ARLA/CLUSTER: The End Fed Half Wave Antenna

[Carlos Fonseca]

The End Fed Half Wave Antenna Steve Yates - AA5TB

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*E-mail <http://www.aa5tb.com/e_mail.html>
Last Update: January 5, 2010 *

*On this page I will try and describe what I know about End Fed Half Wave
Length Antennas. I am always learning so you may see this page change from
time to time as I learn more and correct errors. I don't expect anyone to
take this information as gospel. I just hope that it helps others
successfully use this type of antenna since it does have some unique
advantages. *

An End Fed Half Wave Length Antenna is a variation of the much more common
half wave length dipole <http://www.aa5tb.com/dipole.html> antenna. When an
antenna that is one half wave length long has RF energy applied to it at its
resonant frequency a standing wave
<http://en.wikipedia.org/wiki/Standing_wave>develops on it. This standing
wave consists of both current and voltage that are 90 degrees out of phase.
The end result is a distribution of current that is at a maximum at the
center and a distribution of voltage that is at a maximum at the ends
(fig.1).

[image: Dipole Voltage and Current Distribution]
*Figure 1 - Dipole Voltage and Current Distribution *

The most common method of feeding energy to this type of antenna is at the
center where the current is maximum and the voltage is minimum.
Consequently, the impedance at this point is low and on the order of 72
ohms. This makes it convenient to feed the antenna directly with low
impedance 50 ohm coax cable. To minimize the chance of common mode currents
on the coax that can cause the coax to become part of the antenna a
balun<http://www.aa5tb.com/balun.html>is sometimes used. Feeding the
antenna at the center is by no means a
requirement however. Energy can be fed anywhere along its length and the
impedance will increase as the feed point moves away from the center (more
voltage, less current). Taking this to the extreme is to feed the half
wavelength antenna at its end. At first thought this would seem to be
impossible yet many people have done it very successfully. In practice the
impedance at the end of an end fed half wavelength antenna is on the order
of 1800 to 5000 ohms.
------------------------------

It is often commented that the End Fed Half Wavelength Antenna needs no
"counterpoise" or radials to work. In practice this is often how the antenna
is used in the field. In reality something is always being used as a
"counterpoise" even though it may not be evident at first glance. There are
often heated arguments between folks who really only differed in whether
they sided with the practical aspects of using this antenna or the sterile
theoretical description of the antenna (fig. 2).

[image: No Can Do End Fed Half Wave Approach]
*Figure 2 - Theoretically Will Not Work *

Since myself and others have been very successful at doing the seemingly
"impossible", I will attempt to describe what I think is going on. Even
though there should be no limitation as to what frequency this antenna can
be used on my discussion here is mainly related to HF since this is where
this type of antenna is most practical. Also, keep in mind that I usually
use this antenna at low power levels (<20W). There are high voltage issues
to take into consideration when using higher power levels, mainly with the
capacitor in the tuned LC circuit. Don't try to run 1.5kW through a tiny
coupler <http://www.aa5tb.com/coupler09.jpg> using polyvaricons! There are
some distinct advantages to using QRP <http://www.aa5tb.com/qrp.html>. With
proper component scaling high power can be used. I have successfully used
100W without any problems with this
coupler<http://www.aa5tb.com/coupler3.html>.

------------------------------

Hams have been successful end-feeding resonant half wavelength antennas for
decades. Typically, a parallel tuned
circuit<http://www.aa5tb.com/coupler1.jpg>is used at the end of the
antenna with the feed line link coupled or tapped
to the coil in the circuit. I found empirically that the impedance of this
antenna is about 1800 to 5000 ohms when things are adjusted properly.
Antenna modeling suggests that the impedance is closer to 1800 ohms
depending on the "counterpoise" length used. As the "counterpoise" itself
approaches a half wavelength, the impedance approaches 5000 ohms. In the
graph below I have plotted what MININEC predicts for the impedance of an end
fed half wave antenna versus the "counterpoise" length. The antenna's length
(approx. 0.47 free space wavelength) was determined by modeling the length
required to obtain resonance (jX = 0) when the "counterpoise" was the same
length. In other words, it was modeled as two half wavelength wires
end-to-end and the length of each was kept the same but adjusted to obtain a
purely resistive match.

[image: Impedance versus Counterpoise]
*Figure 3 - Impedance versus "Counterpoise" Length for an End Fed Half Wave
Free Space Antenna*

[image: Two Half Wave Length Conductors End-To-End]
*Figure 4 - Two Half Wave Length Conductors End-To-End for Determining
Resonance in Modeling*

In figure 3, particular attention to the points where the impedance is
purely resistive (jX = 0 ohms) at 0.05 wavelengths and about 0.47
wavelengths. The *0.05 wavelength "counterpoise"* length is what I also
determined empirically as to being the ideal length for the counterpoise.
The coupler can be adjusted to compensate for any reactance that may be
present due to other lengths but it is around this length where the whole
antenna system is the most stable and provides a purely
resistive<http://www.aa5tb.com/efha_wrk.html>load to the coupler.
*Notice that there is nothing magical about the often recommended 1/4
wavelength "counterpoise" with this antenna!*

Below is the ideal setup for an end-fed half wavelength antenna that I have
determined through experiments and antenna modeling (fig. 5).

[image: Ideal End Fed Halfwave Antenna]
*Figure 5 - Ideal End Fed Half Wave Length Antenna*

The exact impedance of an end fed half wave length antenna has been debated
but if you design a coupler <http://www.aa5tb.com/efhw_05.gif> to match
close to the impedance range in figure 3 a good match can be obtained. Some
hams have had great success with this antenna with no feed line imbalance
problems (common mode currents) while others have had very bad "RF in the
shack" problems due to the feed line radiating as much as the antenna. If
you adjust your coupler <http://www.aa5tb.com/efha_wrk.html> into a
resistive load on the bench first and then adjust the antenna for a proper
match then you should have a resistive match that will minimize current
through the coupler and through the "counterpoise".

When you design a coupler for this antenna it helps to know what turns ratio
you should use. Use the formula below to determine the turn ratio. Based on
the predictions above a 6:1 to 8:1 turns ratio will be close enough. I have
often used a 10:1 ratio with acceptable results. The exact ratio will
probably be a function of the inductance value that you need on the
secondary to obtain resonance in the frequency range desired with the
capacitor that you have available.

Za = antenna's resistive impedance
then,
Turns Ratio = √(Za/50)
For example:
6:1 for Za = 1800 ohms
7:1 for Za = 2450 ohms
8:1 for Za = 3200 ohms
9:1 for Za = 4050 ohms
10:1 for Za = 5000 ohms

If you look at the predictions below (fig. 6) you will see that a ratio of
8:1 provides the most stable SWR versus "counterpoise" length. This is
assuming direct transformation of the antenna's complex impedance to 50
ohms. In reality, most couplers will be able to tune out the reactance to
achieve a much lower SWR but you should try to operate at the point where
the antenna's load is resistive, especially if the feed line and coupler are
the "counterpoise" as in figure 15. In circuits where the feedline is
physically isolated (link coupled, fig. 5) there doesn't seem to be much of
an issue.

[image: SWR versus Counterpoise for Various Ratios]
*Figure 6 - SWR vs "Counterpoise" Length vs Turns Ratio for an End Fed Half
Wave Free Space Antenna*

The graph below (fig. 7) is of actual measured data of an end fed half
wavelength antenna with an isolated coupler using an 8:1 turns ratio. The
coupler was tuned to resonance on the bench first using a resistive load.
Although not exactly the same, you can see that the data resembles the
predicted (red) data in the graph above (fig. 6), especially for the shorter
"counterpoise" lengths. I suspect that the difference in data is due to the
fact that the predicted data was for a free space antenna and the measured
data was for a real antenna at a low height in a relatively cluttered
environment.

[image: Measured SWR versus Counterpoise for 8:1 Turns Ratio]
*Figure 7 - Measured SWR vs "Counterpoise" Length vs Turns Ratio Data for an
End Fed Half Wave Antenna*

Keep in mind that even in the real measurement above the SWR could have been
brought down to 1:1 in most circumstances by simply adjusting the coupler's
tuned circuit and/or turns ratio but the idea is to show the result when the
coupler is fixed tuned at the bench first. With higher antennas I almost
always achieve a 1:1 SWR without retuning the coupler.
------------------------------

According to Moxon [1 <http://www.aa5tb.com/efha.html#ref>], because of the
very high impedance (i.e., very low current) at the end of a half wavelength
antenna, only a small counterpoise (1m @ 14 MHz) or a few pF of capacitance
to ground is required to return the current. However, in order to maintain a
balance an equal amount of length is required to be added to the antenna
itself. So even though the antenna is fed at the end, a good balance is
maintained. Below (fig. 8) is what I understand Moxon
[1<http://www.aa5tb.com/efha.html#ref>]
tries to describe.

[image: end fed Half Wave Balanced Feed]
*Figure 8 - Moxon's Ideal End Fed Half Wave Length Antenna (with a bit added
by AA5TB)*

In the above diagram you can see that the half wave portion of the antenna
presents very high impedance to the 0.05 wavelength section added and free
space presents very high impedance to the other end of the 0.05 wavelength
"counterpoise". Therefore the center fed portion between the two 0.05
wavelength sections is essentially balanced. With no physical connection to
the feed line common mode currents along the outside of the feed line will
be at a minimum. The main potential path for imbalance will be through
capacitance to the surrounding environment, just as is the case with any
dipole. Notice that the antenna is no longer a resonant length.

At first I agreed with Moxon because it made sense and my usual couplers
could easily tune out the reactance in the load. For a link coupled antenna
this would all work fine. However, I now believe that the way to minimize
common mode current in circuits where the feedline may not be totally
isolated is to use a *resonant* half wave length antenna adjusted to provide
a resistive load to the coupler. If this is done then the current into any
length of "counterpoise" is minimal except those lengths that approach a
half wave length. If the current is minimized on the "counterpoise", then
potential common mode problems are minimized too.
------------------------------

Below are some photos of an experimental setup for 20m that helped me
understand what is going on with end fed half wavelength antennas. I first
adjusted <http://www.aa5tb.com/efha_wrk.html> the LC circuit using a 4.7k
Ohm resistor (fig. 9) in the CW portion of 20m to obtain a 50 Ohm match
using my MFJ-259B Antenna Analyzer. When the antenna was properly installed
I adjusted its length along with a 1m long counterpoise to obtain the same
50 Ohm resistive impedance without having to retune the circuit (fig. 10).
At this point the antenna worked well. I then removed the 1m long
counterpoise and I was no longer able to obtain a match at any setting (fig.
11). Essentially, the antenna was not energized. I had taken measures to
prevent as much coupling as I could to the shield of the coax (no direct
connection, fig. 12, and ran the coax 90 degrees to the antenna). There may
have been some stray capacitance but there was not enough to "complete the
circuit". It should be noted that increasing the counterpoise beyond 1m made
no noticeable improvement. Also, if this method of adjusting the tuned
circuit (using resistor first) <http://www.aa5tb.com/efha_wrk.html> is not
used then any deviation from the optimum antenna length will cause an
increase in return currents in the "counterpoise" making the "counterpoise"
requirement more critical. I believe this accounts for some of the failures
that have been described in ham reports that were supposedly corrected when
a 1/4 wavelength radial was added. Any deviation from the resonant antenna
length will require a similar increase in "counterpoise" requirement. This
is not to say that the antenna cannot be used at other frequencies. If you
do plan on operating over a large range of frequencies then the
"counterpoise" requirement increases but a match can usually still be
obtained by retuning the coupling circuit.

[image: Proper Tune Diagram]
*Figure 9 - The Proper Method of Tuning an End Fed Half Wave Length Coupler
*

[image: Proper Setup]
*Figure 10 - Proper Feed - 1m return (counterpoise). *

[image: Improper Setup]
*Figure 11 - Improper Feed - No counterpoise at all, no worky. *

[image: Transformer Setup]
*Figure 12 - Close-up of Transformer *

[image: Capacitor]
*Figure 13 - Close-up of Capacitor *

[image: Properly Adjusted]
*Figure 14 - Match when properly adjusted with 1m counterpoise *
------------------------------

In many configurations of the end fed half wave antenna a counterpoise is
not used at all. So how is this possible? The test above proved that this is
not possible. However, in the real world it is possible to connect the
return side of the LC circuit to the ground side of the feed line as shown
below (fig. 15). Even though no apparent "counterpoise" is connected the
feedline and/or rig is actually used as the "counterpoise". This is often my
arrangement for field work <http://www.aa5tb.com/coupler.html>.

[image: Return via coax and rig ground]
*Figure 15 - Return via Coax and Rig *

In some cases simply the stray capacitance of the LC circuit to the
environment and feed system will provide enough counterpoise as shown here
(fig. 16).

[image: Return via Capacitance]
*Figure 16 - Return via Stray Capacitance to Environment and Feedline *

Obviously the above two configurations appear to have the potential for the
dreaded common mode currents if some sort of choke balun is not used.
However, due to the very high impedances involved currents levels are very
low when the antenna is of the proper length and when the LC circuit is
adjusted properly decreasing common mode currents. Given that the impedance
is high designing a choke balun with enough impedance to be effective may be
difficult anyway. However, if this
setup<http://www.aa5tb.com/coupler08.jpg>is being used in the field
with very short or no coax (direct attachment to
rig) then any common mode currents that do exist will be negligible. Only
when the "counterpoise" (and/or feed line common mode path) becomes near a
half wave length itself will the return currents equal that of the antenna.
Another possible problem scenario could be when the antenna is fed through a
1/4 wavelength of coax to a grounded rig. In most field setups where I use
this type of antenna neither of these two cases occurs. It should be noted
that in these cases adding the often recommended 1/4 wavelength radial will
not necessarily alleviate the problem either. The 1/4 wavelength radial
recommendation often given is based on the assumption that a high impedance
at the open end of the "counterpoise" will present a very low impedance at
the return side of the coupler. This is indeed true but not required.
Remember, the impedance is relatively high at this point so a very short
"counterpoise" is all that is required. A longer "counterpoise" offers
nothing to the performance and this can be verified by computer modeling. If
the coupler and end fed half wave antenna are
properly<http://www.aa5tb.com/efha_wrk.html>adjusted then you should
have no problems using a very short "counterpoise"
or depending on stray capacitance for the return.

So far this discussion has had the antenna floating in space for the most
part. Down at earth it can be used in any configuration that an ordinary
center fed dipole can be used. One common orientation is vertical. In this
case many people say that a large radial system consisting of up to 120
radials are required just as is required for a 1/4 wavelength vertical (see
below <http://www.aa5tb.com/efha.html#clar>). As far as completing the half
wavelength antenna is concerned there is no difference between vertical and
horizontal orientation! No radials are required. However, an improvement of
the ground system below ANY antenna (except maybe a Beverage
antenna<http://en.wikipedia.org/wiki/Beverage_antenna>)
will help its overall performance. The closer any antenna gets to the ground
the more current is induced into that lossy ground. Any improvement that you
make to this lossy ground will improve this situation. There is just no need
to unduly require it to "complete" a half wave length vertical, whether fed
at the end or anywhere else. Once again the "counterpoise" requirements are
the same as above. There have been many manufactures over the years that
built half wave vertical antennas that had only a few short radials. For
VHF, anyone who has ever used the old AEA Hot Rod end fed half wave antenna
for an HT know the drastic improvement it made over a 1/4 wave antenna.

Here are some other examples of the end fed half wave antenna. The LC lumped
component tuned circuit described above can be replaced by a 1/4 wave length
of shorted transmission line (stub) as shown below (fig. 17).

[image: LC Circuit Replaced with Stub]
*Figure 17 - Lumped LC Circuit Replaced with 1/4 Wave length Stub (End Fed
Zepp) *

If you take this a few steps further you can see how the antenna evolves
into a classical J-Pole antenna that is often used at VHF with good success
(fig. 18). This shows the general idea anyway. N3GO has a much more detailed
description of the J-Pole antenna
here<http://snow.prohosting.com/~w0rcy/Jpole/jpole.html>.


[image: LC Circuit Replaced with Stub]
*Figure 18 - J-Pole Antenna *
------------------------------

There are many ways to configure an end fed half wave length antenna.
Another very effective DX antenna is a ground mounted half wave length
vertical antenna as shown below (fig. 19). A single ground rod will often
suffice for a ground system to complete the circuit since very little
current has to flow through this ground system.

[image: Ground Mounted Vertical Half Wave]
*Figure 19 - Ground Mounted Vertical Half Wave Length Antenna *

The graph below (fig. 20) gives an example of the feed point impedance of a
vertical monopole versus its height in terms of wave length. The ground
plane is assumed to be perfectly electrically conducting (PEC). Notice the
points where jX = 0 ohms. These are the points where the antenna is
resonant, at 0.25 wave lengths, 0.47 wave lengths, and again at 0.74 wave
lengths. The main difference between the points is the impedance. For a
height of 0.25 wave lengths the resistive impedance is about 35 ohms and a
height of 0.5 wave lengths it is about 2450 ohms.

[image: Impedance versus Monopole Height]
*Figure 20 *

If you design a coupler to feed an impedance of 2450 ohms (7:1 turns ratio),
then the expected SWR for a vertical monopole operated against ground is
shown below (fig. 21). Note that a perfect match occurs when the vertical is
about 0.47 wave lengths tall.

[image: SWR versus Monopole Height]
*Figure 21 *
------------------------------
Ground Losses

Here are my thoughts (right or wrong) about ground losses with end fed half
wave vertical antennas. Let's say you just use a ground rod for a return. To
return the displacement currents that enter the ground from the field around
the antenna there will be a high earth resistance with such a simple ground.
Let's use 35 Ohms for an example. For the 1/4 wave length vertical with 35
Ohms of feed point resistance, the input power will be divided between the
ground resistance and the feed point resistance. The two resistances will
add up to a total feed point resistance of 70 Ohms but the efficiency of the
system will only be 50%. For the 1/2 wave length vertical with an assumed
feed point resistance of 2450 Ohms the feed point resistance will be 2485
Ohms (2450 + 35) and the power dissipated in the earth at the feed point
will only be 1.4%. This gives us an efficiency of 98.6% for the 1/2 wave
length vertical. With an efficiency of 98.6% I don't see any reason to lay
down an elaborate radial system for a 1/2 wave length vertical from an
efficiency point of view. However, at about a 1/4 wave length from the base
of a 1/2 wave length vertical the ground currents once again increase. Of
course this pattern will repeat itself out for many wave lengths from the
antenna. For this reason a bunch of (>1/2 wave length) radials should make
an improvement on ground wave field strengths or on very low elevation sky
wave paths by lowering the pseudo-Brewster angle. However, I am skeptical
about how much of these ground (or surface) wave currents, if any, return
back to the feed point and contribute to the feed point losses. In any case,
to reiterate my earlier statement, improving the ground conductivity out for
many wavelengths will improve the far field of any antenna, at least at some
elevations. This does not mean that an elaborate ground is required for an
end fed half wave length vertical antenna to "have something for the antenna
to push against" or to "complete the circuit" as is often said. Whether or
not you consider ground loss several wave lengths from the antenna as
antenna system loss or not is up to you.
------------------------------

I hope that I have shown that feeding a half wave length antenna on its end
without an extensive ground system or "counterpoise" is practical and it
really works. I use this type of antenna extensively in the field with my
little single band QRP rigs. For example, I have found that my 20m vertical
end fed half wave antenna <http://www.aa5tb.com/coupler01.jpg> with the
bottom 1m from the ground is an extremely effective antenna. During Field
Day activities I can usually rack up 500 contacts using only 4 or 5 Watts.
DX is very easy with this arrangement as well. Give it a try!
------------------------------
Related Links

   - End-Fed Half-Wave Antenna &
Tuner<http://www.vk2zay.net/article.php/115>- by VK2ZAY
   - From a J to a Zepp
<http://snow.prohosting.com/~w0rcy/Jpole/jpole.html>- The truth and
its consequences - Gary E. O'Neil, Raleigh, N.C. (N3GO)
   - An End Fed Half Wave Antenna <http://www.g3ycc.karoo.net/endfed.htm>,
   by the late G3YCC
   - End Fed Antenna Ideas For Fixed Or
Portable<http://www.g3ycc.karoo.net/endfed2.html>,by
   G3WQW
   - J-Pole Antennas <http://www.vk1.wia.ampr.org/bulletins/jpole.html> - by
   Mike Walkington, VK1KCK
   - Portable End Fed Halfwave
Antenna<http://www.qsl.net/oe3mzc/hlfewve.htm>- by OE3MZC
   - A Tiger by the Tail <http://www.cebik.com/gup/gup12.html> - by L. B.
   Cebik, W4RNL
   - Vertical Dipoles and Ground Planes
<http://www.cebik.com/gp/vdgp.html>- by L. B. Cebik, W4RNL
   - end fed vertical j-pole and horizontal
zepp<http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm>-
A reference of End Fed
   *Myths* - by W8JI (he'll dispel any believe that an antenna might work
   ;-)

------------------------------
References

[1] *HF Antennas for All Locations*, L.A. Moxon, RSGB, 1990, pgs. 43,46.
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