On 12/21/2013 6:56 AM, WILLIS COOKE wrote:
Kurt, 20 to 30 feet of RG-213 will give you an acceptable loss even on 6 meters and almost none on the lower bands. I have over 200 feet of RG-213 feeding my SteppIR and it works pretty well on 6 meters. I feed my L with about 125 feet of RG-8X and it works very well on 160 and 80.
Right -- IF the SWR at the antenna is not too high, and if the feedline is not too long. Every edition of the ARRL Handbook since at least the 50s has a graph of the increased loss due to mismatch. It's in the chapter on Transmission Lines.
Study the difference between a balun and an unun. You feed a balanced antenna with a balun (balanced antenna to unbalanced coax feed) and an unbalanced antenna with an unun or unbalanced to unbalanced. Neither is a magic device and both are just transformers.
WRONG! Which is why I so strongly object to the word "balun," which is used to describe at least a half dozen very different electrical components. Yes, there are some transformers, mostly autotransformers, that are called baluns. But the most commonly used things that are called "baluns" are really common mode chokes. The function of a common mode choke is to BLOCK common mode current on the cable.
Vendors enclose them in PVC to keep you from seeing what is really there. All you really need to feed an inverted L is a choke made from six to eight turn coil of coax at the feed point of the antenna to keep the current off the outside of the shield.
A coil of coax is a VERY misguided attempt at a common mode choke, because it is an inductor, and in the common mode circuit, a feedline shorter than a quarter wave is capacitive, so the inductor can resonate with the capacitance of the line and cause common mode current to increase! In other words, there are many practical situations in which a coil of coax does not work at all, and makes things worse.
To make an effective common mode choke (often called a "current balun" to disguise how it works), we need a choke that is predominantly RESISTIVE at the frequency(ies) of interest. We achieve that by winding turns on a ferrite material that is very lossy at the frequency(ies) where we want the choke to be effective. A Fair-Rite #31 core is effective from VHF down to 160M and the AM broadcast band; #43 material is effective from about 4 MHz up to VHF. Study http://k9yc.com/RFI-Ham.pdf
I will stay with my recommendation to forget the external tuner and stay with the K3 tuner.
Agreed -- IF it's big coax, not too long, and the mismatch between the antenna and the feedline is 10:1 or better.
73, Jim K9YC ______________________________________________________________ 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
