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" <[email protected]>
To: <[email protected]>
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: [email protected]
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
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