balun for extended double zepp

(1/1)

Dan Earhart:
I have just constructed an EDZ and need the balun to coax feed from the balanced line. Can I construct one, presumably 4:1, from coax, such as a coil of coax? This seems to be one of those things done frequently in past years. Can you bring me up to date? Thanks 73

George Hicks:
As I understand it, the Extended Double Zepp (EDZ) is actually a center fed dipole with each element fed in phase.  I don't understand why you'd want to use a 4:1 balun.  If I were building it I think I'd run 50 ohm coax to a 1:1 balun and finish the run to the center feed point using 450 ohm ladder line.
You don't indicated which band you're intending to work with your Zepp, but I'll assume you're looking at 20 meters.  I believe that if you cut each leg of the dipole to about 44' 5" and run 9' 5 1/2" of ladder line from the 1:1 balun to the center feed point you should end up with a resonant frequency somewhere around 14.270 Mhz.  You can always cut away about a quarter of an inch or so at a time a time from the 450 ohm ladder line to fine tune it.  Careful with the trimming for tuning.  If my math is correct the "trim allowance" for the top portion of 20 meters (14.225 - 14.350 Mhz) is about an inch.
Like yourself, I'm an amateur and not a professional.  Perhaps some of our more experienced can shed a brighter light on this subject.

73s

Dale Hunt:
The type of balun depends on how you are planning to match your
EDZ, what frequency it is designed for, and whether you are going
to use it for a single band or multi-band operation.

For multi-band use, I'd run balanced line to a tuner.  If your tuner
doesn't have a built-in balun, then you can use either a 1 : 1 or a
4 : 1 external balun.  You may find that coiling your coax to make
a "choke" balun is adequate, but the feedpoint impedance into the
balanced line may be rather high on some bands, which will require
greater inductance in the choke balun than would be the case if
you were just matching a 50 ohm load.

If you are using a series section of open wire line to match the
antenna feedpoint to 200 ohms, then a 4 : 1 coax balun would
be a good choice.  In this case, the antenna will be designed for
single-band use.

If you are using a shorted stub of open wire line with the coax
tapped onto the stub, then either a 4 : 1 or 1 : 1 balun will work,
with the tap point adjusted for either 200 or 50 ohms  respectively.

There are many other ways to feed an EDZ, but these are the most
likely ones if you are trying to connect coax cable to 450 ohm line.

To create the 4 : 1 coax balun, calculate the length of a half wave
of coax times the velocity factor of the coax (foam dielectric is 0.8,
solid poly coax is 0.66 or so.)  For 20m and RG-58, this would be
34.6' * 0.66 = 22.9' (7.0m).  Cut your coax slightly longer to allow
for lead length.  Fold the coax in a half (or wind it into a coil) to
bring the ends together and solder the ends of the braid to each
other.  The ends of the center conductor goes to the wires of the
450 ohm line.  The shield of the main coax feedline is connected to
the common shield, and the center conductor connects to one of
the center conductors of the balun (it doesn't matter which one.)

Good luck! - Dale WB6BYU

Dave Johnson:
I've run a doublet with a 100 foot top and about 25 foot of open wire feeder, for several years.

The balun I made was a 4:1 version, straight from the ARRL handbook, using a stack of ferrite rings wound with enamelled copper wire.

The balun is built into a plastic box mounted on the outside wall of the shack, about 3 feet of coax is fed straight through the wall into an ATU. The antenna tunes on all bands from 80 to 10.

Dave

Navigation

[0] Message Index