Antenna Tuner

From K4EWG Correspondence files

With the wide use of coaxial cable feed lines, the balanced open wire type tuners faded into the radio history books. Coaxial cable became the easy and effective way to transfer RF energy from the transmitter and to the receiver from the antenna. It became easy to manufacture and was self shielding, provided it was matched to its characteristic impedance at both ends (transmitter/receiver and antenna) of the cable. It was flexible and could be installed with little concern for interference from other RF fields. However, in mismatched service, due to its physical construction, coaxial cable simply converts most of the RF energy from a transmitter to heat depending on the degree of mismatch. A mismatched condition presents reactance or resistance to the flow of RF energy to a receiver as well. In fact, the mismatched reactance could form a closed loop series resonance that would short circuit the receiver antenna terminal to ground and prevent little if any RF energy into the receiver. The coaxial cable, antenna and transmitter/receiver output/input circuits form a resonant tank circuit that stores RF energy. In the matched condition, both ends of the coax are resistive and have the same surge impedance as the coaxial cable, all the RF energy available will be radiated or received. This condition can exist only at one frequency or with frequency independent antenna systems (Log Periodic Arrays for example). Today, the need for multi frequency, multi band antenna systems required a matching system that would prevent shut down of solid state transmitters during a miss matched condition using coaxial cable as the feed line. Their solution was the coax to coax ultimate trans-match. And for those die hard old timers using antiquated open wire feed lines, through in a ferrite balun coil with stacked cores to handle the circulating currents! A 1 to 4 ratio unbalanced to balanced aught to work for everything and let the trans-match provide the required conjugate resistive connection to the transmitter! The transmitter will see a purely resistive 50 ohm load from its output circuit to the trans-match and the balun will see an RF circuit containing reactance and impedances nowhere near 200 ohms resistive. The voltage and current relationships along the feed line and antenna remain unchanged.

When there is a miss match at the antenna, a portion of the incident or forward traveling wave front returns back to the generator creating a reflected traveling wave front. Once the reflected wave front reaches the generator it is carried forward again by the next forward wave front. This process will continue as forward-to-reflected-to-forward-to-reflected-to-forward until all the RF energy is either radiated or converted to heat. It is interesting to note that because of this relationship, the feed line will carry more RF energy (RF amperes) than the generator can supply. The greater the miss match the greater the RF energy being carried by the feed line. A miss match at the antenna can create an exponential loss of radiated RF energy in a coaxial cable feed line. RF energy in watts is equal to the RF amperes squared multiplied by the resistance (impedance) of the load. It then is easy to see how even a VSWR of 1.5 to 1 can reduce the effective radiation of the RF energy using a coaxial cable for the feed line.

A parallel open wire feed line system, with no spreader insulators, is the lowest loss connection to the antenna. Since air, rather than polyethelene as in coaxial cable, is the dielectric between the conductors, very little of the RF energy is converted to heat. A mismatch at the connection of the feed line to the antenna has little effect on the forward-to-reflected traveling wave fronts being radiated. However, there must be no losses at the generator due to a reactance such as a mismatch between the feed line and a balun coil.  The reflected wave front must see a non-reactive, conjugant, mirror at the generator. This condition does not exist if a balun coil is used between the feed line and the antenna tuner (trans-match). Also, to reduce radiation from a parallel wire feed line, the RF currents in each wire must be equal and out of phase with each other. The antenna can be an end fed long wire, provided a parallel feeder system, with balanced feed line currents, is used. Some antennas, such as an inverted “L” or “Bob Tail Curtain”, for example, are designed for a vertically polarized radiation, and the open wire feed line connection at the base must contain balanced RF currents in each feed line wire for an optimum radiation  pattern from the vertical portion of this type of antenna system. The horizontal wires of these types of antennas form a capacity hat or top loading to resonate the vertical wires.  Using an open wire feed line for these arrays will allow multi band operation, provided the RF currents are balanced in each feed line wire.  However, the optimum multi-band antenna is a center fed array with physically balanced conductors on each side of the feed line at the antenna connection. My “V” beam, for example, will have a distorted or “warped” pattern if the currents in each feed line wire are not equal. Such distortions can electrically steer the array by un-balancing the currents in the “V” wires. However, this also reduces the antenna’s volumetric efficiency (capture area)  by the distortion of its bi-directional lobes.

The K4EWG tuner, with independent differential variable capacitance voltage dividers, would allow for such steering of an array. Since balun coils are not used, an almost perfect reflection is seen by the feed line, and the reflected wave front is reflected along with the following forward wave front un-impeded. Tapped coils are not used, further increasing the ability of the tuner to provide a more effective mirror for the reflected wave front. Reactance due to the miss match between the antenna-feed line-generator circuit is cancelled by the tuner and does not appear in the circulating current of the tank circuits. A balun coil does not cancel a reactance. It can only provide a transformation of impedances between different alternating current voltages. The impedances must be purely resistive for the transformation to occur. If a reactance occurs at either end of the balun coil, it becomes an active reactance within the circuit. Since an RF current travels closer to the outside of a conductor as the frequency is raised (skin effect), the balun will convert the RF forward and reflected wave fronts into heat due to the circulating currents within the coil wires and its core material.

Close inspection of the circuit and parts for the K4EWG tuner shows that the split stator capacitors C1 and C2 have been physically modified by rotating each rotor half 180 degrees relative to each other on each capacitor. Therefore the overall capacitance of either C1 or C2 will remain the same regardless of their rotor position. This is necessary in order to maintain an electrical rather than a mechanical connection from the feed line wires to the tuner coil. If the antenna has equal conductors on each side of the feed line at the feed line connection to the antenna, the RF currents will be equal when C1 and C2 are rotated so that C1A equals C2A and C1B equals C2B. This is accomplished by rotating C1 and C2 in equal but opposite directions. C4 tunes the link coil L2. It adjusts for reactance, resistance and coupling. It is a critical part of the tuner and may have to be modified if the feed point at the tuner is voltage or current fed. The length of half of the feed line conductor plus half of the antenna conductor must be chosen so that a voltage loop or a current node does not appear at the tuner for each band to be used. A voltage loop will exist at all even half wavelengths and a current node will exist at all odd half wavelengths. A compromise should be reached to avoid these half lengths. If there is no physical way to avoid a curr      ent node, and since the impedance is low and resistive at that junction, it may be possible to directly connect the feed line to the transmitter output and by-pass the tuner. However, the system will be unbalanced for that band. A voltage loop must be avoided by adding additional feed line wire to each side.

This tuner must be placed in an RF tight enclosure to prevent any human exposure to stray RF energy fields.

73’s Pete, K4EWG

One thought on “Antenna Tuner

  1. Hi there,
    Just heard you in QSO on 40m CW at 7029 about 0130 UTC 12/1/11.
    I’ll have to admit I believe you had the strongest signal I have ever encountered. A solid 45 over S9 in Waynesboro, Va. I looked up your call sign and visited your web site. Impressive Amp, is it multiple 4-1000a’s ? I am just returning to active ham radio after being off the air for about 12 years. I was first licensed in about 1976, don’t remember exactly. I have enjoyed homebrewing over the years and have a 4-1000A project still in parts. Would like to see the details on your amp sometime. 73′s Sam WD4FTS

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