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Message-ID: <002401c6d7bd$c432eb60$fb1686d4@captbrian>
From: "captbrian" <captbrian@ukonline.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <000801c6d757$0c6af2e0$60be3b3e@fujitsu> <000801c6d769$a3a3b270$5d423850@MARLEEN>
Date: Thu, 14 Sep 2006 06:22:23 +0100
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Subject: LF: Re: Re: antenna calculation..help needed
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Remember that a short cb antenna has inductive loading in it somewhere,
which means that if you are adding a whip  in series with exixting antenna
then the length for resonance will not be the same as for plain wire. I
regard a cb antenna as being about 9 feet long electrically . You are wrong
to multiply by .96; that applies to wire antennas with large insulators. If
your proposal is to have a self supporting rod without insulator holding it
up then you will need all of a calculated quarter wave at 10.140 minus 9
feet for the cb.

The feed point impedance may well be different from theory. If you make up
the L-match then make it variable  either side of the values proffered by
William.

Another matching system I have used empirically on my boat with good
sea-water around is to feed at a point up about 3 feet from the ground. I
have operated trans-atlantic like this with no matching device at all. Try
it !

If your CB antenna is a plain whip with base loading coil try feeding it at
the junction between inductance and rod.

It gets trickier at 136 tho'  ;-))



Bryan G3GVB


----- Original Message -----
From: "PA0WFO" <pa0wfo@chello.nl>
To: <rsgb_lf_group@blacksheep.org>
Sent: 13 September 2006 20:20
Subject: LF: Re: antenna calculation..help needed


Hello Dick ,
To match a resistive load of 36 Ohms to 50 Ohms you need a L network
consisting of  a coil of 0,4 microH from your TX to the antenna and a
capacitor of 195 pF from the 50 Ohms side to ground ( freq, 10,140 Mhz ) as
calculated with the L matching network calculator ( supplied with the ARRL
antennabook ) . I use this calculator quite often with good results . I will
send you the program ( Old MS DOS ) in a seperate E-mail .
73, William PA0WFO
  ----- Original Message -----
  From: Dick
  To: rsgb_lf_group@blacksheep.org
  Sent: Wednesday, September 13, 2006 7:06 PM
  Subject: LF: antenna calculation..help needed


  Hello all,

  It's off topic, but as I can't find it on the net I'll ask it here.

  I want to modify an old 27MHz CB antenna to an quarter wave vertical for
30m (10.14MHz)

  Diameter is tapering from 26 to 15mm.

  Question: what length should it have? I suppose 0.25 wavelength x 0.96 or
so...

  Sofar not a big problem, but as I will use 36 radials direct on the
ground, it's radiation resitance will be
  around 36Ohm.

  To match this to 50Ohm I can use a series capacitor, but what value this
should be??

  Who can tell me?

  73

  Dick, pa4vhf