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
>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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