[TowerTalk] Re: Vertical Dipoles and Ground Planes

[Joe Reisert]

Hi Jon,

The R7 is not a half wave vertical despite all the hype from Cushcraft. A
base-fed half-wavelength vertical has a very high input impedance and
requires a more sophisticated and frequency sensitive matching system.

I designed the R7 antenna in the the middle of the winter in the back yard
of my new home. It's really a 3/8 wavelength vertical. If you want to prove
it to youself, take all the traps etc. off the structure and place aluminum
tubing out the top tubes until your antenna is about 25 feet high overall.
You'll see that it will have a good VSWR in the 20-meter band (if slightly
off, frequency, readjust the tubing length for minimum VSWR). Furthermore,
this all tubing antenna will be much more efficient than the trap version
albiet only one band!

Now for a little bit of history. Back in the late 1970's I was looking at
alternative 2-Meter mobile verticals that was not so dependent on ground
losses, could be easily matched and that could improve radiation. That is
when I first discovered how easy a 3/8 wavelength vertical would be to
match and how this would put the current or radiation point up off the
ground with a higher overall radiation impedance and slightly more gain.
It's theoretical impedance is a few hundred Ohms (200-300) resistive in
series with an inductive reactance also a few hundred Ohms.
 
Realizing how easy this would be to match, my son Jim (AD1C) and I built a
40 meter model in our back yard. It used a 50 foot long vertical wire
suspended from a pine tree.  We placed a number (not alot of them as I
remember) of 1/4 wavelength radials right on the ground. For matching we
used a 1:4 step-up (bifilar wound toroid, 50 to 200 Ohms). From the top of
the toroid to the antenna we used a small variable capacitor (I think about
100 pfd.) in series to the antenna. Matching was a snap. Just tune for
minimum VSWR and voila, almost a perfect 1:1 VSWR. Jim quickly worked over
50 countries with that wire running only his home brew 5 watt maximum QRP
rig. Later he found it worked great on 10-meters without any changes and
soon had a great DXCC QRP total!

The counterpoise on the R7 consists of seven (7) 4 foot rods and is a poor
ground return but that's the price you pay for small size. If you add
additional or longer ground radials, the antenna as it's build will not work!

I am not familiar with how the HybGain DX-77 works but it's probably a
similar scheme.

Hope this takes some fo the mystery out of the design.

73,

Joe, W1JR

>Remember that the two most common "verticals", the R-7 by Cushcraft and the
>DX-77 by Hy-gain are not 1/4 wave verticals.  The R-7 is a halfwave vertical
>and the DX-77 is a hybrid windom fed 1/2 wave vertical.

>Both the R-7 and the DX-77 use a counterpoise to minimize ground losses, not
>eliminate them.  I would love to see a direct comparison of the 1/2 wave
>verticals.  I have done rather extensive on-the-air comparisons of the R-7
>and the DX-77 as compared to a tribander and a dipole, but the antennas were
>all mounted above 150' in one test and above 240' in the other so they won't
>track most installations.  I did find that the vertical often gave me better
>DX take-offs than the dipole and on some occasions than the tribanders.  I
>expect that was due mostly to polarization.  I, for one, think they are
>pretty good antennas for what they are.  Simple to install and small real
>estate requirements providing you can them up at least 33' in the air.  A
>push-up mast works pretty well.
>
>Jon Hamlet, W4ZW


--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm