Yup. Metal to metal contact with the radials, folks don't have a good mental summarizing device for radials, thinking that "ground" is some kind of sucking everything up magic medium. Text below...
73, Guy. On Fri, Jan 27, 2012 at 6:10 PM, Charles Moizeau <[email protected]> wrote: > Guy, > > Your text is very instructive. I hope I'm not the only follower of this > thread who up until now thought that the shield of the coax feedline was > acquiring undesired current by capacitive coupling to the other radials. > So I therefore assumed that keeping the feedline away from the radials, > either by digging it deeper or routing it far away laterally from the > radials, would mitigate such a problem. > > I hope I'm now properly understanding that the the coax feedline's shield > is hogging a huge proportion of total earth current by virtue of its (much) > lower resistance > compared to the total resistance of all the radials even > If the total parallel radials is "much" higher than the coax, one has a really stinky radial system. But that DOES occur. though those are all in parallel with each other. > > Am I slowly getting on the right track here? > > Charles, W2SH > > > ------------------------------ > Date: Fri, 27 Jan 2012 14:58:32 -0500 > > Subject: Re: Topband: T Vertical feed > From: [email protected] > To: [email protected] > CC: [email protected]; [email protected] > > > Sometimes the only symptom of common mode connection to your antenna is > excessive "ambient" noise, usually from the AC wiring system in the house. > > A on/in ground radial field is not a monolithic single very low Z entity > for purposes of figuring out what is happening in the current division > exercise vis-a-vis how much does the feed line get. > > It is EVERY RADIAL FOR ITSELF. It looks like this for a very well done > radial system: > > Radial number one 100 ohms > Radial number two 100 ohms > Radial number three 100 ohms > ....etc.... > Radial number sixty 100 ohms. > Feedline coax shield 1.7 ohms. > > The single 1.7 ohms lowers the voltage and even in this case of what > appears to be an excellent ground radials system, the coax will carry HALF > the counterpoise current and waste most of that power, besides being a link > to all the household noise, EVEN with a ground rod at the house. See other > material about the RF impedance of groundrods. > > And that is IF the individual radial effective series resistances are AS > LOW as 100 ohms AND if they are all equal, which they usually aren't. They > can have equal length and have widely varying INDIVIDUAL effective series > resistances, all in the same radial field. PARTICULARLY so in a > residential setting. > > What happens if the radial impedances are more like this, more like real > life > > Radial one 60 ohms > Radial two 80 ohms > Radial three 100 ohms > Radial four 275 ohms > Radial five 300 ohms > Radial six 410 ohms > Radial seven 935 ohms (short) > Radial eight 32 ohms (the only one "long enough") > Coax shield 1.7 ohms > > Here the Coax shield for all practical purposes is the ONLY radial, > completely bypassing whatever limited usefulness possessed by the radials. > If you do put an excellent common mode block on it, you might be tempted > to take it out, because the SWR will go a lot higher and higher SWR is > worse, right? > > A good ground radial field gets its efficiency by massive parallelism of > what are in fact fairly resistive single radials, not by innately efficient > radials in/on the ground. A given single ground radial is inefficient, > period. > > Therefore it is a fundamental strategic error to OMIT a proven EXCELLENT > common mode blocking device where the feedline shield connects to any kind > of a counterpoise underneath ANY single pole 160m antenna. The outcome of > the omission is noise in the antenna, and amazing loss in the ground from > the coax shield and whatever incidental connected conductors. > > For the 5/16 folded counterpoise solution, the acceptance impedance of the > FCP is quite highly reactive, and dealt with by means of an isolation > transformer, which is the ultimate common mode current block. > > 73, Guy > > On Fri, Jan 27, 2012 at 11:40 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
