Hope everyone is having an enjoyable holiday season.
My time is always limited, but the recent postings about verticals require a
few, imp6ortant comments. Ken Silverman (K2KW) and I have just completed the
draft of an article for CQ Contest (March, 1997) regarding the antennas used
at 6Y4A. Verticals were used on all bands. The 160 was the only single
element antenna, The other bands used various vertical arrays, including 2el
parasitic arrays (1/4 wavelength elements), 2el ZR's, 2el vertical dipoles,
2X2 using a pair of 2el arrays fed in phase. The article goes into a enough
detail (not just "hand waving") so that you can implement similar systems.
Regarding recent postings to this reflector on verticals, the following is a
quick summarization on a few items:
A full size vertical will not be 50 ohms (1:1), unless the radial system
(the current return) has substantial loss. The ARRL Antenna Book (for
example) in the section on efficiency explains the feedpoint composition
(includes the loss). In general, for a 1/4 wavelength tall vertical, one
wants to continue making the radial system more efficient, which will reduce
the loss and peak the VSWR to its right (unmatched) value of about
1.5-1.6:1. This the antenna system (including the radials) in the low 30 ohm
range, which is correct. A meter, such as the MFJ can be used for this
purpose. The point of lowest dip is the resonant frequency of the antenna
(this includes the radials). After setting the antenna system on the
desired frequency, if the feedpoint is subsequently stepped up to 50 ohms,
such as using a hairpin across the feedpoint, the frequency will drop
because of the hairpin. However, shortening the element length (or radial
lengths), will move it back to the desired frequency, along with the
resultant 1:1 VSWR reading. At 6Y4A, we use 1:1 current baluns right at the
feedpoint, which are attached to the feedline.
When using meters like the MFJ, one can get readings of >100 ohms at the
feedpoint of a vertical that has its radials laying on the ground. The
reading is complex, but it is informing you that there is a real problem,
which is most likely the radials. They are either too few, or they are not
elevated, or both. On our 55' linear loaded 160 vertical last Feb, with two,
full length radials on the ground was actually reading close to 150 ohms on
the MFJ. Whatever this reading really was composed of is interesting, but it
indicated a serious problem. After raising the radials several feet, it was
much better, but still around 40 ohms; not all that great, but better. This
fall, the 160 with 4 elevated radials (elevated higher than before) measured
less than 20 ohms, which was in the right range, and (after being matched at
the antenna to 50 ohms) had a 2:1 VSWR bandwidth at the rig of 40 kHz.
For additional reading, Moxon is very good. In our 6Y4A installation last
month, we demonstrated the effect of laying radials on the ground. On one of
our 20 mtr reflector elements, we laid the two radials on the salt-saturated
coral, complete with salt water filled pot holes along the way, underneath
the radials. The element measured as being resonant at 11 MHz, a full 3 MHz
from the target. Elevating the radials (cut to the same length as the
vertical tubing part) between 18" and 24" brought the frequency right up
close to 14 MHz, with a feedpoint down in the 30 ohm range. We could have
chosen to trim the radial length and keep them laying on the ground;
however, the result would have been a very lossy antenna, since trimming the
radials laying on the ground will not cut the loss (they will just be a
little shorter). Since our goal is efficiency, the radials were elevated.
Two elevated radials running in a straight line (180 degrees from each
other) was sufficient for an efficient current return.
We have done testing of elevated radial height. As a general range,
for 160 mtrs: 16' high; 80 mtrs: 8' high; 40 mtrs: 4' high, and so on. The
antenna itself does not need to be elevated. The 6Y4A article describes this
in more detail. There is also an informative IEEE paper early this year
about elevated radials being added to disintegrating buried radial systems.
Oh, yes, there was a challenge for those who say laying radials on the
ground totally detunes them. Since I am one of those who has demonstrated
that laying radials on the ground can significantly detune a vertical
antenna system, it must apply to me! The challenge was to touch your tongue
to the end of an untuned radial (laying on the ground) with RF applied to
the antenna. Apparently, this was thought to teach that the radial was not
detuned. All it will teach is two, already known things:
1) the end of a radial in a vertical antenna system is most always a
high voltage point (especially for 1/4 wave radiator, 1/4 wave radials);
2) a person touching their tongue to the end of the radial is not
Verticals can perform very, very well, provided they are installed properly.
Since most of us are not right on the ocean as we were at 6Y4A, we cannot
enjoy the benefits of salt water and the low take-off angle. The key is to
emply the most effective radial system (current return) as possible and
minimize other losses as well.
As time permits, information we acquired doing careful vertical antenna
testing performed earlier this year on the salt flats here will be on our
web site. It will be augmented with additional information from the two
operations at 6Y4A.
Hope you have an enjoyable Holiday Season and a terrific New Year!
Have a good day and 73,
Force 12 Antennas and Systems
(Home Page http://www.QTH.com/force12 )
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