Date: Sat, 03 Oct 1998 13:36:31 +0000 Reply-To: Arjen.Raateland@vyh.fi From: Arjen Raateland To: "Low Power Amateur Radio Discussion" Subject: End-fed half wave antenna & Rainbow-type tuner In a private email message sent some notes about an end-fed half-wave antenna with a tuner of the Rainbow type. I expect these notes to be interesting to other Rainbow owners as well. LB kindly gave permission to distribute the notes to the list. Note: 'Tank circuit' denotes the parallel LC circuit in the Rainbow tuner. Quote--------------------------------------------: A 1/2 wl antenna fed at the end will have a high R and a high X. Unlike the center-fed position, where the impedance changes very slowly as you lengthen or shorten the antenna, the end fed position undergoes very large changes of both R and X for very small changes of length--or very small changes in the influences on the antenna from its environment. At precisely 1/2 wl--which no real antenna ever is quite--the reactance is zero, but a tiny bit either side, it goes to 20K+ ohms either inductive or capacitive. Then it falls rapidly down to the 1K region. The resistance can easily reach 5K to 7K ohms at 1/2 wl. Most real antennas, whatever length they are cut, will not be exactly 1/2 wl in use, due to the influence of surrounding objects. What they will be is likely an unknown. The ground plane wires in this system actually form a part of the antenna (a useless part for radiation) and the tank circuit is actually to be measured up from the electrical end of the ground plane wire(s). Because the wires are on the ground, determining the exact electrical end is not feasible. Hence, the tank circuit is actually at some unknown point up the antenna--which I am assuming from your description, is directly fed from the tuner with no parallel feedline in the circuit. Because the tank circuit is up the antenna wire, the impedance values it meets are lower than the 1/2 wl peaks, but how much lower cannot be predicted. Essentially, the system is a cut-and-try. Attempts to reduce this system to a set of equations simply do not work. However, because the tank circuit is not at a pure 1/2 wl peak point, it sees lower values of R and X in most cases--but still usually in the high-Z class. The tuner is simply a tank circuit with the antenna/"ground" terminals across the tank. The transmitter taps make the inductor an auto-transformer. The capacitor can match a fairly wide range of reactances if resonance with a resistive circuit is about midrange. The reactance is tuned out by reducing or increasing the capacitance relative to the amount needed for resonance when the load is resistive, In addition, the capacitor being offset from resonance slightly (apart from reactance compensation) changes the coupling coefficient to the tapped section slightly, altering the impedance transformation from normal, thus giving a wider range of matching capability than just 4 values. In this way, it is similar to link coupling. This generally allows a match under 2:1 SWR on at least one tap, except for the loads with the highest reactance. In essence, there are better antenna tuner circuits and better antennas, but the end fed approximately 1/2 wl wire plus the simple tuner makes an easy field antenna system. And field ease is important on a short outing. Hope these notes help. -73- LB, W4RNL End Quote-------------------------------------------- LB has also written a collection WWW-pages where he explains a lot of antenna/tuner issues with graphs: http://web.utk.edu/~cebik/radio.html 73, oh2zaz -- Arjen Raateland SAS Support Finnish Environment Institute, Helsinki AX.25: OH2ZAZ@OH2RBI.FIN.EU