At 04:12 PM 1/11/2012, you wrote: >Gents, > >I have a ladder line and balun question that I hope someone can guide me >thru.
A 1 wavelength loop represents approximately 100 ohms non reactive impedance. Feeding this antenna on its fundamental frequency is relatively straight forward ... and it can be done with coax ... using some tricks to get it down very close to 50 ohms. However if, as is commonly done the loop is used on harmonics making it multi band the situation is much more complex and the feed is changed from coax to parallel feed line as you have done. The impedance at the feed point increases with harmonics with the addition of an increasing reactive component. It is common in such antenna systems to make the transition from the parallel feed line to coax as it approaches the shack .... since it is generally easier to physically route it through walls etc ... and it is thought that it radiates less than the parallel line (usually not the case). It is possible to actually measure the complex impedance at the end of the parallel line using an analyzer .... but inevitably there will be a mismatch to the coax whether using a 4:1 or 1:1 balun. which will cause radiation from the coax line. It is simply a matter of determining if the amount of radiation from the coax section is tolerable or not. It is possible to bring the parallel feed line right into the shack and connect it directly to an impedance coupler .... and in fact this was the usual method for many years ... and I have done this with no adverse RF effects in the shack. The easiest method of using a large loop on its resonant harmonics is to install the antenna coupler right at the feed point and use coax for the full feed line length. It is possible to make some guesses as to the impedance at the end of the feed line at the different frequencies given the length of the parallel feed line. The experience you describe would indicate the impedance is rather low as the 1:1 balun worked well. The most common strategy for this arrangement is the cut and try method. Stick with what works .... there are no magic rules of thumb. Jim, VE3CI ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
