Posted by
Don Wilhelm-3 on
Mar 02, 2007; 1:24pm
URL: http://elecraft.85.s1.nabble.com/Vetical-Antennas-tp444325p444345.html
Mike,
I suggest you may want to do some reading at L B Cebik's website
www.cebik.com about vertical antennas. It would seem that what you have
been reading/hearing has some 'garbage content' to it, or may be information
specific to one installation. Start with basic antenna principles and work
to apply that to your situational constraints and you will reward yourself
with a better performing antenna system.
For a multiband elevated quarterwave vertical, I recommend using 2
electrical quarter wave radials for each band of use. Each radial pair
should be oriented in opposing directions to cancel the horizontal component
in the radiation pattern. If you do not wish to cancel the horizontal
component, then a single radial for each band is all that is needed.
Since you will have the base of the antenna well above ground, you must
create a ground plane with the radials. A vertical monopole with its base
on the ground will behave nicely with untuned radials buried in the ground
and forming a ground screen, but with an elevated quarterwave vertical
monopole, you must create 'the other half of the antenna' at the base - that
is what the radials accomplish.
An elevated monopole may be tuned by either trimming the length of the
radiator or the length of the radials or both. Yes, think of the vertical
and each of the radials as a dipole, bent at the feedpoint - in other words,
as an upright "L" antenna fed at the corner. If you have multiple radials
for each band, tune each one of them individually. for zero reactance at the
base if you are using an antenna analyzer, but you can also tune for minimum
SWR. Remember that a vertical monopole operating against a perfect ground
(or complete radial screen) will have a feedpoint impedance of 35 ohms. If
the radials slope downward instead of perpendicular to the vertical, the
feedpoint impedance will be increased because the antenna system is
approaching a dipole (which has a theoretical feedpoint impedance of 72
ohms). If you are feeding the antenna system with 50 ohm coax, you can
expect some SWR on the line, so that part can be tuned out at the
transmitter end.
To keep radiation from the feedline to a minimum, run the feedline away from
the antenna in-line with the radiator - it should drop for at least a
quarterwave under the ground plane. Accomplishing that feat is usually not
possible with the antenna roof mounted, so you will have to provide some
in-line decoupling on the feedline. A balun (or unun) at the base will not
accomplish all the decoupling required because the feedline will pick up
radiation beyond the balun and try to act as a radial. A W2DU type current
balun placed along the feedline will help a lot. I would suggest placing it
at the point where the feedline exits the roof and changes direction.
For lightning protection, yes use a heavy wire to ground from the base of
the antenna. The radials should stop the radiation from this ground wire
(BTW, there is nothing wrong with a vertical dipole). If your antenna
already has a DC path to ground at the base, you will be all set, but if
there is no DC path, you can install an RF choke there. An RF choke will
also bleed off any static that will tend to build up on the antenna.
73,
Don W3FPR
> -----Original Message-----
> From:
[hidden email]
> [mailto:
[hidden email]]On Behalf Of Mike Walkington
> Sent: Friday, March 02, 2007 5:56 AM
> To: Elecraft-Reflector
> Subject: [Elecraft] Vetical Antennas
>
>
> Hi All,
>
> I've moved to a new house and I'm trying to get a Hustler 5BTV working
> really well with my K2 in my new location. I plan to install the
> antenna in
> a ground plane configuration on a 2 storey home and feed it with
> coax. I was
> planning to use a few quarter wave radial for each of the bands
> of interest.
>
> I've had problems with TVI using this antenna before, so this time I will
> use some sort of a choke to block currents on the outer surface
> of the coax
> braid. Any suggestions on what I should use?
>
> How should I protect myself from lightning? I initially thought about
> running a lead from the base of the antenna to ground, but the length of
> this would probably turn the antenna into a vertical dipole.
>
> I was reading some doco for this antenna and read the following: Never use
> an antenna tuner to tune the antenna.Use of a tuner only fools the
> transmitter and does not correct a problem at the antenna.
> This doesn't seem right can I use my K2's tuners?
>
> Finally, I've been reading Moxon's HF Antennas for All Locations, and he
> doesn't seem to recommend quarter wave radials for ground planes. I'm not
> sure I comprehend why. Is anyone familiar with his concerns?
>
>
> Mike
> VK1KCK, K2 #2599
>
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