Don,
An HF whip on a motor home is similar to one on any vehicle. Since the
vehicle capacitively couples to the ground, it is similar to a ground
mounted vertical with radials lying on or very near the ground. In these
cases, the notion of radials being resonant is not valid, since the ground
dominates.
As such, it's unlikely that the lack of radials is the problem with your
screwdriver antenna. It's more likely that you have a poor ground
connection to the frame of the vehicle, or the whip is mounted below the
roofline. If it's the latter, it will be fiercely coupling to the metal of
the motor home and you probably won't find resonances on many of the bands.
The current is extremely high at the bottom of a shortened, loaded hf whip
and therefore can induce very high currents in metal nearby if that metal is
not perpendicular to the whip. If this is the case, this is also bad
because you will be mostly heating up the antenna and the motor home metal
rather than radiating rf. This situation would be worse than a 40m dipole
mounted only a few feet off the ground. It would not be surprising that you
couldn't tune it without doing something cruel and unusual to the length of
the dipole.
Since the current is highest at the feedpoint of the whip, its important
to have a really good short heavy duty ground connection to the metal of the
motor home.
If you can accomplish a good ground along getting the antenna free and
clear from any surfaces that are not perpendicular to the whip, you should
have good results.
Since you were considering radials, I gather that you plan on doing a lot
of operating while stationary. You might consider a much larger multi-band
vertical that you would normally use in your yard, but mounted to the motor
home somewhere in such a way that you can tilt it back down, and break it
down into two or three pieces before getting underway. An easier way might
be a vertical wire shot into the trees and loaded with a tuner against the
frame of the motor home.
Good luck with this. It's a very worthy cause, if you ask me.
Dudley - WA1X
------------------------------
Message: 2
Date: Mon, 20 Jun 2005 10:03:38 -0700
From: Jim Lux <jimlux@earthlink.net>
Subject: Re: [TowerTalk] Radial Length
To: Don Josephs <djosephs@beecreek.net>, <towertalk@contesting.com>
Cc: A <djosephs@beecreek.net>
Message-ID: <6.1.1.1.2.20050620092253.01f750e0@mail.earthlink.net>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 08:53 AM 6/20/2005, Don Josephs wrote:
> Folks de K5DEJ
>
>Is there a formula for determining the radial length, i.e. other than 468/f
>MHz ??? I am trying to get a High Sierra 1800 Pro to resonate on my Motor
>Home and have not been successful, as of this writing.
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