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.
There isn't a formula that will work for you. Your motor home is a big
metal object in the reactive near field and will screw up whatever you
calculate.
(unless you're really ambitious and want to build a NEC model and spend
some serious modeling time...)
>
>I have searched the books I have here in the shack on the subject of radial
>length but have not found anything other than the radial length should be
>approximately a quarter wave length. I have cut the radials I have used to
>the lengths specified by the above formula but have not been successful in
>getting the 1800 Pro to resonate.
Given the large size of the motorhome, it's probably going to dominate over
the radials (that is, the radials just act like a capacitor coupling to the
big old motor home). And of course, the resonant frequency of the motor
home is somewhat ill-defined.
>I would like the antenna to cover the frequencies of 4.025 MHz (MARS), and
>40, 20, 17, 15, 12, 10 Meter Bands with a low SWR. The 1800 Pro is mounted
>on the back of a 28 foot Class C Motor Home. Radials will have to be routed
>to use the available space on top of the motor home.
Since your antenna is a "screwdriver" type, you should be able to tune to
resonance at almost any frequency, although resonance might not be an
impedance you like. (that is, an 8 ft whip on 4 MHz isn't going to look
like a resistive 50 ohms)
Some screwdrivers have a 4:1 transformer with a capacitor around them to
help solve this problem.
>Any assistance would be greatly appreciated.
_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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