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@conwellpdx.com>
To: topband@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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