[NCC] [MRRC] EFHW? Any experience?

Tue Sep 24 17:25:18 EDT 2019

Bob, Bob, et al., 

At the end of the day, if it makes QSOs, it’s a great antenna.  

That said, I read the article by John Huggins, but found it flawed in several respects.  He begins with the assertion that it is possible for an EFHW to radiate with no current flow at all in the non-antenna side of the transmission line.  We know that power cannot be coupled from a transmission line to an antenna without current flow in a complete circuit.  Huggins suggests that no counterpoise is necessary and that the transmission line can, “push against the opposite terminal of whatever power source exists at the end (of the transmission line, ed).”  Huggins’ fails to realize that if a structure radiates EM waves containing power, then there is current flow across a voltage difference.  If only one conductor of the transmission line is connected, the return current will flow back to the source through some manner of “counterpoise”.  If the transmitter is made small enough and the feedline short enough, as he did in his experiments, the current will still find its way back to the source through any nearby conductor, the soil conductivity, the space around the transmitter, and anything else available.  Huggins’ EZNEC model does not include the current return means, therefore it is incomplete.  

Huggins used a clever instrument to measure the magnetic field a few inches away from the antenna to determine the performance of his test antennas.  Unfortunately, the near field distribution of E and H fields bears little relation to the far field power density or radiation resistance.  In the far field, EM waves caused by current in the antenna, current in the return path, current in the ground, current on the feedline, and current everywhere all combine through constructive and destructive interference to produce the ultimate power density on the spherical aperture.  This is why professional antenna designs are evaluated by placing a receiver a very long distance downrange of the antenna under test, the AUT is fed with a known power, and the AUT is rotated to map the power density.  

There is a well written and credible analysis of EFHW antennas on Tom Rauch’s web site, https://www.w8ji.com/2end-fed_1_2_wave_matching_system_end%20feed.htm, which I highly recommend reading.  Bob Brehm, AK6R (Palomar Engineers) has written some sound technical articles on EFHW.  

73, 

Jim, W8WTS

From: MRRC [mailto:mrrc-bounces at contesting.com] On Behalf Of Bob Liddy (K8BL)
Sent: Friday, September 20, 2019 1:54 PM
To: Robert Hayes <kw8n at aol.com>
Cc: ncc at contesting.com; mrrc at contesting.com
Subject: Re: [MRRC] [NCC] EFHW? Any experience?

Sometimes, as Hams, we just throw something up that is reasonably close

to an antenna and have fun seeing how it works. When I winter at my Daughter's

home in Louisiana, I put up an end-fed 124 ft wire with a 9:1 UnUn and work

literally all over the World on FT8 with an IC-703 at 10 watts. It starts at the

bottom of an MFJ 32 ft fiberglass mast and goes off across her backyard to

a fencepost 6 ft off the ground. There is a 60 ft counterpoise going off in the

other direction (kinda) and fed with 50 ft of RG-58. I tried a ground rod, but

the counterpoise works much better. This arrangement has been used on 160

through 15, depending on what has signals at the time - HIHI.

GL/73,    Bob  K8BL

On Friday, September 20, 2019, 10:24:59 AM PDT, Robert Hayes via NCC <ncc at contesting.com <mailto:ncc at contesting.com> > wrote: 

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 <mailto:ethan at k8gu.com> >

To: MRRC Mailing List <mrrc at contesting.com <mailto:mrrc at contesting.com> >

Cc: North Coast Contesters <ncc at contesting.com <mailto:ncc at contesting.com> >; Robert Hayes <kw8n at aol.com <mailto: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 <mailto: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 <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 <mailto:kw8n at aol.com> >

> Cc: North Coast Contesters <ncc at contesting.com <mailto:ncc at contesting.com> >; MRRC Mailing List <mrrc at contesting.com <mailto: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 <mailto: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 <mailto:NCC at contesting.com> 

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> >

> 

> 

> --

> *Anthony Luscre*

> 

> *K8ZT*

> Assistant Ohio Section Manager for Education Outreach ARRL - The National Association For Amateur Radio™

> 

> a at k8zt.com <mailto:a at k8zt.com>  (best for Education & Technology) k8zt at arrl.net <mailto:k8zt at arrl.net>  (best for Amateur Radio)

> 

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