[Elecraft] EFHW

[Ron D'Eau Claire]

Bill, you are close to the famous Zeppelin antenna. It was a
single-frequency antenna. The idea was to move the RF field as far as
practical from the airframe. RF arcs between elements of the duraluminum
airframe were to be avoided, especially up near the top of the airship where
there was always some leaking hydrogen finding its way out through the top
vents on the covering. 

The original antenna was 1/2 wavelength long and fed at one end by 1/4
wavelength of open wire line. At the antenna end, one of the two wires in
the transmission line was connected to the 1/2 wavelength long radiator. The
other wire terminated at an insulator, and nothing else. 

Since the 1/2 wavelength radiator presented a very high impedance to the
feed line, very little current flowed in that side of the feed line into the
antenna. Since the other side of the feed line went to an insulator, there
was very little current flowing there too (some current would result from
typical leakage at the insulator). The end result was decent current balance
and minimum radiation from the feed line from the balanced output of the
transmitter to the actual connection to the radiator. Since the feed line
was electrically 1/4 wavelength long, the very high impedance at the antenna
was transformed into a very low impedance at the transmitter. 

The weakness in your design will be the transformer. Even at QRP levels,
very high voltages can occur at the end of a half-wave radiator and so in
your transformer. It's pretty easy to build an open wire line to handle the
voltages without voltage breakdown. Not so easy in a transformer. 

73, Ron AC7AC

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Bill
Frantz
Sent: Saturday, February 11, 2017 2:52 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] EFHW

The next time I experiment with end fed antennas, I want to try a matching
transformer at the end of the antenna wire of about a
50:1 matching ratio. That way I will have an impedance in the feed line of
about 50 ohms and minimize coax loss. Now if I am connecting the antenna
directly to binding posts on the side of the KX(2/3), the loss won't matter.
But if I am using a length of RG174 for a light weight feed line, the lower
loss will be very nice.

Adding Don's recommended 0.05 wave length "counterpose" in the binding post
scenario sounds useful. With a coax feed, the coax should perform that
function.

Now, where am I all wet?

73 Bill AE6JV

---------------------------------------------------------------------------
Bill Frantz        | "I wish there was a knob on the TV to turn 
up the
408-356-8506       | intelligence.  There's a knob called 
"brightness", but
www.pwpconsult.com | it doesn't work. -- Gallagher

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