Keep in mind that the original End Fed Half Wave antenna was the
original Zepp antenna.
A halfwave radiator that was fed from one side of a quarter wave
transmission line.
That antenna was trailed from a Zeppelin airship. The 1/4 wave
transmission line reduced the high impedance of the halfwave antenna to
a low impedance (see transmission line characteristics). The ideal
characteristic impedance for the 1/4 wave transmission line is 300 ohms
- see the work of N3GO - "the J-Pole according to N3GO" at
http://knightlites.org/n3go_workshop/index.htm.
Of course, the transmitters in those airships had adjustable amplifier
tank circuits which could match almost anything except a very high
impedance - that function has been moved to the ATU in recent times and
the PA output stage is designed to work into 50 ohms.
If you want a modern-day example of the EFHW fed with 1/4 wave
transmission line, consider the J-Pole antenna that is popular for VHF -
it is nothing more than the original Zepp antenna oriented vertically.
I have no idea how the "Zepp antenna" designation has been construed in
amateur radio circles to apply to a balanced center fed non-resonant
radiator but it has been so construed that we have to be careful when
saying "I have a Zepp (or Extended Zepp) antenna - it no longer means
what it originally did.
73,
Don W3FPR
On 7/14/2016 8:38 PM, Walter Underwood wrote:
I totally agree. End-fed half waves are tweaky. One terminal is the antenna
wire and the other terminal must be something. The RF will find something to
drive as the other “pole” of the antenna. There are no monopole antennas.
It could be the outside of the coax, it could be a “counterpoise” (not fond of
that term), or it could be your hand on the key. Ouch.
I was using “ground reference” as the negative side of what you measure RF
voltage against. Whatever that is.
There might be an antenna topic with more voodoo than end-fed antennas, but I don’t
know what it is. The EC-130J flies a long wire out behind it and broadcasts on medium
wave. I guess the other pole of that antenna is the airplane itself.
http://www.af.mil/AboutUs/FactSheets/Display/tabid/224/Article/104535/ec-130j-commando-solo.aspx
<http://www.af.mil/AboutUs/FactSheets/Display/tabid/224/Article/104535/ec-130j-commando-solo.aspx>
wunder
K6WRU
Walter Underwood
CM87wj
http://observer.wunderwood.org/ (my blog)
On Jul 14, 2016, at 5:14 PM, David Gilbert <xda...@cis-broadband.com> wrote:
OK, I can buy that ... except that the term "ground reference" as used by you and Don
doesn't really apply. The counterpoise is simply providing some minimal balance to the high
impedance at the end of the EFHW, and it doesn't need to have anything to do with
"ground".
If the counterpoise is located at the rig (as I inadvertently thought Don was
suggesting) instead of at the end of the antenna then indeed the feedline would
radiate, and if located at the rig the impedance transformation effect of the
transmission line (depending upon it's length) could easily put a low impedance
there ... rendering a short counterpoise essentially ineffective.
For the record, I know how electricity works.
Dave AB7E
On 7/14/2016 3:42 PM, Walter Underwood wrote:
The transmitter needs to drive into two terminals. That is how electricity
works. If you do not provide a ground or a capacitive RF ground (counterpoise),
the ground is the chassis of the transmitter. In a big grounded shack, that
might be a stable reference. Otherwise, you need to provide a ground reference.
Without that, the feed impedance changes every time you touch the rig.
The ground reference does not need to handle much RF current, because of the
high impedance feed (around 10 kOhms). Even a little capacitance to ground will
stabilize the feedpoint impedance.
A minimum length of coax feedline would reduce the SWR due to losses, but it
would also provide an RF ground reference with currents on the outside of the
shield. Currents on the inside of the shield are balanced, of course.
Walter Underwood
wun...@wunderwood.org
http://observer.wunderwood.org/ (my blog)
On Jul 14, 2016, at 2:40 PM, David Gilbert <xda...@cis-broadband.com> wrote:
I'm confused why an EFHW should need a counterpoise. If it needs a
counterpoise it isn't actually acting like an EFHW. If it needs a counterpoise
that means there isn't enough choking impedance at the feedpoint, and it means
that the feedline is radiating with the counterpoise acting as ... well, a
counterpoise.
Manufacturers state a minimum length feedline simply to have the feedline
losses help swamp out SWR variations along the line, and of course to
marginally lower the SWR.
Dave
AB7E
On 7/14/2016 1:03 PM, Don Wilhelm wrote:
Andy,
What happens if you connect to a dummy load? Does the KX2 tune it properly?
If so, then you know the KX2 is operating into a 50 ohm load OK.
Do you have an antenna analyzer? If so, then connect it to the antenna
feedline and look at the impedance - both real and reactive - as well as the
SWR.
That will tell you whether the antenna is troublesome.
You specifically said an EFHW - that should be a resonant antenna. If instead
you have a random length wire (not a halfwave), and what is the matching device?
Many EFHW antennas need a counterpoise to work - that is why many manufacturers
state a *minimum* length feedline.
If the problem is *not* the antenna, contact Elecraft support.
73,
Don W3FPR
On 7/14/2016 2:40 PM, Andy Clift wrote:
Just received my KX2, but unable to get it to tune an EFHW wire. I’ve tried a
home made one and then tried a SOTABEAMS MultiBander that I’ve used
successfully with an FT-857. I’ve checked the continuity in the wire and also
the continuity in the KX2 between the BNC connector and where the wire connects
inside the KX2 (as I removed this to fit the end panels). Battery is at over
11v. Any ideas please?
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