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Author Topic: End-fed shortened Zepp with no radials: how does it work?  (Read 2389 times)
JAHAM2BE
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« on: May 02, 2013, 09:09:11 PM »

I've been looking at some of N0LX's interesting portable antenna designs, which are end-fed vertical wires with two loading coils. They sort of look like a loaded dipole, except they are end-fed.

Examples:
http://www.n0lx.com/efhwa.html
http://www.n0lx.com/T_20_20.html
http://www.n0lx.com/short4band.html
http://www.n0lx.com/3band_toroid.html

He matches these with an end-fed-halfwave matching unit: http://www.n0lx.com/efhwa_160.html

I have a couple of questions about this kind of antenna design, which seems rather unusual (at least, I can't find any other web pages with similar designs):

1. Does this kind of end-fed antenna require a "counterpoise" or radial wire, or not? It seems that N0LX doesn't use a counterpoise, as far as I can tell from the photos. In that case, the case of the rig will be hot with RF, right? The electrons pushed into the antenna have to come from somewhere. I suppose this isn't a problem at QRP levels.

2. Would a capacity hat help with this kind of antenna? If so, do you need one at each end, or only at the top of the antenna?

3. Electrically, how is this antenna different from a typical loaded "vertical" antenna that needs to be worked against a ground? Do the dual loading coils in the "shortened EFHW" design somehow make it ground independent, unlike a vertical?

Thanks for any advice.
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KB4QAA
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« Reply #1 on: May 02, 2013, 09:27:05 PM »

To quote a song from the 1920's "Which is the rooster, which is the hen?"

These days people will call an antenna they design by any old name according to what they wish it was, or what they want to fool people into thinking it is.  Undecided

If an antenna is not physically a half wavelength long, it isn't a half wave antenna!!
-These examples are 'Loaded dipoles' or "Loaded verticals".
-Not saying they don't work.

This is one of the big falsehoods that unscrupulous manufacturers foist on hams with little experience.  The classic approach is to wind say 10m of wire onto a 6ft glass pole and call it a 20m half wavelength antenna.  It's not.  It's a 6ft antenna with a lot of wire.  (compare to a certain popular vertical manufacturer with a lizard's name).

-Modifications to these antennas:  You are on your own. Do your own calculations, modeling and empirical measurements!

-Most End Fed Halfwave antennas I have seen don't use a counterpoise.  But, honestly, what antenna won't work better with more wire, in the right places??  But consider, once we start adding counterpoise(s) to this antenna, aren't we better off to just make a dipole or a vertical with some radials?  Smiley

-Loading coils help shorten an antenna.  They don't improve performance (gain), and they can't possible replace radials/counterpoises. Ever.

p.s.  In your title, you asked about End Fed Zepps.   None of these antennas are EFZ's nor do I think the original author calls them that.  To review, and End Fed Zepp is a Halfwave wire, fed with a (vertical) 1/4wl open wire line.  Period.  Change the length or the feed line and it isn't a Zepp anymore.  tah-dah.
« Last Edit: May 02, 2013, 09:31:46 PM by KB4QAA » Logged
K5LXP
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« Reply #2 on: May 03, 2013, 08:49:46 AM »

Bravo on your reply, QAA.  To underscore your point:

Quote
If an antenna is not physically a half wavelength long, it isn't a half wave antenna!!
...
Change the length or the feed line and it isn't a Zepp anymore.  tah-dah.

If you add a "counterpoise" to an end fed half wave antenna, it's no longer an end fed half wave antenna.  It's just an offset-fed somewhat longer than a halfwave antenna, a half wave vertical with a really short radial, or something in between.


JAHAM2BE - It's a given that with an end fed half wave that it will be difficult to eliminate common mode.  But at a practical level they can work pretty well and offer features that make them a useful antenna.

Besides changing the nomenclature and topology of the antenna, if you add loading to a half wave (L or C) you will also lower the feed Z which will pretty dramatically affect how you will feed/match it, so the traditional half wave tuner/matching section will no longer work.


Mark K5LXP
Albuquerque, NM
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WB6BYU
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« Reply #3 on: May 03, 2013, 09:48:30 AM »

If one defines* "half wave resonance" to mean a resonant condition where there is a single
current maximum in the electrical center with high voltages at the ends (similar to a half wave
antenna), then these antennas are "half wave resonant", and will have the same radiation
pattern and properties regardless of where they are fed:  center, off-center, or at one end.

*  Whether this is a useful definition, or one that makes any sense at all, is left for a
separate discussion.  But I think it helps to understand how the antenna operates without
getting overly bogged down in semantics.


A classic example of this type of antenna is the Cushcraft R7:  in that case the "radials" on
the bottom are actually a capacitance hat.  The antenna is fed off-center (essentially between
the capacitance hat and the vertical portion) where the impedance is lower than at the very
end, so more suitable for a step-up transformer.  The original R3 version used a motorized
variable capacitor to adjust a parallel-tuned circuit at the base, with true end-feed, but the
tuning range was somewhat limited.  (The R3 only covered 10 / 15 / 20m.)



Quote from: JAHAM2BE

1. Does this kind of end-fed antenna require a "counterpoise" or radial wire, or not? It seems that N0LX doesn't use a counterpoise, as far as I can tell from the photos. In that case, the case of the rig will be hot with RF, right? The electrons pushed into the antenna have to come from somewhere. I suppose this isn't a problem at QRP levels.



Such an antenna WILL have a counterpoise, whether we provide an intentional one, or
leave it off and let the coax shield serve by default.  Because of the high impedance there
is less current flowing at the feedpoint, so the counterpoise does not need to be as
elaborate as would be the case for a quarter wave vertical.

Yes, you can still have problems with RF in the shack at QRP levels, but how much of a
problem it is depends on many factors, including the coax length.  Typically I've had more
issues with end-fed antennas when the tuner is connected through a length of coax rather
than having the tuner right beside the rig, though in many cases the ill effects are not
noticeable.



Quote

2. Would a capacity hat help with this kind of antenna? If so, do you need one at each end, or only at the top of the antenna?



A capacity will have the same effect as for the same antenna fed in the center:  it allows you
to shorten the antenna and/or use smaller loading coils.  The principle is the same as for the
short dipole you have been considering.



Quote

3. Electrically, how is this antenna different from a typical loaded "vertical" antenna that needs to be worked against a ground? Do the dual loading coils in the "shortened EFHW" design somehow make it ground independent, unlike a vertical?



I would compare it instead to a shortened vertical dipole:  it's the same antenna with the feedpoint
shifted.  (Shifting the feedpoint location will change the radiation pattern of an antenna with multiple
current maxima, but not with "half wave resonant" antennas.)

Compared to a "quarter wave resonant" antenna fed against ground, the efficiency is improved because
any series loss resistances are small relative to the feedpoint impedance.  (Note that the radiation
resistance
is still the same as for the equivalent dipole, in spite of the higher feedpoint impedance.)
That doesn't mean it is "ground independent", or "has no ground losses", as there are still losses due
to the vertically polarized wave propagating over ground with poor conductivity.  And the high impedance
ends also have some fields that extend into the dirt.  But the component of ground loss resistance due
to ground return currents that appears in series with the feedpoint impedance has less impact on overall
efficiency.

For a specific maximum antenna height, it isn't entirely clear which would give the highest radiated power,
as the "half wave resonant" version requires larger loading coils (effectively squeezing twice the
electrical length into the same physical length) and therefore higher coil losses.  The answer will depend
on the extent of the radial field:  for a temporary portable antenna where it isn't convenient to lay a
good ground radial system, the "half wave resonant" antenna may work better.
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W5WSS
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« Reply #4 on: May 03, 2013, 11:14:08 AM »

The resonant 1/2 wave end fed vertical (Wire Zep) for portable use can be decoupled relative to problematic common mode displacement currents  although doing so seems burdensome the return in performance for the work done is worth it.

I operated portable from Tennessee and worked with, investigated, and evaluated this type of both resonant 1/2  wave wireand (non resonant vertical End fed using an L network) and 1:1 current balun together located at the slightly elevated feed point.

Obviously when feeding an end fed 1/2 wave zep and the second part or a radial system such as a flat plane horizontal system of radials is not included then
 Kirchoffs Law will mandate that the rf current and voltage find and involve things" to satisfy the Law to complete the cycle. Called insisted effort towards Balance.

Speaking of voltage and current it is very high with the 1/2 wave end fed Zep.

The end fed 1/2 wave when all the decoupling is done properly develops a pattern and radiates in it's own right.

The real question is pattern development of verticals in general and whether or not the perceived or real of being a 1/2 over some other length such as far less problematic 1/4 wave ever really manifests an advantage over the 1/4 wave in the first place.

With verticals, Secondary lobes can form with relationship to earth surface and really we can utilize this to our advantage.

The question boils down to what we value as pattern for our specific coverage area whether it is worlwide or not etc.

I am not excluding omni directionality but speaking to pattern development relative to power manifested at one dominant trajectory vs subdividing it into two or more trajectories upwards towards the ionosphere.

As Dale pointed out that is an ongoing discussion as to whether we want one trajectory upwards with all the power within it or whether we want more trajectories upward attempting to strike a balance in either case higher angle inclusion is a user choice but often time includes unwanted propagated noise and legitimate signals mixing with other skywave from lower and usually more challenging to hear.

So height matters relative to verticals too and pattern development that which we can intentionally choose to perform a utility.

I found that elevated tuned pairs of radials sloping downwards broadside to the vertical meaning symmetrically routed were very effective and Dc isolated completed the system and when I made the vertical length held to a range of 1/4 to 5/8 wave from the fixed base height of 4-5 ft sloping the tuned radials downwards to the intersect point at ground along with the L network and 1:1 current balun at the feed point I had an excellent hf skywave dx antenna for multi band service.

I found that the antenna was no longer an end fed but rather more balanced relative to feed point position which moved much closer to center.

The system cost about $20 and very gratifying I hope this experience helps with your technical pursuits and evaluations.
 73

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RFRY
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« Reply #5 on: May 04, 2013, 04:15:01 AM »

...The real question is pattern development of verticals in general and whether or not the perceived or real of being a 1/2 over some other length such as far less problematic 1/4 wave ever really manifests an advantage over the 1/4 wave in the first place.

The link below compares the elevation patterns of 1/4-wave and 1/2-wave verticals when both are driven against a set of four elevated radials, each 1/4-wave long.  These are the radiation patterns actually launched by these verticals, as they exist a few wavelengths away from them.
 
For the same matching loss, this elevated counterpoise provides the same radiation efficiency as if these verticals were driven against a set of 120 x 1/4-wave buried radials.

The 1/2-wave vertical has about 1.25 dB more gain in the horizontal plane than the 1/4-wave vertical, but has less gain than the 1/4-wave at higher elevation angles.

Of the two, the feedpoint Z of the 1/4-wave would be easier to match to 50 ohm transmission line.

http://postimg.org/image/6wcycaiuh/

R. Fry
« Last Edit: May 04, 2013, 04:22:28 AM by RFRY » Logged
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