[amsat-bb] Re: Determining the length of a driven element
domenico.i8cvs at tin.it
Sat Aug 23 06:58:30 PDT 2008
If the person asking the question is reluctant to pull the aerial down a
dozen times so that short lengths can be nibbled off each end then a good
method in the absence of any instrument like an impedance meter is to
keep the dipole down in the field and feed it with a coax cable which
electrical lenght is exactly 1/2 wavelenght or any odd or any even numbar
of 1/2 electrical wavelenghts cut for the wanted working frequency of the
No matter if the characteristic impedance of the above feed line has or
not the same impedance of the dipole because the impedance Z = R+/-jX
ohm shown by the dipole will be seen without transformations at the
open end of the above coax line so that any adjustement made over the
dipole down the field to get the lowest SWR will have the same effect
as to work over the roof.
Another advantage to use such 1/2 electrical wavelengts of coax cable
down to the shak is the possibility to connect to it a network analyzer
and measure the same complex impedance R+/-jX of the antenna like
to be directly connected with the instrument to the antenna connector
over the tower.
The amount that needs to be cut off or to add cannot be determined
from the difference in the SWR at both ends of the band.
In addition the same value of SWR can be generated if the frequency
feeding the dipole is too low or too hight with respect to the resonant
frequency of the dipole because the SWR meter alone cannot recognize
if the reactive part jX of the impedance producing the same value of
SWR is inductive +jX or capacitive -jX
If decreasing the frequency with respect to the working frequency the
SWR decreases it means that the dipole has inductive reactance at the
working frequency and is too long
If increasing the frequency with respect to the working frequency the
SWR decreases it means that the dipole has capacitive reactance at the
working frequency and is too short.
When the exact lenght of the dipole has been determined and all
reactances are cancelled out probably the SWR cannot be reduced
anymore because the real part R of the impedance is different from
the characteristic impedance of the feed line and at this point a
matching system must be used to get Z = R +/-j0 ohm
If the 1/2 wave dipole is open at the center as the dipole in question
then a T- match with a suitable balun is suggested.
If the 1/2 wave dipole is not cut at the center then a gamma-match
matching system without any balun can be used.
----- Original Message -----
From: "Phil" <phillor at telstra.com>
To: <amsat-bb at amsat.org>
Sent: Saturday, August 23, 2008 5:18 AM
Subject: [amsat-bb] Determining the length of a driven element
> During a casual conversation recently, the topic of aerial trimming came
> Perhaps this may be a little off-topic here but since I don't know the
> I thought this would be a good place to ask anyway.
> Say a dipole has an SWR of 5:1 at one end of a band and 4:1 at the other
> Further, let's say that the dipole will be used at the centre of the band
> with a reasonable SWR. It has been determined that the dipole needs to be
> shortened. The question is by how much?
> The person asking the question is reluctant to pull the aerial down a
> times so that short lengths can be nibbled off each end and instead wants
> know if the amount that needs to be cut off can be determined from the
> difference in the SWR at both ends of the band.
> A logical question I suppose but is there a reasonable answer?
> Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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