Zepp antenna the ultimate OCF?
 

From time to time I lift my head, blink a few times, and utter some
banalities, so here we go.

The OCF topic got me to thinking, what is a Zepp?

I may even have the wrong terminoligy, I'm speaking of the antenna fed
with ladder or open line, and one end of the line is fed to the antenna,
and the other just sets up there, lonely and tied to nothing.

Is this the ultimate waaayyy off center dipole? I'm assuming the
feedline radiates a LOT.

- 73 de Mike N3LI -

Zepp antenna the ultimate OCF?
 

On Mon, 15 Feb 2010 16:25:12 -0500, Michael Coslo
wrote:

Is this the ultimate waaayyy off center dipole? I'm assuming the
feedline radiates a LOT.

Hi Mike,

For one, as we understand a Zepp, it is very long in relation to
wavelength. This means at some frequency it is a Hi Z resonance we
are feeding and hence a very small current. As current is generally
regarded as the indicator of radiation, any pigtail (that portion of
the remaining off-center) would barely contribute at all.

However, for what we understand to be a Zepp, there is no pigtail;
except, perhaps, the transmitter itself as the Zepp is classically fed
from the antenna connector (or the plate tank). This, of course,
gives rise to RF being in the shack and all the concomittant problems
that follow from that. (How in the world did they ever come to this
design in a hydrogen filled balloon? Such is the anachronism of
naming conventions, actual dimensions, history, and the Hindenburg.)

To return to your question, yes, the Zepp is the degenerate form of an
OCF dipole.

73's
Richard Clark, KB7QHC

Zepp antenna the ultimate OCF?
 

Michael Coslo wrote:
From time to time I lift my head, blink a few times, and utter some
banalities, so here we go.

The OCF topic got me to thinking, what is a Zepp?

I may even have the wrong terminoligy, I'm speaking of the antenna fed
with ladder or open line, and one end of the line is fed to the antenna,
and the other just sets up there, lonely and tied to nothing.

Is this the ultimate waaayyy off center dipole? I'm assuming the
feedline radiates a LOT.

- 73 de Mike N3LI -

Yep, you've got it right.

Here's a way to understand why the Zepp feedline *has* to radiate. If
the feedline didn't radiate, it would mean that the currents on the two
conductors were equal and opposite. At the antenna end, one conductor is
open so no current flows from it. If the currents were equal and
opposite, there would be no current flowing from the other conductor to
the antenna. And if the antenna had zero current, it wouldn't radiate.
So if the feedline didn't radiate, neither would the antenna.

In fact, the radiating and non-symmetrically placed feedline also
couples into the antenna, altering its current flow somewhat.

The Zepp antenna is so named because it was allegedly used in the form
of an end fed trailing wire on Zeppelin airships.

Roy Lewallen, W7EL

Zepp antenna the ultimate OCF?
 

On Feb 15, 3:25*pm, Michael Coslo wrote:
Is this the ultimate waaayyy off center dipole? I'm assuming the
feedline radiates a LOT.

Historically, a Zepp has been defined as a 1/2WL wire end-fed with
1/4WL of parallel feedline, a single-band antenna. MFJ sells such HF
antennas modified for 50 ohm feed. The 1/2WL end-fed wire has a very
high feedpoint impedance thus minimizing the current at the feedpoint
and minimizing the absolute magnitude of imbalance (but not the
percentage of imbalance). For instance, the current for 100 watts into
a 5000 ohm feedpoint impedance is 141 ma., a magnitude less than the
1.41 amps into 50 ohms for the same 100 watts. Seems to me, the low
current at the feedpoint, which can be considered to be common-mode,
ensures a current minimum for both differential and common-mode
standing waves.

P.S. A Double Zepp is the same thing as a 1WL center-fed dipole.
--
73, Cecil, w5dxp.com

Zepp antenna the ultimate OCF?
 

On Feb 16, 10:05*am, Cecil Moore wrote:
On Feb 15, 3:25*pm, Michael Coslo wrote:

Is this the ultimate waaayyy off center dipole? I'm assuming the
feedline radiates a LOT.

Historically, a Zepp has been defined as a 1/2WL wire end-fed with
1/4WL of parallel feedline, a single-band antenna. MFJ sells such HF
antennas modified for 50 ohm feed. The 1/2WL end-fed wire has a very
high feedpoint impedance thus minimizing the current at the feedpoint
and minimizing the absolute magnitude of imbalance (but not the
percentage of imbalance). For instance, the current for 100 watts into
a 5000 ohm feedpoint impedance is 141 ma., a magnitude less than the
1.41 amps into 50 ohms for the same 100 watts. Seems to me, the low
current at the feedpoint, which can be considered to be common-mode,
ensures a current minimum for both differential and common-mode
standing waves.

P.S. A Double Zepp is the same thing as a 1WL center-fed dipole.
--
73, Cecil, w5dxp.com

"ensures a current minimum for both differential and common-mode
standing waves..."

I don't think it guarantees it. Consider the case of end-feeding a
"fat" half-wave-resonant antenna, like a cage or what I recall used to
be called a "flat-top" with several wires running parallel in the same
horizontal plane, connected together at the feed end (at least).
Consider feeding it with high impedance (600 ohm) open wire line. The
data I have suggests it's possible to have a feed line impedance
higher than the feed-point impedance of the end-fed half-wave (fed
against the counterpoise represented by the feeder). In that case,
the differential load impedance at the end of the feed line (the feed-
point impedance of the antenna) is lower than the feed line
differential impedance, and we must be at a maximum current point on
the feed line.

Admittedly, it's not a case you'd run into very often, but it appears
to me to be quite possible.

Cheers,
Tom

Zepp antenna the ultimate OCF?
 

K7ITM wrote in news:1a1dd526-f947-482c-84d9-dd789a6b8f75
@e19g2000prn.googlegroups.com:
....
I don't think it guarantees it.

That is the issue with the Zepp. Radiation is the result of current flowing
in the 'flat top' and the common mode component of current flowing in the
not-standardised feed line.

If the feedline was say a quarter wave vertical section (as is often
depicted) with the common mode point at the bottom effectively grounded
(not usually clearly described), you have a three quarter wave L fed one
quarter wave from the end. If the common mode point referred to above was
effectively isolated from ground, you have a different antenna, different
boundary conditions to satisfy. Still, the are all Zepps to us!

Owen

Zepp antenna the ultimate OCF?
 

Owen Duffy wrote:
K7ITM wrote in news:1a1dd526-f947-482c-84d9-dd789a6b8f75
@e19g2000prn.googlegroups.com:
...
I don't think it guarantees it.

That is the issue with the Zepp. Radiation is the result of current flowing
in the 'flat top' and the common mode component of current flowing in the
not-standardised feed line.

If the feedline was say a quarter wave vertical section (as is often
depicted) with the common mode point at the bottom effectively grounded
(not usually clearly described), you have a three quarter wave L fed one
quarter wave from the end. If the common mode point referred to above was
effectively isolated from ground, you have a different antenna, different
boundary conditions to satisfy. Still, the are all Zepps to us!

Owen

Exactly. People tend to look at the flat top as the "antenna" and the
twinlead as the "feedline". In actuality, the "feedline" is an essential
part of the antenna, and any changes in it modify the radiation
characteristics as well as the impedance. The same binary thinking often
misleadingly separates "antenna" from "ground".

Roy Lewallen, W7EL

Zepp antenna the ultimate OCF?
 

On Feb 17, 2:06*pm, K7ITM wrote:
Admittedly, it's not a case you'd run into very often, but it appears
to me to be quite possible.

You're right, Tom. I was assuming an end-fed 1/2WL "thin wire"
configuration. Here's a question for the experts. If the current at
the feedpoint is 140 ma into the 5000 ohm 1/2WL wire and zero on the
other (open) wire at the feedpoint, what are the currents in each wire
1/4WL back down the feedline at a 1:1 choke-balun, for instance? What
would those currents be when fed from a link coupling not referenced
to ground?
--
73, Cecil, w5dxp.com

Zepp antenna the ultimate OCF?
 

On Feb 17, 9:22*pm, Cecil Moore wrote:

Here's a graphic that might shed some photons on the subject:

http://www.w5dxp.com/ZeppCrnt.JPG
--
73, Cecil, w5dxp.com