[amsat-bb] Re: Fw: Re: 70cm Parasitic Lindy
domenico.i8cvs at tin.it
Wed Aug 22 16:48:34 PDT 2007
----- Original Message -----
From: "Anthony Monteiro" <aa2tx at comcast.net>
To: "i8cvs" <domenico.i8cvs at tin.it>
Sent: Wednesday, August 22, 2007 4:46 PM
Subject: Re: Fw: [amsat-bb] Re: 70cm Parasitic Lindy
> Hello Domenico,
> Nice to hear from you! Unfortunately, I do not understand
> Spanish. I could tell that you were commenting about the BALUN
> but not enough to really understand it - I am sorry.
> Tony AA2TX
Hi Tony, AA2TX
About a simple balun for your Parasitic Lindy I was trying to say you
that a balun-bazuka for 435 MHz with a ratio 1/1 can be made as follows:
Slip 81 mm of coax braid over the PVC jacket of your feed line and live
this braid open and insulated at the point in wich the feed line connects
to the open dipole.
Cut and remove a small ring of PVC jacket from your feed line at a distance
of 81 mm from the studs of the open dipole.
Solder the other end of the 81 mm braid all around the ring of the exposed
braid of your feed line and protect this soldered point from water using a
The braid slipped over the PVC jacket of your feed line is made 81 mm long
because the dielectric constant of PVC II or Vinil Polychlorure is ranging
between 4 to 5 depending of the jacket type so that considering an average
value of 4.5 for the dielectric constant the velocity factor for PVC is
Vf= 1/(SQR 4.5 ) = 0.47
It follow that for 435 MHz a quarter electrical wavelenght balun-bazuka made
of a braid slipped as a tubing over the PVC jacket of a coax feed line
300/435/4 * 0.47 = 81 mm
>From the electrical point of view call I1 the RF current flowing over the
inner conductor of your feed line connected to one arm of the open dipole.
Without using a balun-bazuka the current I2 flowing over the inner surface
of the braid of your feed line as soon arrive to the other arm of the dipole
divide itself in two parts.
I4 is the current that flow in to the arm of the dipole and I3 is the
current that flow back to the generator traveling over the outside surface
of the braid of your feed line and is badly radiated by the line itself but
in any case I2=I3+I4
Using instead the mentioned quarter wave balun-bazuka the current I3 after
flowing back over the outside surface of the braid of the feed line get a
short circuit after 81 mm from the dipole and it is completely reflectet
back to the dipole so that I3 cannot be radiated by the feed line and the
currents flowing in both arms of the dipole are equals and balanced i.e.
I am confident that this type of balun-bazuka 1/4 wavelenght long is much
better than using ferrite beads slipped directly over the jacket of the coax
For more information on this type of balun-bazuka read Walter M. Maxell
W2DU book "REFLECTIONS Transmission Lines and Antennas "
Best 73" de
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