Hi Frank,
Thank you Frank!!
73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources
On 11/25/20 2:45 PM, donovanf@erols.com wrote:
Hi Dave,
Burial prevents common mode radiation onto the the buried portion of
the feed line. This is an excellent common mode suppression technique
for very low signal level receiving antennas such as small loops, high
impedance short verticals (e.g., Hi-Z verticals) and multi-element
receiving antennas with very close spaced elements (e.g., Waller flag
arrays).
Coaxial cables laid directly on the ground provide adequate common
mode suppression when used with more efficient receiving antennas
such as Beverages and low impedance short verticals. A fairly
low impedance ground system at their feed point provides
additional common mode signal suppression.
Transmitting antennas (except ground based verticals) are almost always
elevated. Common mode currents will be radiated onto the outside of
the elevated portion of the feed line, they then enter the coaxial cable at
the antenna feed point. Buried cables can't prevent common mode
signals from entering the elevated feed point.
73
Frank
W3LPL
----- Original Message -----
From: "Dave Cole" <dave@nk7z.net>
To: rfi@contesting.com
Sent: Wednesday, November 25, 2020 8:52:33 PM
Subject: Re: [RFI] coax shield outside impedance?
Jim,
I have an additional question along this line...
If the coax is buried, I have heard, there is no need for a choke. Is
this true?
73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources
On 11/25/20 11:38 AM, Jim Brown wrote:
On 11/25/2020 10:47 AM, Ray LaRue wrote:
*When attempting to minimze CMI on the outside of 50 ohm coax, the
shield is not normally 50 ohms. What shield Z do you use for
calculating the needed choke Z and thus the attenuation to expect?
In the common mode circuit, the coax shield is acting as part of the
antenna, so like any antenna, current and voltage, thus the impedance,
vary along its length. Impedance is highest at ends, lowest a quarter
wave from ends. A good choke has a very high impedance (5-10K ohms), so
forces a high impedance, which results in a low impedance a quarter wave
away. To minimize common mode current on the feedline coupling to the
antenna, the optimum point for the choke is at the feedpoint (that is,
up in the air).
73, Jim K(YC
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