[Elecraft] Antenna Question

[Charles Greene]

At 06:23 PM 3/25/2002 -0800, Ron D' Eau Claire wrote:

Ron et el,

An end fed 1/2 Zepp antenna on 40 will be full wave end fed antenna on 20, 
a 3/2 end fed antenna on 15 and a 2 wave length end fed antenna on 10. All 
can be fed in a similar manner.  You don't need a specific length of feed 
line for an end fed zepp but if you use a multiple of 1/2 wave length, the 
high voltage current node will be in the shack.  If you are using a 
transmatch with a balun, the high voltage will be bad for the balun and it 
could destroy it.  My experience on baluns is that they will work up to an 
impedance of 500-700 ohms or so, but the impedance of the current node is 
several thousand ohms.


>  > 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

73, Chas, W1CG