[Elecraft] K3 & Common mode

[Jim Brown]

On Sat, 01 Aug 2009 12:32:47 +0100, Dave G4AON wrote:

>8 turns on the stacked cores appear to give higher impedance on the low
>bands compared to typical chokes fitted to other transceiver power
>leads.

Who cares -- their impedance is far too low to make a dent in HF noise. The 
multiturn choke on the 2.4-inch toroids is FAR more effective. See measured data 
for typical chokes in my tutorial. 

> Here in Europe the Kenwood TS-480SAT includes a TDK ZCAT3035-1330
>choke which with 2 turns of the K3 lead (maximum that will fit) would
>only give around 90 Ohms impedance compared to 820 Ohms for the
>FT-240-43 with 8 turns, at 1.9 MHz.

Those chokes do nothing more than get them past EMC testing, which essentially 
looks only above 30 MHz. 

>An on-line calculator for various chokes is at: http://toroids.info/

This calculator ignores the capacitance in the equivalent circuit of a ferrite 
choke -- that is, it treats the choke as a pure inductance, assuming an 
additional capacitance will be supplied to resonate with it. It is essentially 
useless for our purposes. 

>(the TDK choke info is at: www.tdk.co.jp/tefe02/e9a15_zcat.pdf)

The TDK parts listed here are designed for suppression at VHF and UHF, and have 
little value at HF. They appear to be a cross between Fair-Rite #43 and #61. 

>80 WAS poor and 160 unusable until i put a common mode choc in DC cable.
>i got my best result with 8 turns on 4x FT-240-43 stacked.

>Is anyone with experience in chocs on DC line ?

I've done it a bit here. If your noise is coupled that way, it can help. For 
this application, you don't need four cores if you can wind more turns. See the 
measured data in my tutorial for 1-14 turns on single #43 and #31 toroids. For 
80 and 160, #31 is far superior to #43. Above 5 MHz the two materials are quite 
similar, with #43 being slightly better. 

It is also possible to put a choke on only one of the conductors -- that is, as 
a differential filter. A small core will saturate, but a large one will saturate 
only on transmit. There's no destructive problem with saturation -- the choke 
simply doesn't choke anything -- so as long as the choke doesn't saturate on 
receive (lower DC current) it can be effective. 

The tutorial is at http://audiosystemsgroup.com/RFI-Ham.pdf  

73,

Jim Brown K9YC