Charles, Joe listed some symptoms of excessive feedine common-mode current such as RF bites and RF feedback. If you do not suffer from any of these effects a balun may be of no use.
If one does experience CM problems that might be cured by a balun (CM choke) by all means give it a try. In my NEC model a CM choke having an impedance magnitude of 300 ohms reduced the coax CM current by one half. My anecdotal evidence from a dozen vericals is that no balun has been needed. Even in the case of a 12' top loaded 1.8 MHz vertical having a base current of 10 amps. Dave WX7X On Jan 27, 2012 9:40 AM, "Charles Moizeau" <[email protected]> wrote: > > My radial field consists of 55 radials, 75' to 150' in length, buried 0.5" > to 1" deep. My coax feedline, encased by a 1.25" gray pvc conduit, is 12'' > deep and 80' long. It passes beneath several radials between the shack and > the antenna base. > > I don't use a common-mode choke at the base feedpoint of my inverted L, > where the only matching element is a series-connected capacitor to cancel > out the inductive reactance of the antenna's total length of 170'. > > I am willing to insert a common-mode choke, but don't know what to measure > beforehand to learn if one is needed. Nor do I know what changed > indications to look for after such a choke has been installed. > > I'd be grateful for any advice. > > 73, > > Charles, W2SH > > > From: [email protected] > > To: [email protected] > > Date: Fri, 27 Jan 2012 10:20:16 -0500 > > Subject: Re: Topband: T Vertical feed > > > > > This is a terrible error in logic. Current on the radials will divide > > > based on the impedance of each radial. If the feedline happens to be > > > a "pathological" length its (outer) shield can carry *all* of the > > > antenna return current. > > > > To Joe's point, I don't think we want the feedline to become a radial. > It > > also seems that placement of the line should occur under the radial field > > and not on top of it, but I have not seen any studies that compare > > measurements. Anyone have this data? My initial thought for base-fed > > verticals is to use a CM choke at the base and also at the perimeter of > the > > radial field, unless by placing the line under the field significantly > helps > > to reduce coupling to the line. > > > > Paul, W9AC > > > > _______________________________________________ > > UR RST IS ... ... ..9 QSB QSB - hw? BK > > _______________________________________________ > UR RST IS ... ... ..9 QSB QSB - hw? BK > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
