[Antennas] End Fed Zep

Mon Oct 18 04:16:07 EDT 2004

Ronnie, 
> I am contemplating an end fed zep antenna. I have 110ft between two tall
> trees (30ft), and one tree is close to the shack. I was thinking of running
> balanced line (300-450ohm) from the antenna, underground in pcv pipe to a
> MFJ-949E tuner. 

... since you intend to run the feeder underground, this setup can only be
recommended if the balanced feeder carries balanced currents and thus
is not radiating. Otherwise the ground losses will be substancial. For the
currents on the feeder of an end-fed Zepp to be (almost) balanced, the
radiator must be resonant. Thus it's length must be a half wave on the lowest
operating frequency and it can only be operated on harmonic frequencies.
If, for example, the radiator has a length of 40m you can operate it on the
80, 40, 20, 15 and 10m bands.
Paul,
> Put up a 133' inverted L, put a ground or wire counterpoise at the base, 
> feed with twinlead along the ground, dump the MFJ tuner and build a
> simple L network with a 4:1 balun, and you're in business on all bands.

... your setup exhibits the same principal problem, but even more severe:
if we assume the counterpoise to be on ground potential (which is true
for non-elevated radials), this antenna is an unbalanced load and the
balanced feeder will radiate more or less on *any* operating frequency.
That feeder radiation causes ground losses if it is run along the ground.
Running a feedline on or under ground is *generally* not a good idea,
because that only works without ground losses if the load is *perfectly
balanced* (symmetrical voltages) in the case of a balanced feeder
(twinline) or *perfectly unbalanced* (one connection on ground potential)
in the case of an unbalanced feeder (coax). A non-radiating feeder
regardless of the type of feeder and antenna can be achieved by
*isolating the potentials* of the antenna and feedline at the antenna
terminals without affecting the current flow and that is exactly what a
current-BalUn ("Guanella") does. Building efficient BalUns for high
antenna impedances is a very difficult task, but that 's another story ...
73
Karl, DJ5IL