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Re: [TowerTalk] Radial Length

To: "Don Josephs" <djosephs@beecreek.net>,<towertalk@contesting.com>, "Jim Lux" <jimlux@earthlink.net>
Subject: Re: [TowerTalk] Radial Length
From: <paul@w8aef.com>
Reply-to: paul@w8aef.com
Date: Mon, 20 Jun 2005 17:16:17 -0000
List-post: <mailto:towertalk@contesting.com>
The biggest problem I have seen with antennas on motor homes is they are 
mounted such that the antenna itself is too close to the motor home frame.

You gotta get the antenna away from the frame, either mount it so the bottom 
of the antenna is even with the roof line or tilt the antenna away from the 
body.

Up high is preferred but not always possible.

de Paul, W8AEF

----- Original Message ----- 
From: "Jim Lux" <jimlux@earthlink.net>
To: "Don Josephs" <djosephs@beecreek.net>; <towertalk@contesting.com>
Cc: "A" <djosephs@beecreek.net>
Sent: Monday, June 20, 2005 5:03 PM
Subject: Re: [TowerTalk] Radial Length


> 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.
>
> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless 
> Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with 
> any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
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> http://lists.contesting.com/mailman/listinfo/towertalk
>
> 

_______________________________________________

See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

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