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 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 ohmsRadial number two 100 ohmsRadial 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 ohmsRadial two 80 ohmsRadial three 100 ohmsRadial four 275 ohms
Radial five 300 ohmsRadial six 410 ohmsRadial 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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UR RST IS ... ... ..9 QSB QSB - hw? BK
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