> Trying to make a hybrid coupler (collins method) for 2
phased verticals.
I never use a hybrid coupler for unidirectional endfire
antenna arrays (or for any other antenna array). Two element
arrays (and four squares) require equal currents at the
current maxima in each element and in a unidirectional array
mutual coupling makes each element have vastly different
impedances. This means each element presents a vastly
different impedance (SWR) to the coupler and transmission
lines, so equal current is not produced by an equal power
split.
Equal current is produced by equal voltages when the line
feeding each element is an odd multiple of 1/4 wl long and
has very low loss. Unfortunately the hybrid coupler does not
deliver equal voltage into unequal load impedances.
I suggest reading some of W7EL Roy Lewallen's articles about
phasing antennas before building an array.
> And what seemed an easy calculation, regarding the number
of turns, is
> keeping me puzzled.
> 10 turns on the T225 core gave 2,50 uH in stead of the
desired 1.13 uH.
> Measured with my MFJ259B.
> I don't expect the MFJ very accurate, but 100 % off, I
don't believe.
Especially with low permeability cores, inductance varies
greatly with winding style. If you bunch the turns up
inductance increases, if you spread them over the core
inductance decreases.
> So where am I going wrong?
Probably the articles and charts and information you see
from multiple sources is not detailed enough for correct
information.
I just measured a 225-2 core with ten turns on both a $250
MFJ269B and a newer $25,000 HP4191A laboratory style
impedance test set.
With 10 turns spread evenly over the core I measured 1.805
uH on the HP4191A and 1.98uH on the MFJ meter at 7 MHz.
When I compressed the turns (but not perfectly tight) I read
3.714 uH and 3.91 uH on each.
I can vary inductance by greater than two going from even
winding distribution to a tight winding area on the core.
Your results do not puzzle me. They could be very normal.
How many articles or calculators warn us about things like
these? I read toroids are self-shielding and have no
external flux (external flux leakage). I read that hybrids
are a good solution for unidirectional arrays. That is what
surprises me. You could have an out-of-calibration MFJ
meter. But more likely it is pretty close. Check it on
resistors and see. The one I used was last calibrated six
years ago.
73 Tom
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