Topband: impedance transformer windings

[amdx at att.net]

  Hi Steve,
  Someone once decreed, "the reactance of the windings should be at least
four times the impedance the winding is designed to look into." So if you
have a 950 impedance, 4x950=3800 ohms of reactance. Using 1.8Mhz as your 
minimum frequency, the inductance would need to be L = 3800 / 2 x pi x 1.8Mhz.
So L= .000336 henrys or .336mh.
  To find the number of turns the formula is N=1000(sqrt(L in mh/A sub L))
Or N=1000(sqrt(.336/8500))= 6.28 turns on the primary.
To find the number of turns on the secondary use Np/Ns = sqrt(Zp/Zs)
or Np/Ns = sqrt(950/50) = 4.36, this is the ratio of primary to secondary 
turns. So Np/Ns = 4.36 or Ns = 6.28/4.36 = 1.44 turns on the secondary.

(NOTE) Np=number of turns on the primary, Ns= number of turns on the secondary
Zp=impedance of primary, Zs= impedance of secondary

 So now we have 6.28 turn pri and 1.44 turn secondary, not good,
I would try a 9 turn pri and 2 turn secondary this is a ratio of 4.5 close to
the 4.36 we calculated.
 I have used a 13 turn primary and 3 turn secondary with a gapped pot core,
I gapped the pot core to give me the desired primary reactance. This was used 
mostly in the MW BCB.

                              Mike





> I've been trying to put together an impedance transformer on the Amidon 
> BM-73-202 binocular core.  It is to be used on a flag RX antenna for 160m so I 
> need about a 950:50 (19:1) transformation.  The published AL is 8500 mH/1000 
> turns.  This translates to 8.5 mH per turn.  From a Handbook formula, I 
> calculated I need 1.4365 mH on the HI Z side and 0.0765 mH on the Lo - Z  side.  
> As you can see, even one turn thru the core exceeds these by a large margin.  Do 
> I need a core with a lower AL?  Seems like many folks recommend the BN-73-202 
> though with a 3:12  (W7IUV) or 1:4 (W8JI) turns.   But I can't arrive at these 
> numbers by equation.  I'd like to learn how this works rather than just build it 
> without learning.
> 
>   Thanks all.  Steve KK7UV.
> _______________________________________________
> Topband mailing list
> Topband at contesting.com
> http://lists.contesting.com/mailman/listinfo/topband