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R: R: modeling qha



----- Original Message -----
From: Wallace Strom <kg4yuw@arrl.net>
To: i8cvs <domenico.i8cvs@tin.it>
Cc: AMSAT-BB <amsat-bb@AMSAT.Org>
Sent: Tuesday, April 20, 2004 5:15 AM
Subject: Re: R: [amsat-bb] modeling qha


> Domenico,
> Thanks for the reply.

Hello Wallace,KG4YUW

I tried to send you a file 3 MB long but it was rejected at your address
kg4yuw@arrl.net

Do you have another address where i can send the above file to you ?

> I am a little confused - I can model the feed point at the top or the
bottom.
> Is the top -endfired and bottom  backfire ?

No !
The feed point on top or the feed point on bottom can be connected to
radiate endfire or backfire and it depends on how you connect the small
bifilar and the large bifilar to the infinite balun and this is why i would
like to send you a drawing to show it.

In simple words

If you connect the small bifilar and the large bifilar following the drawing
fig. 8-17 Chapter 18 page 8-16  in "The Satellite Experimenter's Handbook"
of K2UBC than the QHA radiates endfire upward i.e. in the opposite direction
of  coax connector.

Since the small bifilar and the large bifilar are left hand wounded than the
QHA radiates RHCP upward or endfire in that drawing with that connections.

If you connect the small bifilar and the large bifilar as per fig 22-9 of
W2DU book "Reflections" Chapter 22-15 "supposing" that the drawing is
the same of K2UBC book than the radiation is RHCP backfire downward
or in direction of the book and not in direction of the reader.
>
> Other questions - should the second feed point be 90 degrees out of phase
from
> the first?

At the frequency of operation the perimeter of the small loop is designed
to be 1.016 wavelenght in order to get a capacitive reactance -jX with
the same value of  R wich is the real part of the impedance.

When the impedance is Z=R-jX with R=X than the current lead the voltage
by 45 degrees

At the frequency of operation the perimeter of the large loop is designed
to be 1.120 wavelenght in order to get an inductive reactance +jX with
the same value of R wich is the real part of the impedance

When the impedance is Z=R+jX with R=X than the current lags the voltage
by 45 degrees.

Since the small bifilar and the large bifilar are connected in parallel to
the infinite balun than -jX and +jX cancel out but the total current phase
shift is 45+45=90 degrees thereby generating circular polarization
and the impedance is theoretically purely resistive Z=R+j0 because
of self-matching.

Should I model the antenna's at 40 ohms? (W2DU)  Should the
SWR
> for both feed points match?

To get the above conditions it is mandatory to model the QHA following
dimentions of table 8-6 of K2UBC book or page 22-16 Chapter 22 of
W2DU book with the same results.

More and detailed info on this matter in AMSAT journal May/June 2004

>
> Thanks
> Wally
> kg4yuw
>
73" de

i8CVS Domenico













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