[RFI] coax shield outside impedance?

donovanf at erols.com donovanf at erols.com
Wed Nov 25 17:45:46 EST 2020

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 

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. 


----- Original Message -----

From: "Dave Cole" <dave at nk7z.net> 
To: rfi at contesting.com 
Sent: Wednesday, November 25, 2020 8:52:33 PM 
Subject: Re: [RFI] coax shield outside impedance? 

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) 
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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