The "Zepp" or "Zeppelin" antenna.

[G6Tanuki]

Yes, it's vintage, has been around in various forms for near-on a century.

Basically, a half-wave radiating element, end-fed by a quarter-wave (or multiples thereof) of open wire 'ladder line', but it has spawned other derivatives [which will not be considered].

Les Moxon, in his "HF Antennas for all Locations" book criticises it; he says 'it should not work but sometimes does'.

He analogizes the quarter-wave of open-wire feeder as equivalent to a parallel-tuned circuit with one side connected to the end-fed half-wave dipole [see B in the first image] and then postulates that because there is 'nothing' connected to the other end of the tuned-circuit there is nowhere for any current to go - so he says the circuit is 'incomplete' and it can't work.

The ARRL has a different view in their late-sixties Antenna Handbook; they introduce the idea of a full-wave dipole fed at 1/4-wave from one end [an antenna arrangement that definitely _does_ work!], then they fold the 1/4-wave part back along its other-half so it becomes a transmission-line [see second attachment].

So, which is correct?

The "Slim Jim" and "J" antennas used on VHF have related heritage.

[Keith956]

Originally Posted by G6Tanuki

Basically, a half-wave radiating element, end-fed by a quarter-wave (or multiples thereof) of open wire 'ladder line'

A 'Stairway to Heaven' then

[G6Tanuki]

Originally Posted by Keith956

A 'Stairway to Heaven' then

I see what you did there.

[Radio Wrangler]

The diagrams posted by Tanuki show what happens:

The half lambda top has a high Z at its dead end. Half lambda rotates this one full revolution on the Smith chart, giving a high-Z feedpoint at the driven end. The quarter lambda of wire down to the ATU rotates this impedance half a rev further on the Smith chart and brrings us to a low Z at that node of the ATU. The other node of the ATU sees a quarter lambda vertical wire, the other side of the ladder. This is a dead end at the top, so high-Z. Coming down the vertical ladder rotates us half a rev on Mr Smith, which converts that high-Z to a low-Z at the ATU.

So the ATU is at a low Z place on both connections and can drive some power into the thing.

What I've just called low and high Z are simplifications, the radiation resistance of the currents in the wires bring the impedances away from the zero and infinite values of the simplistic view.

The quarter lambda vertical ladder is carrying fairly similar currents of opposite phases, so most of the fields created will just cancel.

So yes, the Zepp should work, and it does to an extent.

With hydrogen filled, leaky, gasbags and a transmitter, I'd feel rather nervous.

David

[Rowbank]

Originally Posted by G6Tanuki

Yes, it's vintage, has been around in various forms for near-on a century.

Attached is a not quite 90 year old QSL card of John Blair, G5FT's contact using a Zepp to have a duplex QSO.

It was February 1937 before GM was permitted in Scottish calls and GM5FT became his callsign.

Also attached is a photo of John and the perilously close Zepp feed line.

Bruce
GM4BDJ