weber K5IU in Spring 1997 Communications Quarterly.  Basically, Dick 
points out that 1/4 wavelength is the length to be avoided in elevated 
radials.  This is because the impedance of resonant radials is zero, and 
the current distribution in the radials is very uneven and one radial hogs 
the current, drastically modifying the radiation pattern.  He recommends 
using radials of less than 60 degrees or more than 120 degrees to increase 
the impedance of the radials so that a minor variation in the impedance of 
one radial will not affect the current distribution.  Moxon covers the 
subject of elevated radials and recommends using shortened radials with a 
common loading coil to tune them to resonance. He also states that the 
resonance length of 1/4 is the one to be avoided.  Other authors such as 
ON4UN recommend the same approach.  I have experimented with non resonance 
radials by making the radials short and increasing the length of the 
vertical element to tune the system to resonance, and by making the radials 
long and the vertical element short to tune the system to resonance.  The 
reactance of the non resonant radials cancels the reactance of the vertical 
element so that the entire system is resonant.  This may be covered in the 
literature somewhere but I haven't found it.  I got the idea from KC1SD who 
has been using it for years for 20 and 40 meter portable elevated radial 
verticals for field day.  The principle here is the same principal in use 
with an off-center-fed horizontal antenna to a lesser degree.  I have found 
you can't depart very far from the resonant length of either the vertical 
or radial element.  The antenna resistance at resonance with short radials 
and a long vertical is greater than 36 ohms, and the resistance of the 
antenna with long radials and a shortened vertical element is less than 36 
ohms.  I have found this in the modelling and verified it with the actual 
antenna.  Modelling shows the longer vertical element has a fraction of a 
dB more gain than the shorter vertical element, as to be expected.  I have 
modelled the system using an inductance to tune short radials as 
recommended by Moxon, and have also modelled placing the inductor half way 
up the vertical element to tune the system.  The latter configuration shows 
more gain (less loss) than the former for the same reasons that a 
mid-loaded vertical has less loss than a base loaded vertical.  KC1SD and I 
have been using 2 elevated radials.  The model shows they work as well as a 
system with more.  The model shows a surprisingly asymmetrical pattern with 
only two elevated radials, which of course is an advantage for 
portables.  My current project uses two long radials and a shortened 
vertical element.  The radial were elevated only 1' here at home, but they 
were on the edge of a 8' sea wall on the shore of Narragansett Bay.  At the 
special event site, I used the same vertical but elevated the radial to 
7.5' so that people could walk under them.  There was a good bit of 
retuning required from the antenna with 1' elevated and the 7.5' elevated 
radials.

At 06:30 PM 10/18/2002 -0700, Ron D'Eau Claire wrote:

>In the Ham literature, there is an excellent article by Al Christman,
>KB8I, "Elevated Vertical Antenna Systems, published in the August, 1988
>QST. That article also appeared in the ARRL publication, "Vertical
>Antenna Classics". This article is a Ham-oriented version of Christman's
>"Vertical Monopoles with Elevated Ground Systems," published in the
>Proceedings of the Third Annual Review of Progress in Applied
>Computational Electrodynamics by the Naval Postgraduate School,
>Monterey, CA, March 1987.
>
>H. L. Ley, Jr., N3CDR, did some very enlightening and interesting
>experiments comparing a short loaded counterpoise to an elevated ground
>system. He published his findings in "A Multiband Loaded Counterpoise
>for Vertical Antennas" that appeared in "The ARRL Antenna Compendium,
>Vol. 2" as well as in "Vertical Antenna Classics".
>
>Ron AC7AC
>K2 # 1289
>
>
>
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73, Chas, W1CG
K2 #462