[AMRadio] BC-610 terminating impedance

Rbethman rbethman at comcast.net
Fri Oct 28 12:36:56 EDT 2005

The standard connection from the porcelain insulated connections to the 
BC-939 is INDEED 50 ohm coaxial cable.

Those desiring to ELIMINATE TVI issues would NORMALLY install a Low-Pass 
filter in-line between the BC-610 and the BC-939 which is already 
internally wired with 50 ohm coaxial cable.

I also place the ME-165G in-line PRIOR to the BC-939 or the proper 
dipole.  My BC-610 location does not allow for a "wire" exiting from the 
BC-939 to the outside.  So it is coax to the outside.

I ALSO own a T-213, an overhauled BC-610 by depot, in August 1959.  One 
of the changes from BC-610 to T-213, was to remove the porcelain 
insulators and replace ONE with an SO-239.  I can find NO internal final 
circuit mods other than this, and the schematics agree with this assessment.

As Geoff/W5MOR has indicated from his operations, I TOO have had ONE and 
ONLY one gripe about getting into a CHEAP "portable" telephone since 
2000.  I provided the individual with the necessary information to 
contact the phone company to have a filter put in place.  He has NOT 
returned or complained again.

There is NO question that the output of the BC-610 series, my first one 
happens to be an (I) model, is indeed 50 ohms impedance.  It is 
unfortunate that some hams make the ASSUMPTION that since it has 
porcelain insulators for the output, that it is a balanced output of 
OTHER than 50 ohms.

I cannot stress the importance of obtaining the manual, carefully 
studying it, and pay close attention to the details.  The ME-165G is 
unquestionably a 50 ohm input SWR/Wattmeter/dummy load.  It comes with 
"N" connectors for input and output.

Don is ABSOLUTELY correct!

The use of 44ft and 77ft "wires, AND the use of the "term" long wire, is 
the military's AND the manufacturer's.  If you are going to use a dipole 
do NOT use the BC-939!  It's intent was for the specified wire lengths 
and/or the issued whip antenna.

Of course I have not pursued a whip installation.  It would seem to be a 
useless endeavor as the 160, 80, 40, and 20 meter bands are "normally" 
of horizontal polarization.  I see no logic in operating in cross 
polarization, and making communications more difficult than necessary.

Bob - N0DGN

Donald Chester wrote:

> The final is single-ended, but the plate tank circuit is balanced just 
> as in a pushpull amplifier.  This is the classic plate-neutralised 
> triode final.  The other half of the coil functions to generate the 
> out-of-phase rf voltage required to neutralise the circuit.  True, the 
> tube works into only half the coil, but the circulating current of the 
> resonant tank circuit is present in the entire coil, which along with 
> the split-stator variable tank capacitor make up the tuned circuit.  
> The Q of the tank is determined by the LC ratio of the entire coil and 
> capaciter.
> The circuit is inherently balanced.  The flywheel effect of the tank 
> circuit takes care of the fact that the rf pulses are introduced on 
> only one end of the coil.  The link output on the BC-610 came from the 
> factory balanced, with two ceramic insulators at the output.  I have 
> seen many of these converted to an unbalance output by grounding one 
> side to the cabinet and using a SO-239.  But the link in the middle of 
> the tank coil is inherently a balanced output.
> Don K4KYV

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