I've been curious about the insertion loss of the 50:900 ohm matching
transformers we wind for the Flag antenna, especially with the "unofficial"
toroid cores I use. So I did a bit of investigating.
I wound two identical transformers (8 turns primary, 34 turns secondary),
then used the wire from an old 50-pair telephone cable, I'm guessing about
#24 gauge. With the plastic covering, I figured this would space the actual
wires about one wire diameter.
The cores must be a special run for some mfr, but they were identified on
the box as type 43 material. I ran some tests on them to verify, and they
seem to fall between FT114 and FT140 cores. The core size is OD 1.22", ID
.715" and HT .615".
I connected the two test transformers back to back with the 34 turn
windings, and used the 8 turn windings for input and output. Test setup was
a hp 8640B sig gen into a KAY switchable attenuator into the first
transformer. Then from the output transformer into another KAY attenuator
set at 6DB and into my Hitachi dc-100 MHz scope.
At 1.825 MHz, insertion loss was 4 dB even, so I'm saying 2dB per
transformer. I checked the insertion loss of the capacitive coupling
between windings at 49dB.
At 3.550 MHz, insertion loss was 7dB, for 3.5dB per transformer. I had
expected it to be less, but the 43 material must not be as efficient at 80m.
Capacitive coupling insertion loss was 45dB which seemed reasonable.
I also used my LCIIB capacity meter to measure the interwinding capacity. It
averaged 3.75pf between the two transformers. There is about 1/4" of space
between the ends of the primary and secondary windings. Winding with a
finer wire would leave a larger space and the coupling should be less.
That's another experiment for another day.
Using an AEA HF-CIA antenna analyzer, I measured an end-to-end swr of 5:1 at
1.825 MHz when terminated with a 51 ohm resistor. This is consistent with
measurements on a single transformer when terminated with a 910 ohm
resistor.
For my record, I measured the inductance of the transformer windings.
Transformer A had 69.2uH and 1308uH, and transformer B had 75.6uH and
1440uH, again using the LCIIB meter.
I'd be interested in the results if anyone makes measurements using the
"official" FT114 or FT140 cores.
73, Bob N6WG
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