End Fed Half Wave Antennas

End-fed halfwaves have become increasingly popular. They also have become bathed in misinformation, mystique, and contradiction. 

Some people swear by endfed halfwaves, while others swear at them. The reason for that is simple, end-fed half waves have problems that affect consistency of user results.  

The purpose of this article is to show why opinions vary widely by including analysis's of typical antennas and measurements of components. The analysis of a particular style of end fed antenna should include best and worse case scenarios using real-world component values. It is imperative for any analysis to  include data of common mode currents on feedlines and how changes might affect performance.

Styles or Types of EFHW Antennas

There are four basic styles or types of end fed HW antennas: 

• Coaxial feed with a choke
• Coaxial feed with a skirt
• Stub feed
• Direct feed using a lumped matching network or tuner

The four styles above share one common trait. Absent a counterpoise of some type and proper care in construction, they all induce common mode currents on the feed system. The order of feed problem severity may surprise many of us. The list above is actually ranked in order of construction care required or construction difficulty!

The two configurations thought of as "dipoles" are actually the worse of the systems. We'll see why as we analyze each type. I'll start below with the simplest system. 

Direct end feed or traditional longwire style with matching network

The direct end feed antenna brings the end of the halfwave to a transformer or lumped component network of some type. The network is connected to something it "pushes" against so current can be forced into the antenna. The system looks like this:

We can calculate end-impedance of an antenna in the clear with EZnec. The model is:

The model for this antenna consists of the following wires:

The feedpoint impedance (the little circle on the antenna view) is:

EZNEC+ ver. 4.0 6/2/05 6:37:43 PM --------------- SOURCE DATA --------------- Frequency = 7.08 MHz
Source 1 Voltage = 574.6 V. at -1.15 deg.
Current = .1741 A. at 0.0 deg.
Impedance = 3300 - J 66.09 ohms
Power = 100 watts
SWR (50 ohm system) = 66.023 (75 ohm system) = 44.015

The current distribution of the antenna is:

CURRENT DATA ---------------Frequency = 7.08 MHz mean value of current in segment  
Wire No. 1 antenna: 50 segments, .4 ft per segment 
(junction of wire 3, counterpoise wire) .17408a
4 ft up                                                    .34742a
8 ft up                                                    .56407a
12 ft up                                                  .76347a
16 ft up                                                  .93698a
20 ft up                                                 1.0795a

Wire No. 2: 46.5 ft long 24 ft high, .465 ft per segment
at top end of vertical wire 1.0929a
4.65 ft                                1.2016a
9.3 ft                                  1.2744a
13 ft                                   1.2934a CURRENT MAXIMUM
18.6 ft                                1.2542a
23.2 ft                                1.1603a
34.8 ft                                  .70952a
Open End                              .01971a

Wire No. 3 counterpoise 5 ft long 4 ft high .25 ft per segment
at junction to antenna wire  .16418a
1 ft                                      .14095a
2 ft                                      .10931a
3 ft                                       .07673a 
4 ft                                       .04285a 
5 ft Open                             .00584a

One recently proposed theory is this; "if the antenna is not exactly resonant, ground currents will flow." We already know ground currents flow with a resonant end-fed because end-impedance is not infinite. Let's change frequency 10%, since this would be equivalent to a 10% error in length of all sections,  and see how much ground currents increase...

EFHW standard 6/3/05 5:43:47 PM --------------- CURRENT DATA ---------------Frequency = 7.788 MHz

lowest vertical antenna segment  .37542
counterpoise first segment .35496a 

Before at resonance we had:

lowest vertical antenna segment .17408a 

counterpoise first segment         .16418a

Being 10% off resonance just about doubles current. We see it is a definite current increase, but not one from zero to problematic currents! The proposal exact resonance eliminates all current in a counterpoise isn't correct. A 10% length error only doubles current.    

Moving 5% in frequency:

EFHW standard 6/3/05 6:20:14 PM --------------- CURRENT DATA ---------------Frequency = 7.434 MHz
lowest vertical antenna segment      .2049
counterpoise first segment               .19385

With a 5% length error from resonance, we now see only an 18% increase in current. That's negligible since many other things we might do (like moving the antenna a few feet in height) would make a much larger change.  

--------------- SOURCE DATA ---------------Frequency = 7.434 MHz
Source 1 Voltage = 924.9 V. at -58.15 deg.
Current = 0.2049 A. at 0.0 deg.
Impedance = 2382 - J 3834 ohms
Power = 100 watts

We can see there is some merit to maintaining resonance because current is at a minimum value, but we only need to worry when resonance errors are somewhat large. When length errors are modest (under 5%) the error has virtually no effect on ground current. The reason for this is very simple. The reactance or lack of resonance isn't what determines current, the resistive part of the impedance does. We are looking for a resistance peak in the end-impedance of the antenna...not necessarily resonance.   The source resistive part at resonance was 3300 ohms. At 5% error it was 2382 ohms.

With the non-resonant antenna, we have increased electric fields around the feedpoint. Voltage is 925v instead of 575 volts. RF voltages (the electric field) might be an issue with end-fed antennas.      

What else affects Antenna Impedance?

From above we see higher antenna resistance is a good thing for current, and length is not overly critical. We also see lower reactance is a good thing for voltage, and length can affect voltages (and the electric field) surrounding the antenna and counterpoise near the feedpoint.

What about a thicker antenna? With a 1" thick antenna 7 MHz impedance becomes 1684 - J 716.3 ohms and resonance is well below 6 MHz. The reactance problem is because the counterpoise is too short. The drastic resistance reduction at peak resistance occurs because the wire is thicker. Obviously a thicker antenna has higher ground currents!  

Half-wave broadcast towers often have impedances under 800 ohms at resonance. 

What about antenna surroundings? As the area around the antenna becomes more cluttered and/or has more power loss, antenna end-resistance is reduced! Over perfect ground the antenna end-impedance almost doubles from that over average ground. Over lossy ground, especially when the antenna is low in height, feed resistance decreases even more.

With a small counterpoise and end-feed we: 

• need to keep the antenna clear of lossy media including the earth. 
• should try to use a reasonably thin antenna element to minimize current.
• stay within a few percent of resonance to minimize feedpoint voltage and current. 
• always have the same current flowing into a counterpoise (of some form) as flows into the antenna.
• should use the largest counterpoise that can be reasonably implemented, but avoid small counterpoise systems that have wires significantly longer than 1/4 wl

Improvements

The best solution I can think of to common mode or "RF in the shack" problems with this form of antenna is to isolate the counterpoise or antenna ground from the station feed. At low power levels a simple link coupled matching network is a good solution, provided the secondary has no RF path to station equipment. One way to accomplish this is by using two output terminals that float.

In the circuit above:

• Determine the maximum impedance ratio between input and output. 
• The turns ratio is the square root of that ratio
• The reactance value of C1 near 3/4 mesh at the lowest frequency and secondary reactance of T1 should be the maximum expected load impedance over the turns ratio
• C2 is optional, and should be the value of C1 or larger if used. It will allow wide adjustment of matching range   

Assume we have a 5000 ohm load and 50 ohm rig. The turns ratio of T1 is sqrt of 5000/50 or a 10:1 ratio.

The reactance of C1 at 3/4 mesh (so you have adjustment range) should be 5000/10, or 500 ohms. (This is a loaded Q of ten, you need LESS Q with lower transformation ratios and more Q with higher ratios or the circuit becomes too sharp or too "mushy" to tune.)

The reactance of L1 secondary should be 500 ohms in this example. 

U1 should connect to the antenna, U2 to the counterpoise or ground which should NOT connect to the station equipment. The counterpoise should be as long and straight as possible, and directly under the antenna if possible. Ideally the counterpoise, if less than 8 wires 1/4 wl long, should be elevated and insulated from earth. Do NOT make the counterpoise longer than 1/4 wl, especially if it is only a single wire! Most of us, since this is a temporary or compromise antenna, will use a very small ground system. I've found connecting a counterpoise to earth, say a ground rod, actually reduces antenna efficiency. 

If you run low power and don't have a ground or counterpoise, you might just connect U2 right back to the coax shield from the radio. This way you can use the capacitance of the radio and station wiring as a ground system.

To be continued soon!
Zepp or stub-matched antenna

Dipole with coax having a choke at the end

Impedance

Feed Systems

Common Mode Currents

At high power 3 dB loss in even the largest components would mean extreme heat, at low power its difficult to notice several dB loss as any type of component heating. The paradox is while 1500 watt systems could often stand to lose 10 dB or more as heat...low power systems (at least in my way of thinking) should try to squeeze every milliwatt out!

Let's look at a matching system in what I consider one of the most difficult methods of feeding an antenna, the end fed half-wave. If anyone has any other matching systems that are commonly used or recommended, send me one and I'll measure it in my lab and post the data here.  

There is also some discussion of common mode current, and the lack of common mode current because we sometimes can't observe ill effects. 

I remember working with a new graduate engineer, let's call him Simon, on an antenna system. When I asked Simon if he checked for proper feedpoint isolation, he turned an SWR analyzer on and wrapped his fingers around the feedline. Seeing no change in SWR, he declared the system free of common mode currents! Convincing him to use a clamp-on RF meter, we found the feedline that acted "cold" to the touch actually had significant common mode currents. 

Where did Simon go wrong? Pretty simple when we think about what he was actually testing. High voltage points interact with body capacitance at HF, not current points! Had Simon grabbed the coax at a high voltage point of the shield, he might have found interaction with SWR. Unfortunately a significantly high impedance or high voltage point rarely appears along the outside of a "grounded" shield. That's because the cable is often routed near other conductors. The cable is also thick, and that limits surge impedance.

Even if we modify common mode impedance with body capacity, the feedpoint is where multiple paths combine and make the transition to the feedline. Measuring SWR changes is a very poor way to determine proper operation. Even the most basic antenna systems, once the feedline becomes involved, can become a terribly complex web of paths. The myth that we can grab a cable to see if a system needs a balun or has common mode problems is a very BIG myth.  

Measured  Data

Some of my earlier 2005 measured data disagrees with more recent measurements I've made, so I've pulled the data until I have more components to test.