Dean's example is still valid. By placing the CM Choke at the "wrong" distance of 20 feet from the tuner, the 3,000 ohm Z presented at the balun in addition to the extra 20 feet of line can create substantial mismatch loss (i.e., additional loss due to VSWR). The choke simply adds three feet of coax for a total of 23 feet from the tuner. If the balanced line were dragged into the shack to reach the tuner output, loss is much less and the remaining loss is the result of the severe mismatch and heating occurring over just 3 feet of cable instead of 23 feet. Of course, the tuner will have some loss of its own that we haven't considered. But the problem is the extra coax length under a high mismatch conditions -- and not that a CM choke is present in the line, notwithstanding the three feet of coax used in its windings.
Prior authors have focused almost entirely on the CM choke's line balance and common mode reactance attributes and not systematic loss. So, <gulp> if placement of the CM choke at the tuner input results in no better balance, but no worse balance, then clearly, system loss in many situations (e.g., multi-band dipoles and loops) will be less and that configuration may be preferred. OTOH, if balance suffers, then one must look at whether loss or line balance or choke heating is more important, depending on the mismatch magnitude appearing at a CM choke located at the tuner output -- or at some "wrong" distance away from the tuner. Paul, W9AC ----- Original Message ----- From: "Don Wilhelm" <[email protected]> To: "Dean Straw" <[email protected]> Cc: <[email protected]> Sent: Wednesday, December 14, 2011 7:59 PM Subject: Re: [Elecraft] Balun at input or output of tuner > Dean, > > The last two paragraphs of your writeup do not seem relevant to the > discussion of balun (CM choke) at the input or output Those paragraphs > deal with operating coax at a very high 60:1 SWR, and neither support > nor agree with the other points. > > Consider the following: A situation where the windowsill connection is > 20 feet away from the tuner output. The balun has the same loss no > matter where it is placed, so lets assume it is placed at the output. > > Now, consider that the connection between the tuner output and the > windowsill is with 20 feet of RG-213. The balanced line is connected > directly to the coax (no balun). By the analysis presented, the loss > will be exactly the same as with the balun connected at the windowsill > end of the coax. > > Both conditions are electrically the same (If that point is arguable, > then the balun at the tuner input is just as arguable). > > If we can extend this argument, we would be able to conclude that it > makes no difference on a coax fed antenna whether the balun is placed at > the antenna or at the tuner output - no matter whether the feedline is > coax or balanced line. Oh, yes, both the coax or balanced line must be > isolated and run with the same rules normally applied to balanced line. > The point is that while theory says it makes no difference, it is > impossible to achieve that perfect isolation, so the argument falls apart. > > Because most would not consider connecting a balanced line to coax > without an intervening balun because we have been taught that we must > preserve balance in order for things to be correct. > The fallacy I see with the balun at the input vs. balun at the output > argument is that with the balun on the input, everything that follows > must be perfectly isolated from ground - and that is difficult to > achieve when all physical things are considered - if there are *any* > strays, it defeats the principle of perfect isolation. That is also > what makes my example of the 20 feet of coax at the balanced output of a > tuner not a practical consideration - one cannot easily achieve equal > coupling of the center conductor and shield if there are any surrounding > objects. That fact makes the unbalanced tuner with an input balun not > practical because that perfect isolation is just as (or more) difficult > and expensive to achieve than it would be to implement a balanced network. > > 73, > Don W3FPR > > > On 12/12/2011 3:38 PM, Dean Straw wrote: >> While we're at it, let's look at the potential loss due to line >> losses at a CM choke balun placed in the wrong place in an antenna >> system. >> Assume the common scenario where a balanced antenna is fed with open-wire >> transmission line to a 1:1 common-mode choke balun located at the shack >> window. From the balun at the window the ham uses, say, a 20-foot section >> of >> RG-213 to the antenna tuner (which in this case is an unbalanced tuning >> network). Assume again that the CM choke balun uses three feet of RG-213 >> wound on the appropriate ferrite donuts to achieve the target common-mode >> impedance of 5000 ohms so that common-mode currents are choked off >> properly. >> >> The total length of RG-213 is now 23 feet. Again, we'll present the >> balun at the windowsill with a load of 3000 ohms. The overall >> differential-mode loss in 23 feet of RG-213 is 4.534 dB, nearly 4 dB >> worse >> than connecting the open-wire line directly to a tuner with a CM choke >> balun >> at its output! Ouch, that's a lot of wasted power. >> >> 73, Dean, N6BV >> > ______________________________________________________________ > 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 ______________________________________________________________ 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
