[TowerTalk] Radial Length

[Jim Lux]

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.