I fail to see how you can possibly have a VSWR of 5:1 when you terminate
your matching transformer with 900 ohms and look at the 50 ohm side. You
have the turns ratio correct for 50 to 900 ohm impedance transformation. I
wind my Pennant 900/50 ohm transformers with 4 turns and 17 turns. Same
ratio you use but half the turns. My transformers have an inductive
reactance (not resistive but inductive) of about 275 ohms for four turns
thru the core. If I terminate the Hi side with 900 ohms resitive load the
input at 1.840 with my MFJ 259B is less than 1.2:1 Perhaps the cores are
not what you think they are... de gary, kd9sv
----- Original Message -----
From: "Bob Tellefsen" <n6wg@earthlink.net>
To: "Flag and Pennantgroup" <FlagandPennantAntennas@egroups.com>; "Topband
Reflector" <Topband@contesting.com>
Sent: Wednesday, September 11, 2002 6:19 PM
Subject: Topband: Flag Xfmr Measurements
> 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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