Topband: Flag transformer core

Dan Zimmerman N3OX n3ox at
Sun Jan 7 21:31:42 EST 2007

I've got a couple of ~0.75" bobbin diameter type 77 pot cores
(actually type 3B7 from, I think, Ferroxcube, but seems to
cross-reference to 77 material)

I'm looking to build a Flag to be used on 160 and 80.

I wound a 16:1 transformer on such a core with a 4 turn low impedance
winding and a 16 turn high impedance winding. The impedance of the low
impedance winding measures about 0+j150 on my MFJ-259B at 1.8MHz.  Is
this enough?  I'm getting a slightly elevated SWR when terminating
with an 800 ohm resistor, maybe 1.5:1, measuring 50+j20 or so.

(A side note, 800 ohms is the modeled impedance of the Flag of the
aspect ratio that I have a frame for; it was originally going to be a
20m Moxon!  )

I'm measuring about 15pF of interwinding capacitance, for about 6k
ohms of interwinding impedance at 1.8MHz.

Do I need to worry about the slightly reactive impedance I'm getting
when the transformer is properly terminated?  Is it indicative of too
much or too little winding reactance or should I just forget about it
and give it a try?  It seems OK to me.  Incidentally, I'm not going to
be phasing it, maybe this relaxes the requirement for low SWR?


More information about the Topband mailing list