Regarding coaxial cables, RF current flows on the outer skin of the center
conductor and on the inner skin of the shield. Thus the reason for silver
plated center conductors and silver plated shield material. As to coaxial
cable radiation, this is largely dependant on the percentage of shield for
the specific cable. For that reason, tightly shielded application
requirements will use double shield silver plated cables such as RG-9.
Lines made using solid aluminum outer conductor or copper outer conductor
are known as "hard lines" and offer theoretical 100% shielding.
<quote>
"The currents in coaxial cable flow on the outside of the center conductor
and the inside of the outer shield." ARRL Handbook, 2014, page 20.2.
<quote>
"Common-mode currents are conducted and induced into the outside of the
shield of the feed line, which in turn radiates." ARRL Handbook, 2014, page
20.17
My earlier statement regarding using "a 1:1 common mode choke" is a 1:1
current balun at the feed point to attain a balanced to unbalanced condition
for a center fed dipole with a coaxial feed. This is immediately followed
by a common mode choke at the feed point to account for any natural
unbalance of the installation.
With a true flat top dipole and the feed line exiting precisely at 90
degrees from plane, there is little current induced in the coax feed.
However, any angle departure from 90 degrees, including that of a so called
inverted V, is likely to have significant common mode current induced into
the feed line. With the proliferation of inverted V usage, largely to ease
of installation, we see a notable increase in common mode current issues.
<quote>
" Reducing common-mode current on a feed line also reduces:
* Radiation from the feed line that can distort an antenna's radiation
pattern
* Radiation from the feed line that can cause RFI to nearby devices
* RF current in the shack and on power line wiring
* Coupling of noise currents on the feed line to receivers and receiving
antennas
* Coupling between different antennas via their feed lines"
ARRL Handbook, 2014, page 20-18
73
Bob, K4TAX
----- Original Message -----
From: "Mike Schatzberg" <cherokeehillfarm@earthlink.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Sunday, June 01, 2014 5:13 PM
Subject: Re: [TenTec] Trading radios
What I said was:
"With good quality cable, the center conductor is not radiating. However,
the shield may indeed be carrying current."
I don't think you have my meaning and have taken it out of context.
The percentage of braid coverage, the use of foils under the braid, double
braids, and aspects of the cable insulation and geometry, can be factors
in radiation. The common mode current can still be an issue, due to the
direction of the cable run and it's length with regard to frequency.
If you believe that there are no differences in various types of 50 ohm
coax, with regard to radiation from the cable, this does not agree with my
experience.
73,
Mike
W2AJI
Sent from my iPhone
On Jun 1, 2014, at 3:59 PM, "Rick - DJ0IP / NJ0IP" <Rick@DJ0IP.de> wrote:
Mike is doing it CORRECTLY.
My only comment is, the quality of the coax plays NO ROLE in this.
CMC flows on the outside of the coax shield so it makes no difference if
you
are using the best or the worst coax in the world - (as far as CMC is
concerned; of course you should use good coax for other reasons).
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Mike
Schatzberg
Sent: Sunday, June 01, 2014 7:39 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Trading radios
In my case the RF is radiated from my Yagi and the coax shield picks up
the
radiation between the antenna feed point and the radio.
With good quality cable, the center conductor is not radiating. However,
the shield may indeed be carrying current.
Stations with RFI often only experience interference with their
directional
arrays with the antenna in a certain alignment with their cable.
I use a 1:1 current balun at my Yagi hairpin feed point, and the line
isolator at the input to my amplifier, between it and the Orion II.
Seems to work well with all the radios here.
73,
Mike
W2AJI
Sent from my iPhone
On Jun 1, 2014, at 12:42 PM,k4elo@fastmail.fm wrote:
Disagree Bob. Put the choke at the feedpoint of the antenna or your
coax will be radiating rf all the way down to the rig. If you can't
move the antenna, this is the most effective rfi fix.
73
Wayne
K4ELO
For starters, as to RFI issues, yes antenna to radio spacing,
increasing same, will usually improve the issue. Be sure all equipment
is each
bonded to a common point such as the PS ground. Omit the external
ground but keep 3rd pin ground intact. Use a common mode choke on the
output of the radio.
73
Bob K4TAX
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
Sent from my iPhone
On Jun 1, 2014, at 3:59 PM, "Rick - DJ0IP / NJ0IP" <Rick@DJ0IP.de> wrote:
Mike is doing it CORRECTLY.
My only comment is, the quality of the coax plays NO ROLE in this.
CMC flows on the outside of the coax shield so it makes no difference if
you
are using the best or the worst coax in the world - (as far as CMC is
concerned; of course you should use good coax for other reasons).
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Mike
Schatzberg
Sent: Sunday, June 01, 2014 7:39 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Trading radios
In my case the RF is radiated from my Yagi and the coax shield picks up
the
radiation between the antenna feed point and the radio.
With good quality cable, the center conductor is not radiating. However,
the shield may indeed be carrying current.
Stations with RFI often only experience interference with their
directional
arrays with the antenna in a certain alignment with their cable.
I use a 1:1 current balun at my Yagi hairpin feed point, and the line
isolator at the input to my amplifier, between it and the Orion II.
Seems to work well with all the radios here.
73,
Mike
W2AJI
Sent from my iPhone
On Jun 1, 2014, at 12:42 PM, k4elo@fastmail.fm wrote:
Disagree Bob. Put the choke at the feedpoint of the antenna or your
coax will be radiating rf all the way down to the rig. If you can't
move the antenna, this is the most effective rfi fix.
73
Wayne
K4ELO
For starters, as to RFI issues, yes antenna to radio spacing,
increasing same, will usually improve the issue. Be sure all equipment
is each
bonded to a common point such as the PS ground. Omit the external
ground but keep 3rd pin ground intact. Use a common mode choke on the
output of the radio.
73
Bob K4TAX
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|