Please note - this post is not meant to be an ad for DX Engineering - if
something works for me - DXE will carry it for others to use if they like
I have researched high performance feedline chokes for 35 years. Many years
ago I ran across an article written by Bill, W0IYH now an SK. This is his
feedline choke (also known as a very good 1:1 HF balun)
<https://www.dxengineering.com/parts/com-cfc-50k>
https://www.dxengineering.com/parts/com-cfc-50k - chokes designed by K9YC
are also VERY good.
Recently, #31 beads that accept RG-213 size coax (.405 outside diameter)
became available - so this is what I use now - as I don't have to have a
double female in line. I simply install these beads over the coax , then
install the connectors.
<https://www.dxengineering.com/parts/dxe-rfc-160-20>
https://www.dxengineering.com/parts/dxe-rfc-160-20 the choking impedance is
better than the W0IYH design.
To interface coax feeds to Yagi beam antennas W8WWV designed this Feed Point
Connector (very robust).
<https://www.dxengineering.com/parts/dxe-fpc-so239>
https://www.dxengineering.com/parts/dxe-fpc-so239
W8WWV also discovered that taping ferrite chokes to aluminum booms or tower
legs causes the common mode impedance of the bead choke to decrease. So I
designed these stand offs to fix that problem
<https://www.dxengineering.com/parts/dxe-bcso-3-100>
https://www.dxengineering.com/parts/dxe-bcso-3-100
So how are common mode chokes installed at K3LR? Every feedpoint of every
Yagi and vertical element has a high impedance common mode choke installed.
The OWA Yagi's here feed into a custom antenna switch (monostacker) that
matches the combination of antennas (1,2,3 or 4) being used and the output
of the antenna switch also has a #31 ferrite common mode bead choke before
going into the hardline that goes to the station. Once inside the station -
every feedline has another common mode bead choke (or similar ferrite core
for RX) before going into the amplifier or receive antenna matrix for that
band.
This installation may seem extreme to some - but it is not. When using
multiple transmitters (SO2R, multi operator with multiple bands) this level
of choking is very important. If a feedline becomes an antenna (not
intended) it makes your station less efficient. I have measured the RF that
is on the outside shields of hardlines and coax cables with a spectrum
analyzer. It is amazing and scary. Good common mode chokes knock this stray
RF (from AM broadcast, short wave TX, adjacent amateur transmitters and
others) down significantly, which lowers the noise floor on the amateur
bands of interest.
Why anyone would consider using a HyGain BN-86 puzzles me. Now that it is
easy to measure its poor performance - the BN-86 has very low common mode
impedance. Today there are much better solutions - see K9YC's cookbook - or
BN-86 readymade high common mode impedance solutions at DX Engineering.
73,
Tim K3LR @ DX Engineering
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