end fed 1/2 wave matching system end feed
end fed 1/2 wave matching system end feed
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End Fed Half Wave AntennasEnd fed half wave antennas were once very popular. Along came coaxial lines, and popularity faded. Now they are increasingly popular, mostly because of convince and simplicity. Unfortunately end-fed antennas have also become bathed in misinformation, mystique, and outright contradiction. Some people swear by end-fed half-waves, while others swear at them. The reason for that is simple, end-fed half waves have problems that affect their performance and cause inconsistency in results. One commonly repeated myth or "theory" is that end-fed half wave antennas, being resonant, do not require a counterpoise. Styles or Types of EFHW Antennas There are four basic styles or types of end fed HW antennas:
The five styles above share one common trait. Absent a counterpoise of some type and proper care in construction, they all induce significant common mode currents on the feed system. This means they are subject to feedline radiation and "RF in the shack", as well as increased TVI/RFI and receiving system local noise pickup. The actual severity of feed system common mode currents may surprise many of us. Low power, long feedlines, or even just a lucky length of feedline, can hide common mode problems. Some very small antennas actually work because of feedline radiation! Two configurations, thought of or called "end fed dipoles", are often the worse of the end-fed systems. We'll see why as we analyze each type. Direct end-feed or traditional longwire style antenna with operating position matching networkThe 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 basic inverted L antenna system or "half wave longwire antenna" looks like this:
We can calculate end-impedance of this antenna with EZnec. The model is:
Wire 1 is the vertical length Wire 2 is the horizontal length Wire 3 is the small counterpoise
With 100 watts applied, current distribution of the antenna is: Wire 2, horizontal wire 46.5 ft long 24 ft high
Wire 1, vertical wire 20 feet high
Wire No. 3 counterpoise 5 ft long 4 ft high:
The feedpoint impedance (the little circle by the 3 on the antenna view) is:
The problem with this system isn't so much the current and magnetic field around the antenna feedpoint. The problem is the extreme voltages and the very strong electric field surrounding the antenna feedpoint. The base of the antenna has several hundred volts even at 100 watts, and that voltage gets higher at the open end of the counterpoise. As a matter of fact the end of the short counterpoise wire has almost 3,000 volts of peak voltage with only 100 watts of applied power!! One commonly repeated myth or "theory" is that end-fed half wave antennas, being resonant, do not require a counterpoise. That isn't true. 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 the antenna, and see how much ground currents increase.
At resonance we had: lowest vertical antenna segment .1741a counterpoise input .174a Being 10% off resonance just about doubles current. We see it is a definite current increase, but at resonance current was not zero! The idea that "exact resonance eliminates all current in a counterpoise" is not correct. A 10% length error doubles common mode feedpoint current, but current was never zero to start with. This is actually common sense because the end impedance of a 1/2 wave antenna is not infinite, nor can we apply power to an infinite impedance load. The end impedance of any half-wave is finite, and varies with antenna diameter, length, and surroundings. Thin wires have higher impedance, thick antennas have much lower end-impedance. Moving 5% in frequency:EFHW standard
6/3/05 6:20:14 PM
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CURRENT DATA
---------------Frequency
= 7.434 MHz
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 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 longwire 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 feed 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. (End-fed 1/2 wave feed impedance decreases, while center fed half-wave antenna impedance increases with added loss!) With a small counterpoise and end-feed we need to:
Longwire ImprovementsThe 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.
U1 goes to antenna U2 goes to counterpoise, do not connect counterpoise to station ground
In the circuit above:
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.
PAR Antenna and other End-fed Half-wavesMany antenna designers and experimenters assume we can end-feed a half-wave without having the potential for significant feedline common-mode current. The antenna view below shows common mode currents in a feedline grounded 3/4 wl away from a perfectly resonant end-fed 1/2 wl antenna:
Current tables are shown below. I fixed the power level at 100 watts. I deleted all but minimum and maximums: ---------------
CURRENT DATA
--------------- Maximum antenna current is .758 amperes, while the coax common mode current maximum is actually 1.02 amperes! It is actually possible to have MORE radiating current at the common mode current maximum in the feedline than in the antenna itself. This is because standing waves on the OUTSIDE of the cable transforms voltage and current. (This has nothing to do with standing waves INSIDE the feedline that an SWR meter measures.)
That was worse case. Now here is best case feedline length to the ground point:
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CURRENT DATA
--------------- Now we see the current maximum in the antenna is 1.12 Amperes, while the undesired radiation current maximum in the feeder is 0.219 amperes. The antenna now radiates considerably more power more than the feedline. This is a good thing! This problem doesn't mean the antenna won't work or is a bad antenna. It does mean that the feedline is a part of the radiating system, and how the feeder is routed and grounded will drastically affect system performance. It is electrically impossible to end-feed a 1/2 wl antenna without a good counterpoise at the feedpoint and have a totally "cold" feedline. A ten-turn choke or normal current balun at the feedpoint certainly won't change anything. The only way to prevent common mode on the feedline is to pick the "lucky feedline length" or feedline grounding point. |
