Don, W3FPR wrote: I wonder why you are using a 9:1 balun? You may be better off using a 1:1 balun instead. Yes, I know that 50 times 9 is 450, but that is only the characteristic impedance of your feedline - the feedpoint impedance will be quite different than 450 ohms, and may vary from very low to very high depending on the frequency. It just may be that your 9:1 balun is creating more RF in the shack. If you have no 1:1 balun available, try removing the balun and connect the feeders to the center conductor and shield of your coax - and connect your counterpoises to the shield at that point too - you may find your RF in the shack will drop dramatically.
------------------------------------ To follow up Don's comments, I've fed open wire line from a so-called "unbalanced" output with *no* balun and RF ammeters in each leg of the feeders showed that the currents were still balanced. The currents *inside* a coax line are balanced. The current flowing inside the shield is equal and opposite the current on the outer surface of the center conductor. Where things go awry is where the coax shield ends and RF can flow around the end onto the outside. Now there's two loads on those currents: one is the antenna and the other is whatever is connected to the outside of the coax shield, including the rig. If that sounds confusing, remember that RF does not flow *in* a conductor, but only along the surface. So the RF currents flowing along the inside of the coax (or inside your rig near the antenna connector) are totally independent of any RF currents flowing on the outside of the coax (or on the outside surface of the rig). It's those outside currents that tend to cause most of the problems. It's those outside currents you upset when you touch the rig and so detune the system or give yourself an RF "bite" (Ouch!). It's those outside currents that get inductively coupled into things like speaker wires, telephones, microphones, etc., and cause mischief. A balun is just a length of transmission line, sufficiently long that any unbalance at one end is "smoothed out" by the interaction between the RF fields around the wires before RF currents get to the other end. Years ago, we made a balun by winding up transmission line in a big coil. Sometimes we wound each leg into its own coil and sometimes both legs were wound next to each other in one huge 'bifilar' coil. For HF an open wire balun might be 4 to 6 inches in diameter and a foot or two long. (It was common to mount them on standoff insulators on a large board and screw it to the wall!) Nowadays we use ferrite cores that give us the inductance needed in a much smaller space, but the operation is the same. If you look at a 1:1 balun schematic, it's easy to see how it's a coiled-up two-wire transmission line. Other baluns, offering various impedance conversions, are also simply coiled-up transmission lines; usually two or three interconnected to provide the impedance conversion wanted. You could achieve the same impedance conversions with sections of open wire stretched out and interconnected. It would work exactly the same way, but take up a fair bit of space! So all you're doing when you add a balun to the end of a piece of coax (or at the "unbalanced" output of a rig) is you're isolating one end from the other by imposing a long section of transmission line between them. The simple fact is that, if it's long enough, the open wire transmission line itself will do the same thing *if* the antenna is balanced. If it's an off center fed doublet or other inherently unbalanced antenna, the feeder currents won't be balanced no matter what you do. That doesn't make the feeder inherently lossy. The worst is that the feeders will tend to radiate a bit, depending upon the degree of unbalance. The solution to unwanted RF on the outside of the rig is to stop the currents from flowing on the outside of the coax shield and the rig. Usually they get there by flowing out of the open end of the coax at the transition to the open wire line and around the edge right onto the outer surface. Sometimes a balun will provide enough isolation that the surface currents don't cause trouble, but sometimes not. Another way to reduce those currents is to put some ferrite beads over the coax. A number of companies sell ferrite beads for just that purpose. Some even provide a complete cable assembly with the beads installed. Those beads show a very high impedance to any RF currents on the outside of the shield, stopping or reducing them drastically. Your rig still is not "grounded" for RF, but you don't care. There's no RF to cause trouble on the rig anyway. Another approach many installations use is to have a long section of coax coiled up to act as an effective balun because the currents on the outside experience the coil as an inductor with reactance that stops them while the currents on the inside of the coax only "see" the transmission line and are unaffected by the fact it's wound up in a coil. (It's important the coil be solenoidal and single-layer so the input end is well isolated from the output end. A jumble of coax is not very effective). That's *not* a good idea for you because you're operating the coax at a high SWR, so you have significant losses for every foot of coax you have in the system. Keep the coax as short as possible! 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
