ARLA/CLUSTER: The End Fed Half Wave Antenna

[Carlos Mourato]

Ora aqui está um exemplo de uma boa matéria divulgada no cluster. É para
estas e outras materias dentro do contexto que este cluster existe.
Muito interessante Carlos.

73 de CT4RK



2011/2/26 Carlos Fonseca <ct1gfqgrupos  gmail.com>

> The End Fed Half Wave Antenna Steve Yates - AA5TB
>
> HOME <http://www.aa5tb.com/index.html>
>
> *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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>


-- 
*Best 73 from: regards from: CT4RK Carlos Mourato - Sines - Portugal*

 *Warning*
*Save the Radio Spectrum! Eliminate Broadband over Power Line. *
*
*
*Salve o espectro electromagnético!. Não use a rede electrica para
transmitir dados. Os "homeplugs power line" e a tecnologia "power line"
causa fortes interferencias noutro serviços sem voce se aperceber. Diga não
à tecnologia power line. Proteja o ambiente electromagnético. Utilize
tecnologia de redes sem fio, denominadas wireless.*
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