A half-wave antenna has the maximum current flow in the center and 0
current flow at the ends. Ideally, it would be fed in the center. Current
flow is what produces an electromagnetic field, and at the ends there is
nowhere for the current to flow. So in an ideal half-wave antenna, the
impedance at the endpoints is infinite. An end-fed antenna would have to be
either a little longer than a half wave or a little shorter than a half
wave for it to work. That would give a very high (but not infinite)
impedance at the ends. But the antenna will not be resonant. It will still
work, for our purposes. But I'd rather put up a center-fed half-wavelength
dipole.
73, Zack W9SZ
On Sun, Feb 25, 2024 at 2:33?PM W0LEV <davearea51a@...> wrote:
Thanks for posting, Barry!
So you are discovering there really is no "end fed" resonant antenna. The
proof is in the measurements as you show. So why is this abortion so
popular??? ONLY, and ONLY because it's so easy just to throw up a wire
into the trees.....
Sure, a wire thrown up into the grees is what I typically use out camping
and boondocking, but I would never consider it for home installation where
I have the options of optimizing efficiency and fitting the HF antenna(s)
to my local environment.
Dave - W?LEV
On Sun, Feb 25, 2024 at 7:55?PM Barry K3EUI <k3euibarry@...> wrote:
At the end of a month of testing - I conclude "there is no such thing
as
a purely end-fed antenna" (hi) using UNUNs.
All end-feds using UNUNs to match impedances are really very
"off-center
feds" with a counterpoise of some sort.
I wanted to see where the natural resonances occur on my new end-fed:
about 155 ft wire (#12 stranded) with 9:1 Palomar UNUN
It is fed with 100 ft RG213 coax with a choke at 90 feet from UNUN.
So perhaps it has a 90 foot "counterpoise" (whatever that means)
Here is the graph plotting RETURN LOSS (dB) and SWR from 3 to
20 MHz.
Looks like the biggest RETURN LOSS (almost 50 dB) and lowest SWR
occurs at 9.3 MHz
If we use the formula L (feet) = 468 / f (MHz) = 468 / 9.3 =
50.3 feet for a 1/2 wave length
and for the 3/2 wave resonance that gives about 151 feet (close
to
my measured 155 ft)
So the formula works.
The shape of the wire is a giant horseshoe (running around my own yard)
about 20 ft high.
I am only interested in NVIS for 80m and 40m NBEMS nets (and CW).
Might I mess with the counterpoise length or the wire antenna length to
get a lower SWR on 80m and 40M?
I can live with a 2:1 SWR on 80m and 40m, or even a 3:1 SWR on the
100ft RG213 feed line, since the "added" attenuation due to higher SWR
(3:1) on 100 ft RG213
is only about 1 dB of "additional loss" on 80 meters - only a
fraction
of one S-unit.
This 155 ft length is what was recommended by Palomar Engineering in
their handouts.
It can also tune up nicely on the 30m (10.1 MHz) and 20m (14.1 MHz)
and 17 meter (18.1 MHz) bands, which is great for DX
de k3eui barry
Feb 25, 2024
--
*Dave - W?LEV*
--
Dave - W?LEV
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