TopBand: Toroids For Ununs

[Jeffrey Maass]

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
uning 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
or 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?

   2) Is there any reason to use teflon tubing on the windings?

> 
> 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. 

The newer book (don't have it handy: the large-format, purple cover,
"Construction of..." or somesuch) hardly mentions powdered iron at 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. 

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). 


> 
> 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! 

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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