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Refining an End Fed AntennaWard Harriman—AE6TY [email protected]
34 · Winter 2013 The QRP Quarterly www.qrparci.org/
Ihave always been a fan of end fed anten-nas. Why? They are easy to deploy
because the feed line doesn’t weigh downthe middle of the antenna. You can feed atthe bottom of a vertical or one end of a hor-izontal. You can even hang the antennafrom your hotel room balcony… and thereare never any counterpoises or radials toworry about.
This interest in end fed antennas caus-es me to wander around the web lookingfor ideas on what designs are available andhow best to construct them. There are awide variety of popular designs includingautotransformers, Zepps, and JPoles. Oneof the less common end fed antennas iscalled a “Resonant Feed Line” antenna andwas described by James Taylor (W2OZH)in a QST article back in ‘91. Over theyears, I have found lots of articles aboutthis antenna: how it operates, how to buildit, and what to expect. In assimilating allthese articles, I slowly came to the conclu-sion that this antenna was misunderstoodand so I decided to try to design one myselfstarting from scratch. And perhaps, add atouch or two of my own.
Getting startedWhenever I start out an antenna project
I fire up my antenna modeling software.Usually, this is EZNEC (by W7EL)because it is so simple to use. There is afree version of EZNEC which is adequatefor the purposes of this project.
Nearly all of my portable operationstake place on the 20 meter band and so I’lltalk about my 20m antenna here. TheResonant Feed Line antenna (here-afterRFL) is a half wave dipole. Electrically,this antenna is ‘center fed’. So to get ahandle on what I’m looking at, I will startwith a vertical half wave antenna driven atthe center. Figure 1 is the EZNEC drawingof the antenna. For modeling, the antennais described by a single wire as shown inFigure 2.
Once I’ve described the antenna, I runa frequency sweep to compute theimpedance the antenna provides across therange of frequencies. As a confirmationI’m on the right track, I check the SWR tomake sure things are basically correct.Figure 3 shows the resulting SWR plot.
The minimum SWR is at 14 MHz with animpedance of 93 ohms.
The Resonant Feed LineThe RFL antenna is unique in ‘end fed’
antennas in that it is truly a ‘center fed’antenna. It is called ‘Resonant Feed Line’because the feed line is actually part of theantenna. To see how this works, consider apiece of coax. We generally think of coaxhas have two conductors but at radio fre-quencies, the coax actually has three: thecenter conductor, the inside of the braidedshield, and the outside of the shield. Forthe RFL antenna, the center conductor andthe INSIDE of the shield are, in fact, thefeed line while the outer surface of the
coax is a radiating part of the antenna. It might help to walk through where
various currents flow. Referring to Figure4, the feed line begins at the left. Feed cur-rents are injected onto the inner surface ofthe shield and the center conductor. Thesecurrents proceed down the inside of thefeed line until they reach the midpoint ofthe antenna. At this point, the current onthe inside of the shield does a ‘U-turn’ andgoes back to the left on the outside part ofthe coax shield. The current on the centerconductor proceeds to the right.
From an RF radiation perspective, thecurrents on the center conductor and theinside of the shield cancel each other outand generate no radiation. However, thecurrent on the conductor to the right of themid point and the current on the outside ofthe shield do NOT cancel out and, as aresult, radiation is created.
Impedance Matching the AntennaHaving obtained the impedance data
from EZNEC I can see that this antennahas an SWR below 2 to 1 over an extreme-ly limited bandwidth. I would like to makethis bandwidth much wider for two rea-sons. First, my portable rig works muchbetter with a reasonable impedance match.Second, since I deploy my antenna in somany different environments, I would liketo have a good match even when the anten-
Figure 1—EZNEC drawing.
Figure 2—”Wires” definition in EZNEC.
Figure 3—VSWR plot of the antenna model.
www.qrparci.org/ The QRP Quarterly Winter 2013 · 35
na is detuned. How should I go about matching the impedances?I could, of course, simply use a portable antenna tuner. However,this tuner is an extra box to carry around and causes additionallosses in power. I’d like to leave the tuner behind if possible.
One of the easiest ways to solve matching problems is to usea Smith chart. The Smith chart allows us to view the impedancedata graphically and to create simple circuits which will affect thematch. Figure 5 is a Smith chart of the impedance data obtainedfrom EZNEC. The goal of matching is to move as much of thetrace from EZNEC to ‘inside’ the 2:1 SWR circle. There are manytutorials (including several on my web side www.ae6ty.com)describing a wide variety of techniques for doing this match. Hereis how I did it in this case.
My matching is done by using the feed line itself to first rotatethe impedance curve around the center of the Smith chart. Since Ialready have a feed line, using it to rotate the impedance curve is‘free’. Figure 6 shows an updated Smith chart with the impedancedata rotated clockwise around the center of the chart using the
feed line. It is important to note two things about this ‘rotation’.First, the impedance curve now looks substantially different; theoriginal curve did not curl back on itself. We will take advantageof this curl later.
The second thing to note is that while the shape of theimpedance curve has changed, the shape of the SWR curve hasnot. I won’t bother to show the graph but trust me, it didn’tchange.
Now comes the real matching part, which is illustrated inFigure 7. Remember, using the feed line, I rotated the impedance
Figure 4—Current flow on inside and outside of coax shield.
Figure 5—Smith chart plot of feedpoint impedance.
Figure 6—Impedance plot “rotated” by additional line length. Figure 7—Impedance shifted by a shunt capacitor.
36 · Winter 2013 The QRP Quarterly www.qrparci.org/
curve to a special place on the Smith chart.I can now move the impedance curveDOWN to the center using a single, shuntcapacitor. A significant piece of the curvenow lies within the SWR 2:1 circle. Figure8 shows the data on an SWR chart. TheSWR is below 2:1 from 12.7 MHz to 15.85MHz. (Remember that curl that occurredbecause of the feed line? Well, that curl hashelped make even more of the curve fitwithin the 2:1 SWR circle.)
Figure 9 shows how the capacitor isattached.
Construction of the Basic RFL AntennaMy antenna is constructed using
RG174 coax. This coax is small and veryflexible. Some folks frown on it because itis considered lossy. In this antenna, I’vecalculated that I lose about 0.4 dB in thecable.
Since the antenna is a half wave lengthlong, I will need somewhere around 34 feetto make my antenna. Employing the stan-dard practice, I will leave a couple extrafeet in length because it is always easy toremove length… adding it is painful.
The first step in constructing the anten-
na is to cut a piece of RG174 to 15.4 feetlong. These 15.4 feet will be the ‘feedline’ which goes from my rig up to the‘center’ of the antenna. Of course, 15.4feet is NOT the center of the antenna, butI’ve found it is close enough for these pur-poses. At one end of the feed line I put mycapacitor and my connector. I use BNCconnectors. Figure 10 is a picture of therig end of my antenna constructed usingBNC plumbing hardware. The T is a‘shunt’ so the feed line, the capacitor andthe AIM4170 are all connected in parallel.This approach lets me experiment withcapacitor values but is really too expen-sive and fragile for deployment.Ultimately, the end of the feed line will beconstructed as shown in Figure 11.
Figure 12 shows the other end of thefeed line. Here, the center conductor of thefeed line is connected to the shield of theother half of the coax which extendsupwards another 18 feet or so completingthe half wave dipole. In this way, the radi-ating currents always flow on the outersurface of the shield.
Having constructed the antenna it istime to tune it up. I do this in two stages.
In the first stage, I leave the matchingcapacitor out and adjust the length of theantenna in the traditional way: I loft theantenna and measure the frequency of theminimum SWR. If the frequency is too low(which it will be if I start out with such along dipole), I take it down and shorten thedipole. Make sure not to cut off pieces ofthe feed line, it should remain 15.4 feetlong through this whole process.
Once the dipole length has been adjust-ed, I then start to adjust the matchingcapacitor value. Fortunately, this phasedoes not require me to take down theantenna. The binding posts I use for devel-opment make this stage very quick.However, the goal of this tuning phase is alittle different. Instead of tuning for mini-mum SWR I am now going to tune formaximum bandwidth. This is a simple pro-cess using a modern antenna anlyzer suchas the AIM4170B. However, it is a bitmore complicated with older SWR meters.With these meters you have to searcharound to find the minimum and maximumfrequencies where the SWR is below 2:1.
At this point the design is complete.Once you pick the capacitor value you can
Figure 8—VSWR vs. frequency of the antenna, as shifted bythe capacitor. Figure 9—Location of the added capacitor.
Figure 10—Antenna measurement connections. Figure 11—Installation of the shunt capacitor.
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finalize the construction in whatever wayyou might like. Again, Figure 11 shows mysurgery for putting the matching cap inparallel with the feed line. No doubt thereare better ways to do this.
Of course when possible, it alwaysgood to ‘close the loop’ and measure thefinal impedances. Figure 13 is a compari-son between theory and practice. As youcan see, there are some differences but as awhole, theory and practice compare quitefavorably.
IsolationSo far I have described an antenna
which is attached directly to my rig. Thereare times, of course, where I would like toseparate my antenna from my rig and herethings get a little more interesting. Theproblem is, if I just extend the feed linefrom the ‘antenna’ further to my rig, I amalso extending the ‘radiating’ part of theantenna and this will change its resonantfrequency. In order to avoid this, I need tosomehow stop the radiating shield currentfrom continuing down the feed line whileallowing the internal feed currents to con-tinue unaffected. Said in a more familiarway, we need to stop the ‘shield currents’.Traditionally, we do this by using a com-mon mode choke also known as a 1:1 cur-rent balun. Sometimes this traditionalbalun is as simple as a half a dozen loopsof coax. Many web sites propose exactlythis, a coil of coax, as the right ‘trap’ forthe RFL. Unfortunately, these don’t workvery well.
The problem is that these coils of coaxhave inductances on the order of 1 to10 uH. At 14 MHz, this translates to a reac-tance of between 80 and 800 ohms. This
may seem large, but it is small when com-pared to the impedances seen at the ends ofantennas which is often 3 to 6 kohms. Inhis original work, W2OZH made thesecoils ‘resonant’ which increases their reac-tance to several thousand ohms, roughlycomparable to the impedance at the end ofthe antenna. Because these coils of coaxare large and must be tuned, I’ve searchedfor another way.
My solution to stopping the radiatingshield current has been to introduce an oldfashion, ferrite toroid. I construct thistoroid using additional RG174 coaxwrapped 12 to 16 times through a 2.25 inchsurplus ferrite toroid. The measured induc-tance of this toroid is 150 uH. This deliversa calculated impedance of over 10,000ohms. I have found this particular chokeadequate to my needs. However, I wouldwarn the non-QRP folks that this simplechoke may prove inadequate at higherpower levels where significant voltageswill be generated across the winding andlosses may cause excess heating. Figure 14is a picture of my choke.
Improving the MatchWhile the SWR bandwidth is quite
good, the quality of the match within thatbandwidth could stand some improvement.Interestingly, I can sacrifice a little of theSWR bandwidth and improve the matchusing otherwise wasted resources.
Remember that the feed line uses allthree conductive surfaces of the coax. Sofar, I’ve only used the outer surface of theshield of the other half of my coax. Theham in me begs to figure out how I can usethose other two surfaces to my advantage.The ARRL Antenna Book, chapter 9
describes a mechanism for broadbandmatching dipole antennas. One of the tech-niques is to connect a resonant LC circuitto the feed point of the antenna. The ideais, as frequency increases and the dipolebecomes inductive, the LC networkbecomes capacitive thereby compensatingfor the dipole’s inductance. Equivalently,as the frequency decreases below thedipole resonance the dipole becomescapacitive and the LC network becomesinductive; again a compensating effect.
While the mechanism is explainedusing an LC network as an example,implementing an actual LC network wouldbe awkward, adding weight and complexi-ty. Fortunately, there is an alternative cir-cuit which acts much like an LC network:a quarter wave long, shorted coax stub.And remember those two surfaces whichwere unused? Those were the very thingsneeded to implement a shorted coax stub!Figure 15 shows how the two pieces ofcoax are now connected. Figure 16 showsmy final antenna. Figure 17 shows the dif-ference in SWR matching.
As you can see, there is some improve-ment in the match with the introduction ofthis stub. Deciding if it is worth theimprovement is open to debate, but it is funto see what can be done with a little inge-nuity.
TolerancesOne of the chief reasons I like this
antenna is that the wide bandwidth makesit easy to deploy in varying environments.A great example of this is comparing itsperformance when connected directly tomy rig versus the performance with thetrap and an extended feed line. When the
Figure 12—Connectionbetween coax sections.
Figure 14—Toroid common-mode choke.
Figure 13—Modeled and measured VSWR curves. Althoughthere are some differences, the general patterns are the same.
38 · Winter 2013 The QRP Quarterly www.qrparci.org/
antenna is connected directly to the rig, therig acts as a capacitor and detunes theantenna. Figure 18 shows the two SWRcurves. Notice the rig has shifted the curve500 kHz to the left (downward) but thishas no substantial impact on the matchover the operating band.
Another reason I like this antenna isthat the wide bandwidth allows great toler-ances in construction as well. For example,a few inches in length here or there makelittle difference. Coax velocity factor can
vary between manufacturers but thosevariances have little impact in this antenna.The matching capacitor, also, can be off byup to 10 percent, again, with little effect.While you can tune this design to yourheart’s content, such tuning is often justpolishing a brick—this antenna is truly for-giving.
SummaryThe “Resonant Feed Line” antenna is a
very versatile, single band antenna. It has
extremely wide bandwidths, can bedeployed in a wide variety of configura-tions and environments, and requires notuner. Optimizing the antenna can be veryeducational but even the most casual con-struction can result in a rugged,lightweight and efficient antenna. It is myantenna of choice for portable operationand an integral part of my “go bag.”
Figure 15—“Reversed” coax connection. Figure 16—Diagram of the complete antenna/feedline system.
Figure 17—Antenna VSWR with and without coax stubmatching.
Figure 18—Comparison of VSWR with direct connection tothe rig, and with the trap and extended feedline. Capacitancebetween rig and antenna is reponsible for the differences.
MARK YOUR CALENDARS
HF Grid Square Sprint9 March 2013, 1500Z to 1800Z
Spring QSO Party6 & 7 April 2012, 1200Z April 6 through 2359 April 8
Hoot Owl Sprint26 May 2013, 8:00 PM to Midnight LOCAL time
QRP Shootout — 15 & 16 June 2013CW 1500Z to 2100Z on June 15SSB 1500Z to 2100Z on June 16