On of the topics often discussed here in the past is the importance of
having good common mode rejection on unbalanced coax feeds for
Beverages, EWES, and Pennants etc. Most of my Beverage feeds are
between 350 to 500 feet long and are made from consumer quality RG-6
cable. Within 20 feet of the shack I place two separate 2.5 inch Type
31 ferrite rings and connect to a common ground all such feedlines thus
hoping to reduce common mode pickup from the unbalance medium. Each RG-6
feed has two ferrite rings and with twelve separate Beverages and at $8
to $10 per ring you will see why I raise the the following issue. I am
told that a simple 1:1 blaun would work as well as a common mode breaker
but so far I prefered the idea of winding the coax on the rings,
grounding the shield of the coax between the rings, rather than start
drilling up some plastic boxes with F connectors which would have been
simple enough. I just didn't like the idea of another pickup point for
noise ingress.
Recently I purchased some video baluns to use for an FM station to
remove hum from cables feeding a composite audio single from the
stations processor rack across the room to a bank of 5 KW transmitters.
The on air hum disappeared and I was amazed how such a simple ferrite
device did such a great job om a very high RF environment, although
designed for video monitors. I see that several vendors make them in
various forms and the specs show very little insertion loss and the
ability to work to 5 Mhz. I put them to use so fat that I never thought
till now of using the same little device on TB as a replacement for
expensive large toriods. The come with a variety of connectors. Mine
were BNC. Has anyone else ever tried these devices? MCM and M.P. Jones
have them in the catalog in different varieties. They are also found in
video security vendor's catalogs as a plug and play click on solution
for pesky hum rolling through video monitors.
These leads me to another question of using CAT 5 cable for feeding
Beverages. It is balanced, cheap, and some are even designed for self
sealing and shielded versions for direct burial. The reason I ask is
that a similar balun, but designed for long runs of CAT 5, is also
available in a special on line flyer for $10 each if you buy four of
them. The MCM part is 50-7725 with source Code DD1252. (sale ends
midnight May 14, 2012) These baluns are designed to send two separate
audio signals using CAT 5 up to 1000 feet. The unbalanced inputs are
RCA connectors and the CAT 5 is a RJ-45. Since the wire in CAT 5 cable
is a series of twisted pair, and used for years for longer runs of
balanced pair audio and video, perhaps they would have a suitable lower
cost application of feedings Beverages and Two wire reversible Beverages
some distance away from the shack and tower. I have yet to be able to
find out what the impedance of a single twisted pair is on CAT 5 cable.
Nor do I know if these baluns double up on pairs.
There must be a downside to this suggestion or I would have heard some
comments of the A/V balun used for TB RX situations by now. So I pass
this along regardless as someone may have had an chance to try them.
Herb Schoenbohm, KV4FZ
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|