How about a normal L-type network? Connect one side to the balanced line, and the other side via a current balun to the 50 coax. The tuner is 'RF-floating'. (You can run the coax through a toroid or use two seperate wires to make the current balun.)
73, Arie PA3a ---------------------------------------------------------- I've done a number of experiments with this setup, measuring the amplitude and phase of currents into the transmission line for a center fed doublet antenna using balanced and unbalanced networks. The unbalanced networks were tested to compare the effect of a balun at the input or at the output. They all worked FB. I came to these conclusions: 1) The tuner components must be well isolated from surrounding objects if they are part of the balanced feed line (balun at the input of an unsymmetrical network or an inherently balanced network) to avoid excessive stray coupling. Inherently balanced networks can be arranged to provide equal coupling to a box (e.g. the famous Johnson "Matchboxes"). An unsymmetrical network needs more care (isolation is good!). 2) There was no practical difference in the balance available from either a "single ended" (unbalanced as in an L-network) tuner or an inherently balanced tuner (link-coupled circuit with split stator tuning capacitor). After all, when the system is adjusted to provide a 50-ohm non-reactive impedance to the rig, the various coils and capacitors are "transparent" to the RF. The RF "sees" a resistive 50-ohm load at the input to the tuner. There is a possibility of imbalance caused by different losses in the components. For example, coils tend to greater losses than air-dielectric capacitors at RF, so a large coil can introduce ohmic losses that affect balance in an unsymmetrical network. 3) When the balun is placed at the *input* to the tuner, it must be properly isolated. Putting it inside a metal box with proper conductors is important in most situations (i.e. inside the tuner enclosure if a metal enclosure is used). The point is to keep the RF flowing along the inside of the coax shield from finding its way to the outside of the shield and back to outside of the equipment enclosures. Remember, to RF, a shield is two isolated conductors: one in each side. The RF needs to stay *inside* the enclosure of the balun. Simply connecting the coax to a balun laying in the open is sure to cause problems. This issue becomes more important at higher frequencies. 4) A high degree of balance in a wire antenna is almost impossible to obtain at HF. Differences in the proximity of buildings, trees and the ground to each side of a center-fed wire effect the balance in the currents in the feeders. Of course, unbalance can skew the pattern of a beam antenna, but they are usually better isolated (and few beams use open wire lines these days anyway). Pattern skew of a typical wire antenna caused by trees, buildings or other sources of unbalance in the matching network is seldom of concern. That's why so many installations work FB with no balun at all. It's only needed if common mode currents cause "RF in the shack" issues. Remember that the RF output at the coaxial connector on the rig is already inherently balanced: the current flowing on the inside of the shield is equal and amplitude and opposite in phase to that flowing along the center conductor. The challenge is to preserve that balance where the coax shield comes to an end by not allowing the RF to flow onto the outside of the shield, adding the equipment on the operating desk, the station ground and possibly the power lines to one side of the antenna system! Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
