----- Original Message ----- From: "Anthony Monteiro" [email protected] To: "i8cvs" [email protected] 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.
73, 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 heat-shrink tubing.
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 is long
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. I1=I2
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 cable.
For more information on this type of balun-bazuka read Walter M. Maxell W2DU book "REFLECTIONS Transmission Lines and Antennas "
Best 73" de
i8CVS Domenico