Roy Lewallen wrote:
J. Mc Laughlin wrote:

I looked through saved messages and the only reference to type 31
ferrite (Fair-Rite) was in March of 2005. That reference had to do
with ignition wires.

Recommendations for HF choking ferrites have centered on the 70 series
with type 43 being favorably recommended for the upper portion of HF
and VHF. However, Fair-Rite seems to be recommending type 31 for HF.
See Figure 26 in:
http://www.fair-rite.com/newfair/pdf/CUP%20Paper.pdf

Am I missing something? It seems to me that for the purpose of
choking common mode current (on the outside of the outer conductor of
coax) from 3 to 25 MHz one should prefer the use of type 31. This is
a receiving application with a long run of coax.

73, Mac N8TT

P.S. I will also appreciate ideas about places to purchase ferrite in
addition to Mouser.


A fairly brief look at the graphs (Figs. 6 - 8) shows that type 31 has
roughly the same or a little higher total complex permeability than 73
over most of the lower HF range and same or higher than 43 on up to 100
MHz. So it should be a good ferrite to use for very wide band
applications. However, Table 1 shows that it's available in large parts
only. Type 43 is very widely available in a wide variety of sizes, so
you might have a lot more luck getting cores of the sizes you need in
type 43.

Large, in this context, isn't all that big. You can't get it in tiny
ferrite beads to slip over a AWG30 wire. You can get it in 0.485" OD
0.5" long beads with a 0.2" hole in the middle.... $0.42 each from
Mouser. Similar snap on cases too..

Small, for Fair-rite, means things like those little 1/4" diameter multi
hole cores for making wound EMI filters, or chip components.

I buy it in 2.4" toroids so I can put multiple turns of coax on.