Wire Antennas part 2 of 3 End Fed Wire

End Fed Wire Antenna

Another simple wire antenna is a piece of wire that is fed power at one end. Typically it is called an end fed antenna or random wire antenna. Power is supplied to the end of the wire through 50 ohm coax as in the dipole, but one or two other items are necessary. 

First of all there must be a ground connection. If the impedance between the end of the antenna wire and ground is low or near 50 ohms, coax may be directly connected. For this to happen the antenna must be 1/4 wavelength long or an odd multiple of 1/4 wavelength. The center conductor of the coax is connected to the end of the wire and the shield of the coax to the ground. This will be the case only if the length if the wire is one quarter wavelength long or an odd multiple of one quarter wavelength. In this special case the reactance will be zero and the resistance will be somewhat greater than about 35 ohms. In most all cases, if the load is resistive and anywhere from 25 to 100 ohms it may be connected to 50 ohms coax. 

If the wire is longer than one quarter wavelength a simple matching network is necessary. For slightly longer than one quarter wave a series capacitor may be used. In other cases or for multi and use, a series coil (inductor) and shunt capacitor works very well. 

In general the desired directive pattern comes first and sets the length of the wire. In a typical amateur radio station, it may just be the distance between two available trees. In any case, once the length is known the next thing is to determine the input impedance. This impedance will, in general, be different for each frequency. The impedance will start with a low resistive and zero reactance value at the frequency for which the wire is one quarter wave long. As the frequency is increased, the impedance increases to a very high value when the wire is one half wave long. As the frequency continues up the impedance decreases until it reaches a minimum when the wire is three quarters wave long. The impedance continues to vary from low to high every quarter wave length. To summarize: When the wire is an odd number of quarter waves long the impedance is low and non reactive. When the wire is an even number of quarter waves long the impedance is very high. With the right matching circuit, any of these impedances can be matched to 50 ohm coax.

In practice tuning circuits are frequently tuned by trial an error, however a basic knowledge of the fundamentals helps speed the process and give a clue as to which circuit configuration to use.

The precise values are not usually necessary. Knowing if the impedance is low or high is necessary. Knowing if the reactance is inductive or capacitive is necessary. 

One length that probably should be avoided for any end fed wire is a half wave length or any integral multiple of a half wavelength. The impedance will be very high, resistance very high and the reactance can flip flop from very high inductive to very high capacitive reactance. Reactance is zero when the wire is a half wave long and resonant. This tends to make the system unstable at certain critical frequencies.

Starting with a quarter wavelength, the resistance starts at about 35 with no reactance. As the wire becomes longer in terms of wavelength, the resistance goes up and the reactance is inductive, increasing from zero to a very high inductive value when the wire reaches half wave length. At that point the resistance decreases while the reactance flip flops to a large capacitive value and then decreases. At three quarters wave length, the resistance becomes low again, maybe 40 or 50 and the reactance goes to zero. Every half wave length the impedance values will repeat. Not exactly, but very close. At each successive resonant point (reactance is zero) the resistance value will increase slightly. 

Any time the reactance is inductive, a series capacitor with the equivalent reactance can be used to cancel the inductive reactance leaving just the resistance. When the reactance is capacitive, an inductance may be used in the same manner to leave just the resistance. If one component will not result in a close enough match, adding a shunt reactance will complete the transformation to 50 ohms. There is usually more than one combination of components that will match any two impedances. The most common being a series coil and a shunt capacitor at the antenna side. One other complication is the ground lead. The shorter the better. If it is two long it will become part of the antenna. In any case it will have some effect on the antenna impedance. Certain lengths can cause the tuner and radio chassis to be "hot" or have higher than desirable RF voltages. These lengths should be avoided. If not, a quarter wave counter poise can solve the problem.  If there is a low voltage on one end of a wire or cable a quarter wave long, there will be a high voltage at the other end. A quarter wave wire connected to a RF hot chassis will shift the hot spot out to the open end of the wire.

In general RF problems increase as the power level is increased. What works fine for 10 watts may not with 100 watts and what works for 100 watts may not for 500 watts or more.