[Elecraft] OT: G5RV's antenna

[Kim Bottles]

OK, I amend my post to say 130 foot doublet.

Kim - K7IM

-----Original Message-----
From: Ron D'Eau Claire [mailto:ron at cobi.biz] 
Sent: Friday, March 09, 2012 8:09 PM
To: 'Ken G Kopp'; don at w3fpr.com
Cc: elecraft at mailman.qth.net
Subject: Re: [Elecraft] OT: G5RV's antenna

The feeder was exactly 1/4 wavelength long so it transformed the very high impedance at the end of the radiator to a very low impedance at the rig in the Zeppelin, minimizing "RF in the Shack" issues. 

Since the radiator was 1/2 wavelength long, very little current flowed from the feeder into the radiator. It was a voltage "loop". That meant that, although the other side of the feeder was terminated in an insulator, the currents along the feed line were well balanced. Most radiomen of the time considered that the leakage current into the insulator closely matched the current into the radiator so the balance was very good indeed. 

The next step, coming back to Don's comment, was to connect two Zepps back to back for some additional gain. Extending the radiators beyond 1/2 wavelength enhanced this effect and, since it was now a center fed antenna, feedline imbalance was no longer a danger, hence the popularity of the "extended double Zepp".

I am always careful to call my center fed wires a "doublet" to avoid confusion but many Hams today incorrectly call any wire fed at the center with open wire line a "Zepp". 

Feeding a traditional Zepp only minimizes feed line radiation when the radiator is exactly 1/2 wavelength. (The feeder can be any length if you can deal with the feed point impedance at the rig end.) However, many Hams have reported excellent results with Zepps with not-1/2-wave long radiators. In those cases the feed line is also part of the radiating antenna. That can be good when the feeder is in the clear and less so if it's not. The same is true of most off center fed antennas.

73,  

Ron AC7AC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ken G Kopp
Sent: Friday, March 09, 2012 7:34 PM
To: don at w3fpr.com
Cc: elecraft at mailman.qth.net
Subject: Re: [Elecraft] OT: G5RV's antenna

Yes Don, I knew when I used the term "classic Zepp" that I was taking a bit of liberty with the term.  The original Zepp was indeed end-fed from the cabin of a Zepplin.  I've often wondered how long the feeder actually was.
They may have been closer to a simple end-fed wire.

I'd also noted the concept repeated in the J-pole.

We "old ops" have certain advantages ...

73!  Ken

On Sat, Mar 10, 2012 at 3:23 AM, Don Wilhelm <w3fpr at embarqmail.com> wrote:
> Ken,
>
> I will agree with all until I got to the end of your post - "classic zepp"
> The original (and classic)  Zepp antenna was a 1/2 wave wire fed 
> through a 1/4 wave transmission line connected to the end that was 
> trailed behind lighter-than-air aircraft (Zepplin).  If you look at 
> the J-pole, and turn it horizontally, you will see exactly the same 
> thing - a 1/4 wave transmission line with one side connected to a 1/2
wavelength radiator.
>
> I am not sure how the "Zepp" term became associated with any kind of 
> center fed dipoles, but it has in ham circles, and I find it confusing
> - there is the Center Fed Zepp (2 halfwaves in phase) antenna, and 
> then there is the Extended Center Fed Zepp (5/8 wavelength each side 
> of center), and then there is the "classic Zepp" that is the antenna 
> designed for trailing the aircraft.  So we must be careful to explain 
> which kind of Zepp antenna we are referring to when we write "Zepp"
> There is a lot of difference.
>
> 73,
> Don W3FPR