[amsat-bb] Re: Testing a UHF antenna

i8cvs domenico.i8cvs at tin.it
Sat Jan 21 08:30:04 PST 2012

> Hi all.
> Am in the process of building a quadrifiliar helix antenna using this
page: http://jcoppens.com/ant/qfh/calc.en.php .
> How much cable should I use between the device under test and the antenna
analyzer? I've got a Times T-100
> 73 de Norm

Hi Norm

I was looking at http://jcoppens.com/ant/qfh/calc.en.php .

but unfortunately there are not informations on how to connect the
4 by 1/2 wavelength loops to get End-Fire or Back-Fire radiation 
from the quadrifiliar helix antenna as well no information to get a
50 ohm match properly connecting the 4 terminals of the loops to
the infinite balun so that I suggest you to read the article:

"Experimental Investigation of Quadrifilar Helix Antennas for
2400 MHz" published into the AMSAT-Journal May-June 2004 

BTW, about your question the lenght of coax cable between the device
under test and the antenna analyzer must be any even or odd numbar of
1/2 electrical waveleght of coax cable for the frequency in use where
the 1/2 electrical wavelenght of the coax cable is the 1/2 free wave 
multiplied by the velocity factor Vf of the coax used i.e.Vf = 0.66 for

When the lenght of any transmission line is 1/2 electrical waveleght the
impedance at one end of it is the same at the other end of it i.e. the
impedance of the antenna will be seen unchanged at the antenna analyzer
input connector no matter the value of the VSWR.

73" de

i8CVS Domenico

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