[Alexandria Radio Club Reflector] Common-Mode Chokes for Field Day

[Don Lewis (KI4D)]

This note follows up on comments I have made regarding the importance of
using common-mode chokes in the Field Day environment to prevent mutual RFI
caused by the close proximity of several emitters, transmission lines, and
antennas running power.  It is a little late but I  hope some use can be
made of the references  so that some of the RFI from common-mode currents
can be suppressed or eliminated this year.  In the Field Day environment
common-mode chokes should be placed on the emitter end of a transmission
line as well as on the antenna end of a transmission line, but especially on
the antenna end of a transmission line to prevent the transmission line from
absorbing RF from the antenna and  becoming a radiator.  These chokes are
effective because they absorb common-mode current.  Use of a ferrite core
chokes are also be beneficial on  power cords of emitters.  I remember being
surprised that Icom actually supplied a ferrite core choke for the PW-1 I
once owned, and it was effective for   preventing RF from exciting the power
lines in my townhouse complex. I now use them on the power cords all of my
exciters, especially amps.  I am including some quotes from the RFI
reference below by K9YC called  "RFI-Ham.pdf":      

 

". Receiving Antennas benefit significantly from the addition of a common
mode choke to isolate the antenna from its feedline. Both ends of the RG6
feedline to each of my Beverages takes 8 turns through one of the Big
Clamp-Ons.

 

"Egg Insulators:" Use chokes as "egg insulators" on a feedline that is
longer than about λ/4 at the frequency of an antenna that it could interfere
with. Use the chokes recommended for the frequency of the antenna that you
suspect might be interfered with. Use enough chokes that the longest piece
of un-choked line is less than about λ/4. Chokes are most important on lines
that are

approximately in parallel with the antenna in question. 

 

Example: A feedline is running where it might be interfering with an 80
meter vertical. Try to break it down into pieces that are no longer than
80/4 meters (about 67 ft).

 

Table One summarizes cost and weight considerations for various chokes and
construction methods. Traditional "string of beads" chokes are more
expensive and heavier for equivalent performance. Cost and weight includes
for the added length of coax for the choke. Weight can also be reduced by
using specialized coax like RG303, rated for high temperature and high
voltage. Costs assume group purchase in large quantities by a ham club.

 

Table One - Weight and Cost of Ferrite Parts and Complete Chokes (see
reference)

 

The Big Clamp-Ons appear to be expensive but they're not - one is roughly
equivalent to three or toroids! They are wonderful for portable and Field
Day operations, because you can easily apply them to coax without taking the
connector off. There is another advantage - chokes for 160-40 meters using
the Clamp-On weigh about a half pound less than one using toroidal cores."

 

There are many references, but these two seemed most pertinent:

 

 

http://audiosystemsgroup.com/RFI-Ham.pdf

 

http://www.nonstopsystems.com/radio/article-G3TXQ-chokes.pdf

 

73,

 

Don