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" <[email protected]> To: [email protected] 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 _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
