Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE0EYJ, Sep 11, 2019 at 3:04 PM.
Can I add a reflector to a 1/2 wave zepp antenna fed with ladder line? Will it work like a yagi?
Yes, that should work. The reflector can be adjusted for the best pattern and you can use a transmatch instead of adjusting the zepp/feedline for impedance matching.
I'm literally thinking of just throwing a proper-length wire behind it, stretched over poles like the zep would be. Spacing would be similar to a 40m yagi (around 5 to 5.5 meters of spacing between, if memory serves). I have a 10-story roof that I could put the zepp at the edge of, and room for a reflector. The thinking is that not having to feed the antenna at the center will be a lot easier, given the way the roof is at my workplace.
Les Moxon, in his book "HF Antennas for all locations" says that the traditional Zepp feed, as shown in your diagram above, does not work. It should be pretty clear that the current in the stub cannot be balanced because there is an antenna on one leg and nothing on the other! There are solutions to fix this problem - Moxon suggests several.
As to the question of use for an end-fed in a beam, yes indeed. However you feed a half-wave antenna you can obtain directivity by adding a suitable parasitic reflector or director to make a yagi beam.
What modern hams forget/miss is that the "other half" of the traditional balanced line fed Zepp is the EARTH GROUND. In a dirigible it would have been the metal chassis of the cab and balloon framework to which the radio was grounded.
Now earth ground is a terrible RF conductor, which is why Radials should be used. With that, the balanced line should stay generally balanced and not radiate.
However, we know that they did work on the Zeppelins. Modeling (EZNEC) shows us why they work with balanced differential currents and no common mode current at the XMTR. Common mode current exists at the antenna feedpoint but it is only ~10% of the current at the XMTR or at the current maximum on the antenna wire. With an antenna system dangling from a Zeppelin, who cares if the feedline radiates?
Note: Adding a resonant parasitic element to an end-fed antenna is likely to change the feedpoint impedance and thus change the SWR on the coax.
It certainly is not. I would suggest reading the original patent application from 1909;
Where it is stated that the
"Zepp antenna" is specifically designed to avoid using the dirigible's frame as a counterpoise.
This is done by using inductive coupling between the transmitter and the antenna circuit, which results in a very high common-mode impedance.
Good old "No. 2!"; die Erregerspule! How could I have missed it? Link coupled feed.
A J-Pole is a type of "Zepp" antenna, there is plenty written about it. You can certainly mount it 1/4 wave from a reflector, like a mounting pipe, and get good directivity. The J-Pole, like all Zepps, requires additional measures to decouple feedline, if that is important.
Yep, the J-pole and Zepp are both EFHW (end-fed halfwave) antennas. The input impedance of such antennas is quite high, (2-3 kOhms or so.)
The MyAntennas EFHW uses a ferrite-core transformer to match this high impedance to a Coax feedline. The J-pole uses a quarter-wave parallel-conductor section to perform the impedance match. The Zepp typically uses open-wire line with a tuner to perform the match.
All these feed methods will work, provided means are taken to avoid excessive common-mode current on the feedline.