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## Re: [TowerTalk] Balun Recommendation

 Subject: Re: [TowerTalk] Balun Recommendation Steve Hunt Sat, 21 Apr 2012 08:38:40 +0100 mailto:towertalk@contesting.com>
 ```Gerald, You analysis is correct! Of course, the CM impedance "looking into" the coax braid at the feedpoint varies depending on the coax length: if the coax is well grounded at the shack end and is an odd multiple of a quarter-wave long, the CM impedance will be very high and you probably don't need a choke; but if it's a multiple of a half-wave long the impedance can be very low and far more current will flow along the braid than into the dipole leg. I've been doing some work over the past day or so to try to understand better what the common-mode impedances are; a typical worst case with the coax 3/4 wave long was 16+j0 - that made the CM current about 10 times the current into the dipole leg!! Understanding the range of CM impedances we're likely to encounter has implications for the choke design. I'll start a new thread to outline what I've been doing on that topic. 73, Steve G3TXQ On 21/04/2012 02:56, TexasRF@aol.com wrote: > Hi Ian and others: > > Is common mode current the same as rf current flowing on the coax shield > because it is connected to one half of the antenna? > > If so, no wonder the feed point is unbalanced; there are two return current > paths for any voltage present on the shield side of the antenna. The "hot" > side of the antenna reflection sees the only the coax center conductor and > associated impedance. The other side of the antenna reflections sees the > inside of the coax and associated impedance in parallel with the outside of > the coax and it's associated impedance. The total impedance then has to be > less than the "hot" side. > > So, there is a balanced antenna feedpoint with say 50 ohms referenced to > ground on one side and maybe 35 ohms on the other side reference to ground. > The voltage would have to divide 35/50 in that case. > > Providing a means to reduce the current on the shield has to improve the > voltage ratio on the two halves of the antenna as that makes the impedance > referenced to ground more nearly equal. Series choke losses also have a > parallel equivalent that would tend to degrade the balance. So, it seems the > choke can help balance to a large degree but never completely restore > balance. > > Any corrections to this understanding are cheerfully accepted. > > 73, Gerald K5GW > > > > > > > In a message dated 4/20/2012 11:57:10 A.M. Central Daylight Time, > gm3sek@ifwtech.co.uk writes: > > Gerald wrote: >> Hi All, at the risk of really showing some ignorance, a question: >> >> If the choke is doing a good job and located at the feedpoint, isn't the >> common mode circuit quite short? >> >> Seems that the common mode current is the cause of the feed point > imbalance >> rather than the imbalance causing the common mode current. Chicken first >> or egg first? >> > Neither; or both. > > The only parameters that can truly be labeled "first causes" of antenna > behavior are the physical dimensions of the antenna elements and > feedline, their location relative to ground and other nearby objects, > and the frequency and power level of the applied signal. > > These are the parameters that constrain the antenna's behavior according > to the laws of EM physics. It is no coincidence that these are also the > inputs required by an antenna modeling program, which then computes the > one-and-only solution that will fit all the facts supplied. > > Antenna currents, feedline currents (both CM and differential) and all > the related voltages and impedances are all related parts of the > antenna's behavior - but they are all *consequences*, not causes. > > Because these are all *related* consequences, a change in one will be > accompanied by a change in all the others. But "chicken or egg" > arguments are futile because none of them has been the root cause of the > changes we're talking about. > > > So here we are, with an antenna system in which some unwanted CM current > is flowing on the feedline. If we now install a CM choke, we are adding > completely new constraint upon the system's behavior, in the form of a > high impedance at that particular location (which modeling software > would call a "load"). > > The key concept is that we are deliberately forcing the ENTIRE > antenna-feedline system to behave differently from the way it did > without the choke. Everything changes to accommodate this new > constraint, so NONE of the RF currents and voltages will be the same as > before. > > Many of the "chicken or egg" arguments that we've seen in the past few > days contain a hidden assumption that CM voltages or currents will be > the same both before and after the choke is inserted. They won't! The > only valid solution is to recompute the behavior of the entire > antenna-feedline system. Unfortunately that will only produce an answer > which is valid for a particular situation; but it also explains why > anecdotal reports can be so variable. > > > > > -- > > 73 from Ian GM3SEK > http://www.ifwtech.co.uk/g3sek > _______________________________________________ > > > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk > > _______________________________________________ > > > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk > > _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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