Many of you were present at the presentation I made last Friday at the
Dayton HamVention, describing the new three element vertical parasitic
array in use at K1VR. Designed by W1FV and built by the two of us, it
uses the shunt-fed tower as the driven element, with half-wave
reflector/directors, reversible.
During the presentation and afterward, some questions popped up. For
those of you who just can't wait for the published article, W1FV has
provided the following answers to the questions that I was able to
remember. They are probably better answers than those I provided.
Fred Hopengarten K1VR 781/259-0088
Six Willarch Road
Lincoln, MA 01773-5105
permanent e-mail address: fhopengarten@mba1972.hbs.edu
From: John Kaufmann <kaufmann@ll.mit.edu>
To: <towertalk@contesting.com>
Date: Wed, 20 May 1998 09:37:29 -0400
Subject: Re: Dayton Q's
For the record, here are my responses:
>1. How can you measure the resonant frequency of a piece of wire
>off-center? If the braid of the BNC connector goes one way and the
center
>conductor goes the other, aren't you just measuring the frequency of the
>center conductor piece of wire?
The resonant frequency is not sensitive to where you measure it. EZNEC
demonstrates this--you can measure resonance almost anywhere along the
wire and get the same result. You can reverse the center conductor and
braid and measure the same resonant frequency--in fact I did this with
the
MFJ-259 at one point just to double-check the measurements on your setup.
>
>2. Using the MFJ-259, exactly what are you doing? Do you set the
>frequency and then snip your way toward 1:1? When taking a measurement,
>do you sweep frequency to determine the present resonant frequency?
>
The SWR, in general, is NOT 1:1, at resonance (this is a common
misconception). The definition of resonance is zero reactance in the
impedance. This happens to show up as local minimum in the SWR, which
you can find by sweeping the MFJ-259 frequency, but the minimum on
the Spitfire is something like 3:1 or 4:1 when measured at the corners.
>3. Aren't those relays at a high voltage point? Won't they blow up?
Yes, it's a high voltage point, and no, ours haven't blown up (yet).
>
>4. What happens when you use a higher tower? Do you need to have wires
>parallel to the tower up top?
You can shorten or even eliminate all the folded segments if the tower is
tall enough. The thing that matters is the resonant frequency of the
wire.
>
>5. Did you measure the forward gain by field tests? or just by model?
The gain numbers in the talk are all computed. We have yet to make
real field strength measurements.
>
>6. Will I have to break up my guy wires or use Phillystran in order to
>hang this thing and make it work?
>
It's always a good idea to have nonresonant guy wires. If anything,
this is more of an issue for higher band operation than on 160, though.
>7. When tuning, just how did you make the other wing disappear
>electrically? Is grounding enough?
The computer model says grounding is enough. I still need to convince
myself that what we did with the ground rod is a good grounding system.
>
>8. If you had all four wires, wouldn't you be better off leaving the
>additional two wings in line somehow as additional, off-center line,
>reflectors?
>
I tried hard in the modeling program to make use of all 4 wires at a
time,
but I just couldn't see any improvement by having the side wings "active"
once I had optimized the tuning of the in-line wings.
>9. If I had TWO gamma rods, one for 80 and one for 160, could I use ONE
>set of wings for a two band 3 el vertical parasitic array?
Probably yes, but I would need to investigate this in the computer model.
>
>10. My shunt feed is only 20 feet above ground, why is yours at 80'?
No response necessary.
>
>11. Isn't this the same antenna as described by someone in QST back in
>1940?
>
I'll try to check into this, but I would be surprised to find anything.
>12. So, what were the actual dimensions after all?
>
As was said before, the only critical dimension is the spacing of the
corner
of the wing from the tower (1/4 wavelength). All the other dimensions
are
whatever they need to be at one's tower to achieve the proper
resonant frequencies of the wings. The beauty of this thing is that
geometry simply is not that critical. You fold the wires until they fit
and
resonate.
>13. Tell me again why you picked 1.9 and 2.0 MHz for the resonant
>frequencies of the director and reflector respectively?
That's what I determined by trial and error from computer modeling.
>
>14. Do you have to accurately figure out the resonant frequency of your
>tower with all the antennas, or can you just measure it?
The resonant frequency of the tower doesn't really matter, as long as
you can load it. Only the resonant frequencies of the wings matter.
>
>15. Tell me again how you lifted the grounds and feedline, and why mere
>total grounding wasn't enough to make the tower go away while you were
>tuning the wings?
>
A grounded 1/4-wave tower is a resonant antenna! The way to de-resonate
it during tuning is to unground it, so it becomes a floating 1/4 wave.
>16. Can you build a one wing version? What is the gain of that?
Yes. Gain is TBD, but probably a dB or two less that the 2-wing version.
>People still get wound up in dBi vs. dBd. Article should stress gain or
>F/B in dB1v (dB in relationship to a single vertical).
Probably a good idea. N6BV was a little unhappy with quoting absolute
gain numbers in dBi because it's very dependent on one's ground
conditions.
73, John W1FV
.
.
.
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