[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

R: R: Single Feed crossed Dipole



Hi Bob,

To get 45 degrees phase difference between voltage and current the
roule is that the real part of the impedance and the imaginary  part
of the impedance must have the same values.

For example a dipole wich present an impedance Z= 70+j70 ohm
at the frequency of use is longer and has an inductive reactance
of 70 ohm wich has the same value of the real part (70 ohm ) and
in this case the current lag the voltage by 45 degrees.

A dipole witch present an impedance Z=70-j70 ohm at the frequency
of use in shorter and has a capacitive reactance of 70 ohm wich has
the same value of the real part (70 ohm) and in this case the current
lead the voltage by 45 degrees.

You can get the exact lenght for both dipoles with cut and try having
access to a vectorial network analyser wich show you the dipole complex
impedance while you decrease or increase the lenght of the dipole.

If you don't have a network analyser tune your dipole for the maximum
return loss of -20 to -30 dB at the frequency of use and this condition
correspond to an impedance of about Z= R+j0 where R is close to 70
ohm if you make measurements in a 70 ohm system.

Now without changing the resonant frequency increase simmetrically the
lenght of your dipole until you read a return loss of exactly -7.1 dB wich
correspond to a SWR of 2.6

Generally speaking if you look at the Smith Chart and you introduce an
impedance having the same real part and same imaginary part you will get
always a SWR circle of  SWR=2.62  corresponding to infinity points having
each a return loss of -7.1 dB

BUT.........

Provided that the termination of your impedance bridge is 70 ohm than a
return loss of 7.1 dB correspond to an impedance of 70+j70 ohm if the
dipole is longher than at resonance.

If the termination of your impedance bridge is 50 ohm than a return loss
of 7.1 dB correspond to an impedance of 50+j50 ohm if the dipole is longer
than at resonance.

Having got a return loss of -7.1 dB increasing the lenght of the dipole with
respect of dimensions of resonance you are now close to Z=70+j70 ohm

Take reference of the above dimensions.

Do the same shortening simmetrically the dimensiond of the dipole with
respect of resonance until you get again a return loss of -7.1 dB and you
will be close obviously to an impedance Z=70-j70

If you read Terman you will find also that for a resonant circuit the points
at -3 dB or half power points have both the same real part and the same
imaginary part.

Now parallel both dipoles as per the indications of the RSGB VHF/UHF Manual
page 8.45 in order to cancel out +j70 and -j70 to get a resultant impedance
Z=70+j0 and get  a total out of phase 45+45=90 degrees to generate circular
polarization.

After maching to a transmission line of 50 ohm make a circularity
measurement to establish also if  the semidipole connections that
you made to the slotted balun generates a  RHCP or a LHCP

It may be a good exercise for some student at the Academy.

Best 73" de

i8CVS Domenico

----- Original Message -----
From: Robert Bruninga <bruninga@usna.edu>
To: <amsat-bb@AMSAT.Org>; <domenico.i8cvs@tin.it>
Sent: Wednesday, February 18, 2004 1:13 AM
Subject: Re: R: [amsat-bb] Single Feed crossed Dipole


> About the single feed turnstyle:
>
> >>> "i8cvs" said:
>
> Read the RSGB VHF/UHF Manual fourth edition
> Chapter 8.44 and 8.45 figg-113 and 114
>
> Perfect!  Thanks...
> but it doesnt tell how to compute the length to
> get the needed 45 degree phasing differences..
>
> still looking...
> hummh...
>
> Bob
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
> Not an AMSAT member? Join now to support the amateur satellite program!
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>












----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home