[NCC] [MRRC] EFHW? Any experience?

[Robert Hayes]

All,
I may be way off on this, but my theory on this is that since you are constantly changing the voltage at one end of the End-fed dipole (at the rate of the frequency of the signal).  It takes time to transfer to the other end of the dipole (basically a 1/2 wavelength's amount of time).  That means the voltage at the other end is a half wave behind the feed point voltage.  It is that ever present voltage difference (except that instant when it crosses 0) that results in a current flow through the antenna wire.  And of course the direction of the current changes every 1/2 cycle.
Same thing does not happen at DC of course, but with RF frequencies (and their resulting relative short distances between 1/2 waves) it should.
Glad to hear some attestments and measurements to confirm.
73 Bob KW8N


-----Original Message-----
From: Ethan Miller K8GU <ethan at k8gu.com>
To: MRRC Mailing List <mrrc at contesting.com>
Cc: North Coast Contesters <ncc at contesting.com>; Robert Hayes <kw8n at aol.com>
Sent: Fri, Sep 20, 2019 12:55 am
Subject: Re: [MRRC] [NCC] EFHW? Any experience?

All,

For many years, I would have said exactly the same thing as Jim,
W8WTS; and everything he said is 100% correct.  However, I was
recently made aware of an experiment that puts some actual numbers
against the problem and the experiment suggests it's not quite as bad
as you would think:

https://www.hamradio.me/antennas/electrically-isolated-end-fed-center-fed-dipole-radiation.html

If you assume that the transmitter was delivering the same amount of
power to the load (antenna plus matching unit) both times and ratio
the peak value of the magnetic field for each test, you get about 1.2x
or 1.6 dB "gain" (I did 20*log10 for dB because we are comparing
fields/currents, not powers) from the center-fed dipole.

In summary, it's not my preferred way to throw power away, but if you
can tolerate the losses, go ahead.

73,

--Ethan, K8GU (recently relocated back into MRRC country from
PVRC-land; dues, meeting attendance, and contest participation will be
forthcoming).


On Mon, Sep 16, 2019 at 12:01 PM James M. Galm, W8WTS <jim at w8wts.com> wrote:
>
> Hello Anthony, Bob, et al.,
>
> End fed half wavelength antennas will radiate, but their performance will essentially always be inferior to a center fed half wavelength antenna.  There are two main reasons for this.
>
> In order to radiate EM waves, there must be a complete circuit in which current flows where part of that circuit is a radiating structure (antenna).  This is easy to picture in a center fed half wavelength antenna, where any current flowing into one conductor of the feedpoint flows out of the other conductor, thereby making a complete circuit.  When you feed a half wavelength antenna at the end, the radiating structure has only one conductor.  You cannot make a complete circuit with only one conductor.  Any current flowing into the end fed half wavelength antenna must return to the source by some means via some other conductor (often called the couinterpoise).  The return current conductor will be the outside of the feedline, the matching network, the station, the ground, and any other conductor in the vicinity.  The return current conductor will not be controlled nor will it have a consistent impedance.  If you block the return current by adding sufficient choke inductance to the outside of the feedline, you will simultaneously block the ability of the half wavelength antenna to radiate.
>
> Second, the feedpoint impedance of an end fed half wavelength antenna is very high.  The matching network must be designed carefully because the high voltage needed to match such a high impedance is likely to saturate a ferrite transformer core and cause high losses.  Choking the counterpoise further increases the feedpoint impedance, exacerbating the matching difficulty and losses.  A non-ferromagnetic transformer core mitigates the saturation, but can be unwieldly.
>
> I hope this helps everyone understand some of the engineering aspects of an end fed half wavelength antenna.
>
> 73,
>
> Jim, W8WTS
>
>
> -----Original Message-----
> From: NCC [mailto:ncc-bounces at contesting.com] On Behalf Of Anthony Luscre
> Sent: Sunday, September 15, 2019 2:57 PM
> To: Robert Hayes <kw8n at aol.com>
> Cc: North Coast Contesters <ncc at contesting.com>; MRRC Mailing List <mrrc at contesting.com>
> Subject: Re: [NCC] EFHW? Any experience?
>
> I have used one for the last 3 years as part of my portable FD setup.
> I used the 132 feet MFJ-1982HP EndFed 1/2 Wave 800W 80M-10M Wire Antenna.
> It was up about 25 ft in surrounding trees. It worked well on 80, 40 & 20M.
> I had no problems with RF getting back into the radio, but as usual, I was only running 5 watts.
>
>
> On Sun, Sep 15, 2019 at 2:10 PM Robert Hayes via NCC <ncc at contesting.com>
> wrote:
>
> > Has anyone tried any of the End-Fed Half Wave antennas?
> > I have been a current feed kind of a guy, but this could handy for a
> > simple quick temporary all band antenna when travelling.  (And
> > recognizing the patterns on the higher frequencies will not be broadside to the wire).
> > 73 Bob KW8N
> > _______________________________________________
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> > NCC at contesting.com
> > http://lists.contesting.com/mailman/listinfo/ncc
> >
>
>
> --
> *Anthony Luscre*
>
> *K8ZT*
> Assistant Ohio Section Manager for Education Outreach ARRL - The National Association For Amateur Radio™
>
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>
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>
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