Kevin Williams asked: >> Could someone tell me what kind of balun is used on a typical reference dipole? <<
This is the kind of question that will get you in trouble, Kevin. The message thread may never die! A balun has two functions in a reference dipole. It has to decouple -- balance - the coax to dipole transition, and it has to match the (nominally) 73 ohms dipole impedance to 50 ohms. This is easy at spot frequencies. Sometimes it's done with shorted parallel lines; I've a couple of dipoles like that, older ones, in my garage. Along with a tape calibrated in Mc. But such a balun must be adjusted to the correct length as frequency is changed, just like the elements. It can be damaged -- bent -- or become corroded. If you open a biconical dipole, especially the older ones, you may find a balun made by winding coax around a ferrite rod. These have the advantage that they are (theoretically, anyway) broadband, and sometimes antenna sets will use the same balun on their reference dipole and wideband bicon. But this is a problem. First, the balun is not perfect. Its windings have capacitance of their own, capacitance to the (usually metal) housing in which they are mounted, and a certain physical imbalance that can't be avoided, only minimized. Then too, the balun wound this way may not do much to match 73 to 50 ohms, and with standing waves between the balun and the dipole, self-resonances and parasitic reactance, its match and transmission can differ across the band of interest. Still this is a common balun, as it's handy for broadband measurement. The Roberts Dipole was invented by a United States Bureau of Standards engineer. What he did was to come up with a compensated, linear, coaxial matching and balancing device whose characteristics were predictable and stable, and whose construction made changes extremely unlikely. This is the preferred design in the USA because it is accurate, well known, accepted by the US government (they invented it) and the FCC uses it. The design is published; you could easily build them yourself. Or you can buy one. One of the simpler and yet still effective baluns is a sleeve of ferrites on the outside of a coaxial cable. While this does not match 73 to 50 ohms, if it is long enough, and absorbs enough, it will prevent current from flowing on the coax shield, and give a good balance. It does add loss to the antenna, but a some loss is tolerated anyway. I understand there is at least one broadband antenna using this balun; I don't know it it's used in a reference antenna. By the way,some antennas use a resistive, minimum-loss pad to perform matching. This will raise the antenna factor, but it does keep the SWR down. The Europeans take a slightly different approach. I have a Schwarzbeck dipole; its balun tube is entirely of metal, and since it does work (and I have elements) I am loathe to open it up and see how it is built. Someone else here will likely be able to tell us. And I have seen a paper, somewhere, which described an antenna whose balancing is done entirely by resistive attenuation. Do you have a particular application, or are you looking for information to build your own antenna? Cortland --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], or [email protected] (the list administrators).
