[Elecraft] PAR endz-fed

[Ron D'Eau Claire]

The PAR End-Fedz (Par's spelling) is a Fuchs type antenna; an end fed 1/2
wave radiator.  

Its advantage over shorter radiators is that it requires very little ground,
if any, since it's fed at a voltage loop. Efficiency is not compromised by a
mediocre ground as is so common with radiators 1/4 wave or less in length
because virtually no current need flow into a ground system. 

If it's mounted vertically it will still have the induced ground losses all
verticals have but, being half wave long means it can have the current loop
some distance above the ground. If fully vertical the loop will be 1/4 wave
away from the lossy earth, helping reduce induction losses. 

The disadvantage of a Fuchs is that one must decouple that high-impedance
voltage loop at the feed point from the rig. Modern solid state gear is very
unforgiving of even a little RF voltage floating around. That's part of what
the matching network at the end must do as it transforms several thousand
ohms impedance to 50 ohms. 

The matching network must also handle the extreme RF voltages involved at
the voltage loop - often in the thousands of volts of RF - requiring
component spacing and capacitors designed for those voltages. Notice the PAR
antennas with relatively small matching units are strictly power-limited.
Many compact solid state tuners won't handle those voltages or impedances.
Old hamshack photos from the 20's and 30's often show a large coil/capacitor
"tuner" mounted high on the shack wall, out of casual reach. That was the
sort of tuner commonly used with the Fuchs antennas then (as well as with
open wire feed for multiband doublets). 

Sometimes a Fuchs is called a "Zepp", but a Zepp offer as low impedance feed
using 1/4 wavelength of open wire transmission line as an impedance
transformer converting the very high impedance at the end of the 1/2 wave
radiator to a low impedance at the transmitter. So a "Fuchs" can be thought
of as a true Zepp without the feed line. 

I've not used a PAR design but I currently use a 130 foot homebrew Fuchs set
up as an Inverted L with 40 feet vertical and 90 feet horizontal at my home
QTH. It does a great job on 80-10 using an external tuner. The design works
well on the harmonically-related Ham bands where it shows a high impedance
at the end. As the frequency goes up, making it longer in wavelengths, the
impedance at the feed point drops, making the voltages a bit less but still
quite high. 

On other bands a Fuchs can be used as a simple random-length end fed wire if
the matching network allows. Then a ground becomes more important,
especially as the length drops to 1/4 wavelength or less. I use my Fuchs as
a simple 1/4 wave end-fed wire on 160 with good results working against a
counterpoise, but where it really shines is on 80 and the higher
frequencies. 

Ron AC7AC


-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jim Brown
Sent: Monday, March 02, 2009 1:32 PM
To: Elecraft at mailman.qth.net
Subject: Re: [Elecraft] PAR endz-fed

On Mon, 2 Mar 2009 10:16:27 -0800, Randy Cook wrote:

>HOW MANY OF YOU ARE USING PAR ELECTRONICS END FEDS 

This is a VERY easy antenna to build. Why would anyone want to buy 
what they can build in an hour for a fraction of the cost? See 

http://audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf

One half of the dipole is a quarter-wavelength of wire, connected to 
the center conductor of the coax feedline. One quarter-wave from the 
end where the wire is connected you add a ferrite choke that 
functions as an "end insulator" for the coax, which makes the 
section of the coax between the wire and the choke the other half of 
the dipole. You do NOT apply a velocity factor when determining the 
length of the coax between the wire and the choke. 

If you're running 100W or less, the choke can be as simple as 10 
turns of the coax around two 2.4-inch o.d. toroids, #31 or #43! I 
used much bigger chokes to run 1.5kW. For 20W or less, a single 
toroid is plenty.

73,

Jim Brown K9YC