David, I have enjoyed the dialog on this subject immensly as it reminds me of the jouney I travelled to opitmize my wire antenna. The two major sources I have studied include www.dxengineering (their articles on baluns and coax losses in multiband antenna installations are excellent) and www.cebik.com. Without going through the entire story (it is LONG) I have determined that:
1) All antennas need to be grounded via lightning arrestor at the entrance to the shack. I use arrestors made for balanced line from www.iceradioproducts.com mounted on a ground rod at the entrance of the antenna wires into the shack. 2) For a multi-band wire antenna the SWR on some bands like 30m and 15m are so high (50:1) that using any kind of coax will reduce both transmitted and received signals by a huge amount. That is why the G5RV doesn't really work well on these bands. Check out the math in the articles by Cebik. Run as MUCH ladder line as you can and as LITTLE coax as you must with the balun at the shack wall on the outside. In my case the coax from the balun to the antenna switch in the shack is 6 ft. 3) The best balun could very well be 1:1 current type depending upon your other factors. With the feed point impedance of a multi-band antenna varying all over the map between bands in some cases it could be so low that reducing it further by using a 4:1 balun could make it very hard for the tuner to match it. Thus, it seems that using a balun just to make the transition from balanced-to-unbalanced without any impedance transformation is the best solution. That is what DXengineering recommends and they (of course) sell baluns for just that purpose. 4) If one is going to use a tuner designed for coax lines and a balun between the tuner and the feedline of a multi-band antenna, look for those that are rated for 1K even if you never plan to run anything near that power. The larger components used therein will have lower losses at the 100W and QRP levels. After all my research I converted my G5RV coax fed antenna to a 110 doublet by replacing the coax feed with 450 ohm ladder line into a DXEngineering balun (type AT). I liked the pattern of this length antenna based upon its direction and height above ground. It's working very well with 170 countries confirmed with 100w (about half of them since I made the change) and, most notably, it works much much better on 30m. I have sense added a 2nd wire antenna at about 45 degrees fromt the first one to get different azimuth angles from other parts of the world. Both have their own balun by DXE which is a little pricey but makes switching them easy using a coax switch inside the shack. It is easy for me to keep them separated except where they connect to the arrestors at the shack wall. A few lessons I've learned. First, any antenna is better than no antenna and a carefully researched and understood antenna (direction, pattern, feed point impedance) is better still. Second, if I had it to do over again I'm sure I would use a coax tuner with the baluns. Instead, I would have either built or bought a balanced tuner. While I like having coax from the arrestors into the shack I'm not sure how much the baluns are reducing the received signal (have to measure that some day). I will say this: the baluns are built like a tank! (Full disclosure: I have absolutely nothing to do with DXE except that I am a happy customer!). _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
