In a message dated 1/5/01 9:22:28 PM Pacific Standard Time,
ron.stordahl@digikey.com writes:
<<
I would like to put up a full size 80 meters vertical without guys.
I will have space for a buried radial system of as many wave radials as
is needed for good performance. Would prefer an insulated base design,
however I believe some successfully use a gamma match grounded base system.
I would like to not have to tune it to cover from 3.5 to 3.85 if possible
with max 2:1 SWR.
The reason for no guys is to keep the neighbors and city planning commission
happy, thus it would also be important that the tower be as small in
diameter as possible consistent with the no guy design.
This tower need only support itself. I will be hanging nothing on it.
I would imagine the top would consist of a pipe section pushed up to the
proper length to resonate the antenna midway between 3.5 and 3.85.
Any suggestions?
Ron N5IN
>>
Ron: There is a concept that will give the necessary bandwidth from 3.5-4
MHz. If a 1/4 wave vertical is floating or insulated at the base it will
require a big strong insulator to take the wind leverage. Assuming you have
a self supporting vertical and insulator that will take the wind and ice
loading, try this. Resonant the vertical at 3.6 MHz. Connect a coax that is
a 1/2 wave at 3.6 MHz (90.2" for .66 VP) or a multiple. In the shack install
a BC 3 gang variable Xc in a box that is in series with the center lead of
the coax. Have an insulated shaft coming out of the box. Bend some plates
over so that the 3 gang variable Xc shorts out at full capacity. As you
operate higher in frequency the vertical is inductive reactive and it
reflects the same at the end of the coax. Use the variable Xc to tune this
out leaving the Resistive part which rises very slowing going toward 4 MHz.
You will have a low SWR at 4 MHz as the reactance is tuned out. 3.6 MHz is
selected as a resonant frequency as it gives the lowest average SWR over the
whole band. It's called "Proper SWR Management". One felt it should be
resonated at 3.5 MHz for reasons that didn't full fill the main goal. I've
used this concept in a variety of ways--even with open wire line. Although
it has its critics--they never used it.
I also had a 5 band trapped vertical with each resonant frequency set in the
lower part of the band such that with the Magic Length of .66 VP coax of 92'
2" or multiples (1/2 wave or multiples on all bands), the SWR can be reduced
with the Series XC as one operated higher in the band. Each band could be
fully covered with a low average SWR with the proper selection of the lower
resonant frequency. Be sure to have a way to short out the variable Xc in
the box. Only one component tunes each band over usually less than about 160
degrees of rotation. Calibrate it over the bands. A wider spaced capacitor
will be needed for power levels over 250W. If a variable of less than 500
uufd is used, have switching provisions to parallel equal values of surplus
Micas.
I'm not sure a gamma matched vertical will have as good a SWR curve with the
series Xc. It's worth a try. Sticking the pipe in concrete for support
without an insulator is a simple way to mount this vertical to eliminate guy
wires. There is an unused 65' steel flag pole I'm looking at I'd like to
try. I don't have an insulator to support it and I don't know of any.
Another "Flag Pole Vertical" technique is resonate it at 3.75 MHz by grid
dipping it. Have 2 gammas. Set one for 1:1 SWR at 3.6 and one at 3.9 MHz.
Have a relay at the feedpoint to select the proper gamma. K7GCO
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