Topband: Common mode choking of beverages - which side of grounded shack entry panel?

[donovanf at erols.com]

Hi Bill, 


The antenna feed point is where common mode signals on the outside 
of coaxial cable shield most easily and severely degrade antenna 
directivity. This a critical antenna performance issue for very low 
signal level compact receiving antennas such as small loops and 
short high impedance verticals and somewhat less critical for higher 
signal level receiving antennas such as Beverages and resonant 
short verticals (such as umbrella verticals). 


Common mode signals less severely affect the performance of 
high signal level antennas such as Yagis, horizontal dipoles and 
quarter wavelength verticals over extensive radial systems 
laid directly on the ground. However, multi-transmitter and 
SO2R contest stations should use common mode chokes at their 
antenna feed points to minimize cross-station interference. 

Double shielded coaxial cable or hard line coax with properly installed 
high quality coaxial connectors greatly minimize the penetration 
of common mode signals into the coaxial cable and the equipment to 
which it is connected. In most cases common mode chokes will not 
be needed in or near your shack. 


If common mode signals are penetrating into your transceiver 
-- especially in a multi-transmitter contest station -- the most 
effective common mode choke location is directly at the coaxial 
cable connection to the transceiver. 


73 
Frank 
W3LPL 

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

From: "Bill Conwell" <bill at conwellpdx.com> 
To: topband at contesting.com 
Sent: Sunday, October 4, 2020 5:24:05 AM 
Subject: Topband: Common mode choking of beverages - which side of grounded shack entry panel? 

Sorry if this has been asked and answered previously, but I don’t find it 
in a check of the archives. 



I’ll soon have an aluminum entry panel in my shack for all entering TX and 
RX lines (as well as rotator lines and misc control cables). The panel is 
copper-strapped to ground rods fifteen feet away, and to station ground. 



My beverages have common mode chokes near the antennas, followed by 250-500 
ft of RG6 coax to the entry panel. (TX antennas are also choked, near 
their feedpoints.) 



If I were to add further common mode choking at the shack end of the 
beverage coaxes, where should such choking best be applied – to the RG6 
cables on the outside, or the shack side, of the entry panel? 





Tnx, 



/Bill, K2PO 

Portland, Oregon 
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