Last Update: November 04, 2009
Here is a sure fire way to make end fed half wave antennas fed with a 50 ohm coupler work - without long radials, elaborate grounds, chokes, voodoo, etc.
First, setup your end fed half wave tuner (ex., parallel tuned circuit link coupled) with a resistor across the output as shown on the schematic below. The resistor should be the value that is appropriate for the turns ratio of the coupler. For example, use something close to these values:
Make sure the resistor is rated for whatever power you are going to use to make this adjustment. I use my MFJ-259B antenna analyzer so power isn't an issue.
Now turn your analyzer or transmitter to the desired frequency that you want your antenna tuned to. Adjust the circuit to resonance (dip in SWR, 0 reactance). At resonance you should be able to get a 1:1 SWR. If not adjust the coupling (turns ratio) until you do. Once the point of 1:1 SWR is achieved make a mark on your tuner where the parallel capacitor is tuned. This is the point where the tank circuit is resonant and the transformation from 50 ohms to the desired resistance value is correct.
Next, cut your wire to a half wave length at the desired frequency and be sure to make it a little bit long. Install the antenna in a manner similar to the way you are going to use it. I like to use wire with a soft insulation and loop the extra length at the far end back upon itself and tie it in an adjustable knot. This allows me to adjust the antenna's length easily in the field if necessary.
Now remove the resistor from tuner and connect the antenna. Either use a "counterpoise" at least 0.05 wave lengths long (ft = 49.2/MHz, m = 15/MHz) or connect the side opposite the antenna to the system ground (return). If you are setting up a portable station and plan on using no additional "counterpoise" then just hook up all of your equipment. Adjust the tuner to resonance. If everything is well the parallel capacitor will be tuned to exactly where you marked it when using the resistor. If you have to use more capacitance then you did with the resistor then the antenna is too short. If you need less capacitance then the antenna is too long. Yes, the tuner can probably easily compensate for length differences in the antenna but the idea is to provide the same load as was provided by the test resistor. This is to minimize the current in the "counterpoise". There is no need to adjust the "counterpoise" length as long as it is at least 0.05 wave lengths long or has enough area to provide some capacitances (QRP rig, key, operator, etc.). There is no advantage to making the "counterpoise" a magical 1/4 wave length long.
With a resistive (resonant) high impedance load very little current flows into the antenna and therefore very little current is required to go through the counterpoise. This means that only a very small amount of "counterpoise" is required to return the current (via displacement currents) and this is easily taken care of with stray capacitance of the coupler or a short radial. With a resistive end fed half wave antenna load you will find that touching the tuner, radio, coax, etc. will NOT change the SWR. I have not had any common mode current problems on the outside of the coax but under certain ideal conditions this is said to be a possibility. The most current will flow into the counterpoise when it is itself 1/2 wave length long or in someway resonant at the same frequency as the antenna. My experience has indicated to me that providing radials doesn't do anything noticeable to improve the system once tuned properly. This not to say that zero current (impossible) is flowing into the limited return system but in real practice with QRP portable systems that I have used these antennas for there is not a problem. When I perform A/B comparisons of this antenna against a similarly orientated center fed through a balun dipole and I can detect no differences. What more can you possibly ask for? As I've mentioned on other pages this doesn't mean that improving the ground below the antenna won't help performance since improving the ground helps almost ANY antenna. It just means that a large return ("counterpoise") is not necessary to complete the antenna.
Now where most folks have problems is when the antenna is not adjusted to true resonance and excessive current will flow through the tuner and more and more of a "counterpoise" is required for the return current. Without a good ground system then everything that you touch will cause the SWR to change, because more return current is flowing through YOU. This should tell you that things are not right. Operating in this mode without a more elaborate return ("counterpoise") is problematic. Essentially, the feed point is moving away from the current node (voltage loop) of the antenna when the antenna is not resonant and more and more current will flow into whatever is being used as a return.
In practice I've found that I can set things up in the backyard and fine tune the system and when I get out into the field things don't hardly change. I do leave provisions to change the length of the wire just in case the environment changes the resonance significantly. When the tuning capacitor is set to some value other then the value found at home with the resistor then I know I need to change the antenna's length.