From: Jim Cummings (email@example.com)
>Here's another vertical antenna idea. I have used it in several fixed and
>portable configurations and it works.
>A vertical mast or a wire may be used. The length should be no greater than
>5/8 wavelength at the highest frequency you will use. That's about 20 feet
>at 28 MHz or 41 feet for a 14 MHz top frequency. The antenna will load on
>higher frequencies, but the radiation pattern will not be optimum for
>low-angle work as with 1/4 to 5/8 wavelength antennas.
>The antenna is nothing magical, it has a little gain towards it's top
>frequency range where the length is 1/2 to 5/8 wave. Electrically it is very
>similar to a groundplane antenna (either elevated or ground-mounted) or your
>VHF mobile 5/8 whip with the tuning coil at the base.
>Tuners: I have used the Icom AH-2 antenna tuner. Other whip and longwire
>autotuners will work, such as the SGC. Most manual tuners will work at the
>base of the antenna, but cheap manual "coax-only" tuners may not have enough
>L/C inside to tune out some reactances.
A nice application of using auto tuners... I used an AH-2 and an SGC at my
old station location where I was living in an apartment building, but I had
access to the roof for my antennae. After much ruminating, I fed the tuners
to a homebrewed 4:1 toroidal balun into a 40 metre long balanced dipole...
worked great on 40 and 80 and the other bands in a pinch....
If anyone wishes to use an AH-2 with a rig other than and Icom 735 or Icom
radioes of that era, just apply 13 V dc to the appropriate pin on the DIN
plug that would otherwise be plugged into the 735. When you wish to tune,
put the rig in low power transmit (AM, FM, RTTY, or CW is fine - any of the
SSB modes are useless!) and push the button. When the great LED comes on,
then put the rig back on the mode and power setting that you intend to use.
Granted, it is a bit tedious, but it is a great little tuner and I see no
reason why it couldn't be used with other rigs.... Of course, the SGC is
not as tedious....
Anyways, Jay's article folds in nicely with a conversation that I have been
having the last couple of mornings on one of the local 2 metre repeaters on
the run into work. My friend wishes to install a vertical on the roof of
his house for HF operation. He has a Cushdraft AP8, but he is disatisfied
with its narrow bandwidth of operation (40 kHz or so) on 80 metres. It just
so happens that I am thinking of building, with another amateur, one of
those screw driver antennae (a commercial version is the High Sierra HS-1 -
and there are equivalents). I invited this friend to participate in
building his own with us, but this has gotten me thinking...
To explain.... the screw driver antenna was developed by Don Johnson, W6AAQ
(I think that's his call) and he called it the DK3... It is a centre loaded
whip, where the coil can be adjusted for the frequency of operation by means
of cordless screw-driver... the screw driver and threaded rod are inside a
2' in copper/brass/aluminium tube, and this extends or retracts the coil for
lowest VSWR on the band. The coil is roughly one metre long (about 36
inches for the metrically challenged) and has a cut-down CB stainless whip,
about 165 cm (66 inches) long, mounted on top of the coil.... If you really
want the dirty details, drop Don a line and he will send out to you the
sheets that will guide you into building one of these beasts. Better still,
contact Worldradio and get a copy of Don's book 45 + 5 years of mobiling -
or something like that. No doubt about it, Don's ego is strong, but it is
well worth the read.
The point being that not only is the antenna a pretty good mobiling antenna
(I used an HS-1 for a while and was quite impressed with it), but I would
imagine that it would make a fine "low visibility" or
"My-property-is-the-size-of-a-postage-stamp" type of antenna. However, like
all quarter wave antennae, it is subject to the caveat that one must have a
good ground system in order for it to work properly. I really don't want to
get into that because there are a lot of different themes that could be
discussed on that topic. However, it has occured to me that instead of
building a quarter wave version, why not build a half wave version?
If we look at all the advertisements of these half wave antennae, they all
claim that ground radials are not required. This would seem to make sense
because theoretically, at a half wave, the feedpoint would have a maxima
voltage, with a minima current. Since there is no feed-point current,
ground loss currents are trivial and should not be a concern. The problem
is, however, that the feedpoint impedance of these antennae is infinite and
the problem is how to transform this infinite impedance down to 50 ohms.
Obviously, the impendance of these antennae is not infinite, but is
nonetheless quite high. Although it would seem to be the perfect
application to use one of these remote tuners to feed one of these beasts,
this is not the case. The accompanying literature states that these tuners
are unable to resolve the impedance to 50 ohms of an antenna that is
one-half wave length at the frequency of operation.
It would seem however that both Cushcraft and Hy-gain have solved that
problem. I don't know how Hygain does it, but Cushcraft uses a black box
for the R-5 and R-7 anennae. I have popped open the box on my R-5 for a
quick look and it appears that Cushcraft have use two toroids in some manner
to achieve this impedance transformation. The question is - Does any body
have a schematic for this box, or indeed have another solution to transform
the high impedance of the half wave radiator to 50 Ohms?
I hope there is someone out there who can help. If nothing, I guess I will
have to ask this question on the newsgroup on antennae....
73 and live better digitally
DON'T GET TOO EXCITED...
because remember, today is the first
day of the rest of your life.
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