[CW] Off-Topic: EFW vs Dipole?

[D.J.J. Ring, Jr.]

Most of the writing on the Internet isn't about EFHW "End Fed Half Wave"
antennas, but about EFW "End Fed Wires".

A half wave wire fed at the center is low impedance and has high current at
the center, at the ends where the current goes to zero (very low current) ,
it becomes "voltage fed" or high impedance.

The radiation of any antenna is maximum at the current peak which occurs at
1/4 wave points - which is the center of a 1/2 wave dipole.

See the gif showing the standing waves on a half wave dipole, the RED is
voltage - maximum occurs at the ends where current goes to ZERO because
there is NO more conductor to travel, so current flow goes to ZERO.  When
current is zero, voltage by necessity is maximum.

Blue is the current, where there is maximum current that is where maximum
radiation occurs.  The center of the center fed dipole is at a high current
point and a low voltage point. A high current point / low voltage point is
low impedance.  A high voltage, low current point (the opposite) is a high
impedance. So the ends of the 1/2 wave wire are high impedance which is an
End Fed Half Wave.  The practical measurement of a half-wave end fed wire
at a half wave is around 3,000 ohms at the center it will be about 70 ohms.


Half wave antennas will have larger bandwidth because the impedance doesn't
change as rapidly as the low impedance at the center does.

Although resonance is important, for practical radio working, it's much
more important for power to be accepted by the antenna, which means that
the characteristic impedance of the feedline (or feeder) matches the
impedance of the antenna feed point.

A half-wave antenna end fed (impedance = 3,000 ohms) by 50 ohm coax will
have a VSWR of approximately 60:1.
A half-wave antenna center fed (impedance = 70 ohms) by 50 ohm coax will
have a VSWR of approximately 1.4:1.
A half-wave antenna end fed (impedance = 3,000 ohms) by 50 ohm coax using a
49:1 transformer to elevate its impedance 2,450 ohms will have a VSWR of
approximately 1.2:1.

Antenna books will have more information on transmission lines - high
tension lines are high voltage and are best for long runs - high voltage =
high impedance - which is why open wire feeders are lowest loss.

Shane Nelson, KD8UJM who sells excellent quality transformers like this one
https://www.ebay.com/itm/191782023426 for antennas (also available in all
stainless models)  has an extremely well researched and tried document
about EFW antennas.
http://www.hamuniverse.com/randomwireantennalengths.html

<http://www.hamuniverse.com/randomwireantennalengths.html>
His recommended lengths are:
*29  35.5  41  58  71  84  107  119  148  203  347  407  423*

EFHW is probably the WORST name for this antenna, because you need to AVOID
half wavelength lengths for the bands you wish to use!

The antenna is really a End Fed Wire - EFW.

https://www.kb6nu.com/playing-end-fed-wire-antennas-91-ununs/
https://udel.edu/~mm/ham/randomWire/
https://www.hamuniverse.com/randomwireantennalengths.html

AVOID half-wave and harmonics of half-wave!


the following chart the WHITE areas are the desired lengths!  You cannot
find one that is an "f" because it doesn't exist, but you can find an e.

I bought one of Shane Nelson's excellent transformers (the stainless
version, but the plated version should be fine for people that aren't
obsessed with stainless like former salt water radio officers like myself.

Highly recommended.

73
DR
N1EA
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/cw/attachments/20220628/b4fc4b9c/attachment.html>