TopBand: Toroids For Ununs

[km1h @ juno.com]

On Tue, 14 Oct 1997 09:20:06 -0400 (EDT) Jeffrey Maass
<jmaass@freenet.columbus.oh.us> writes:
>On Fri, 10 Oct 1997 km1h@juno.com wrote:
>
>> 
>> 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.
>
>I had ordered an Amidon unun (Multi-match: 5 ratios available), and 
>I'm
>using it right now for testing. I don't trust that little core, 
>although
>it's a thicker beast than 1.5-inch cores I've seen of other materials 
>(a
>"beefy-boy"). I'm going to keep the multi-match unun for testing and
>tuning of new antennas. I've put 1500W though it key-down for five 
>minutes
>without seeing any variation in the match, although I didn't get out 
>to
>check for temperature after the test. No obvious visual signs of abuse 
>in
>the box.
>
>I have some FT240-K cores on-order, and should have them today or
>tomorrow, along with some #14 Thermaleze wire. I'll double these cores 
>up
>for my 2.25:1 unun. I haven't found a value for Al for these cores. 
>
>Two questions:
>
>   1) Is there any value to/problems with to wrapping the two cores 
>with
>       glass tape (#27?) prior to winding?

IMO you should always wrap with #27 tape. It provides protection from
sharp edges and most important HV punch-thru. Ferrite is not an insulator

The Teflon provides HV insulation and conductor spacing.  Typical magnet
wire insulation is only good for a few hundred volts or so....cant find
my reference right now. 
 
Since theb large-format, purple cover,
>"Construction of..." or somesuch) hardly mentions powdered iron at 
>all.

I know. He got a lot of flack over that and the CQ articles were after
the book. 
He grudgingly admitted that powdered iron might be a better choice in
highly reactive situations such as a nominal 4:1 balun for some open wire
line antennas. 
 
>> 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. 
>
>We've used the old 4:1 ferrite (wound on #61) for over 10 years or 
>more
>(not at my station, but at various contest operations), and it's held 
>up to
>some catestrophic events ("antenna falls down, keyed at 1.5KW", " top 
>hat
>falls off: keyed at 1.5KW",  and the like). 

Typical contest enviroment Jeff!  I have no experience with ferrite
failures, just what I've heard over the years. 


>> Have you considered just using coax to do the matching since it is a
>> single band affair?
>
>Although I specified 160M only, I'm considering using this antenna for 
>80M as
>well, possibly trapped, but more likely as a "switch for the evening 
>to
>catch a DXpedition" or "switch for the weekend contest" arrangement. 
>
>Thanks for all the good information over the months I've been 
>rebuilding
>the station! 

Tnx !  
73   Carl  KM1H



>
>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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