In message , Geoffrey S.
Mendelson writes
richard8123 wrote:

Hi. I've seen construction details of a longwire magnetic UnUn made with
a T130-2 toroid. The T130-2 is trifilar wound, each of the 3 windings
are 9 turns. The windings are connected in series, so that the turns in
total are 27: Turns ratio is :1:3 and impedance ratio is 1:9.

Okay, but do I keep the same number of turns for each winding (9) if I
use a T68-2 toroid? Thanks.

In theory, you have to calculate the exact impedance of the primary or
secondary and match it to where you want it to go. For example, if you
are connecting a 50 ohm coax to the primary, you have to match it to
50 ohms, which would make the secondary 450 ohms.

In reality, there is not much that a few turns or an impedance mismatch
will do, so wind it the same way you would on the other toroid.

While you are at it, save yourself a lot of money and use RG-6 coax.

Also in reality, you may not notice any difference at all using the
transformer.

The famous article floating around usenet said that it was not needed
for "any modern receiver" and that was written 20 years ago.

You may also try a straight 1:9 transformer with the primary (high number
of turns) connected to the antenna and the other side to ground, and then the
secondary connected to the coax. This reduces noise by having a DC path to
ground and no DC connection between the antenna and the receiver.

I can't totally disagree with you, but you seem to have totally failed
to answer the OP's question, which was "Do I keep the same number of
turns for each winding (9) if I use a T68-2 toroid?" I think that the
answer could be a "maybe".

From what I can quickly see, the T68-7 is twice the diameter of the
T30-2. See the data at
http://toroids.info/T30-2.php
and
http://toroids.info/T68-7.php

I don't think that the diameters (and hence the magnetic path lengths)
will matter too much, but the permeability (AL) of the T68-7 is somewhat
greater than the T30-2, and its cross-sectional area three times
greater. I would therefore guess that with the same number of turns, the
T68-7 would noticeably more inductance than the T30-2, and would
therefore be expected to work down to lower frequencies, and roll off
more at high frequencies.

However, a more scientific approach would be to compare the
characteristics of the two inductors by using calculators below the two
sets of data information.

In the calculator for the T30-2, enter (say) 10MHz and 27 turns, then
'calculate'. Note the resulting
3.13uH, 81pF and 197ohms.
[I presume that the pF is the capacitance required to resonate it at
10MHz, and the ohms is the shunt loss resistance.]

Next select the T68-7, and repeat. You get
3.79uH, 67pF and 238.2ohms,
so there's not a lot of difference in the results. [I'm rather surprised
that the inductance is only x1.2 greater than the T30-2.]

However, now try reducing the inductance to match the T30-2. Try 24
turns (3x8) instead of 27 (3x9). You get
3.00uH, 85pF and 188.2ohms
Hey! that's pretty close!

Next try (say) 1Mhz and 100MHz. You will see that the match is
maintained pretty well.

So I guess that the performance of the two cores is (or can be) very
similar. As I said, with 27 turns, the T68-7 will probably work to a
somewhat lower frequency, and have a lower HF roll-off, but it seems
that with 24 turns, it should be almost identical.
--
Ian