TopBand: Toroids For Ununs

[km1h @ juno.com]

On Fri, 10 Oct 1997 10:57:38 -0400 (EDT) Jeffrey Maass
<jmaass@freenet.columbus.oh.us> writes:
>
>I'm reading through W2FMI's book on Baluns and Ununs, looking to 
>replace
>the 4:1 transformer used to feed my inverted-L (originally used with a
>short, top-loaded vertical) with one for 1:1.78 (50:28.13) or 1:2.25 
>(50:22
>ohms) ratio (BTW, there seems to be some disagreement about what the
>impedance of a practical inverted-L is. Some sources show 28-ohms, 
>some show
>22-ohms).  
>
>Most of his Ununs are wound on 1.5-inch ferrite toroids with 
>permeability
>of 250. Problem is, Amidon doesn't sell such a core, the closest being
>their "K" material with permeability 290 (which he specifies in some 
>other
>of his Ununs).

There is virtually no performance difference between those values Jeff.
Since it will be a very narrowband application lots of cores will work.
What is the Al value of that K core? 
I would recommend against using a 1.5" core at QRO levels. Sevick's book
is good for the lab but those small cores DO NOT like reactive real world
loads. 
I would consider working the formulas a bit and wind up stacking a pair
of FT240-61 cores at least. The Al value roughly doubles when you stack
them.

It took several years but Sevick finally admitted in CQ Magazine that
some of his designs had real world problems and maybe powdered iron was
not so bad after all. 

There are many out in the RF world who recommend against using any
ferrites at those QRO levels. I try to stay away from any of that stuff
except for QRP and RX levels. 

Have you considered just using coax to do the matching since it is a
single band affair?

73   Carl   KM1H


>
>So, what to do? Should I use the Amidon "K" material in exchange for 
>the
>core specified? My use is strictly on 160M (I don't need it to have
>bandwidth to cover 1-30 MHZ), and I want to have it "comfortably" 
>rated for
>2.5KW+ to provide some safety margin. 
>
>73,
>
>Jeff Maass       (jmaass@freenet.columbus.oh.us)      Amateur Radio 
>K8ND
>      USPSA/IPSC # L-1192   NROI/CRO          NW of Columbus Ohio
>      25000 Members in 2000!
>
>
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