[Elecraft] Antenna Question

[Ron D' Eau Claire]

 > Kurt reviewed all the cautions, then built a 40M Zepp end fed.  He
> reported
> "NO RF in the shack!" from the feeder.
> Yes, you do have to use a transmatch on other bands than the resonant
> one.
> I am
> thinking of trying one to see if it holds true for my location.
> Location
> and nearby conductors can cause problems for any antenna.  Yes, high
> impedance end feed might have some issues, but it is a low current point
> as
> well.

Stuart's absolutely right, Jim. At the resonant frequency, it will have NO
r-f in the shack because the feeders offer a good balanced termination for
the transmitter. That was the whole idea behind it for use on the Zeppelins.

But remember that at any frequency other than the one where the antenna
section is exactly 1/2 wave, the antenna is a simple random wire coming down
to the ATU. The "other side" of the feedline is now non-functional. If you
are using a balun feed, disconnect it. It can do nothing for you but cause
problems (like I had with arcing) and losses.

Actually you need some matching device on the resonant frequency too. It is
a low impedance feed when the feeders are 1/4 wavelength long, but not
necessarily anything near 50 ohms. That depends upon the exact electrical
length of those feeders. Of course, back when the Zeppelins were flying
everyone used an ATU and open wire feeders <G>.

The fact that the feedline is just part of the radiating antenna on other
frequencies is certainly no problem in most installations. Such a 'random
wire' antenna can work GREAT, especially if a good part of it is up in the
clear.

You will need a good counterpoise on any but the resonant frequency though.
Either to ensure good efficiency (when the feedpoint impedance is low) or to
keep r-f from floating around the shack on those frequencies where the
impedance is high. That's what gave "end-fed" wires a bad reputation. Many
Hams did not understand the need for a good "r-f" ground and found
themselves getting r-f bites off of their rigs, etc. They blamed the antenna
instead of the lack of a good r-f ground.

One way of looking at the Zepp is that it has a built-in "counterpoise" for
the frequency for which it is designed. That 'unconnected' side of the
feeder is exactly 1/4 wavelength long at that frequency. The far end will be
at a high impedance since it is insulated and not attached to anything, so
that forces the other end - at the rig- to be at a LOW impedance. Hence it
is at a good 'r-f' ground.

There really is NOTHING unusual or 'magic' about a Zepp. It works exactly
along normal antenna principles. The idea was to provide a good antenna for
an airship aloft that had a minimum of r-f radiation within the airship
itself and which could still end-feed a 'trailing wire' antenna floating out
below and behind the airship. But, like many circuits in the Ham world, it
developed a 'mystique' all its own <G>.

Ron AC7AC
K2 # 1289