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 <w2sh@msn.com> 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: olinger@bellsouth.net
> To: w2sh@msn.com
> CC: w9ac@arrl.net; topband@contesting.com
>
>
> 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 <w2sh@msn.com> 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: w9ac@arrl.net
> > To: topband@contesting.com
> > 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
>
>
>
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