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TopBand: Transformers for pennants and flags

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Subject: TopBand: Transformers for pennants and flags
From: (Earl W Cunningham)
Date: Tue, 15 Sep 1998 01:03:04 EDT
Hi all,

I previously was direct-feeding my point-to-point Pennants with 75-ohm
coax.  Results were disappointing.  Jose, EA3VY correctly showed via
modeling that this would severely degrade the antenna's performance.

After settling on Larry, W7IUV's separate primary and secondary windings
on a toriod core and Tom, W8JI's recommendation for a 77 mix (ui = 2000)
core, (Larry's successful xfmr was on a 43 mix core with ui = 850), I
used an Amidon FT-82-77 toroid (O.D. is 0.825" and about 0.25" thick) and
wound a transformer using #24 AWG wire with primary and secondary
windings separated as much as possible.  After playing around with
different number of turns for the low-Z winding with my MFJ antenna
analyzer, I finally concluded that 5 turns looked best.  This gives about
5.3 uH of inductance for the that winding.  I used 18 turns for the
high-Z winding because I'm using 75-ohm coax (21 or 22 turns would be
more appropriate for 50-ohm coax).

After installing the transformer, Will, K6NDV (who is about 4.5 miles
distant and directly in line with my Pennants) provided a signal source
to check the F/B on my Pennants.  Alas, still poor results.  Suspecting
that my shunt-fed phased towers were influencing the directional patterns
of the Pennants, I detuned all four gamma matches on the towers and ran
the tests again.  On 160m, F/B was only about 3 dB, about the same as
before detuning the towers.  On 80m, F/B was 6 to 6-1/2 S-units, or
better than 36 dB (which is what EZNEC predicts).  Before detuning the
towers, F/B on 80m was only about one S-unit.  On 40m, F/B was perhaps
only one S-unit, however on sky-wave signals it appears to be
significantly better than that (3 to 4 S-units).

For fear that I might miss some rare DX, I retuned the gamma matches
immediately after concluding the test with K6NDV, so I haven't yet
checked the 160m and 80m F/B on sky-wave signals.

It was very obvious that (at least on 80m) the vertical tx array was
severely degrading the F/B (and therefore the S/N) of the Pennants.  One
tower is directly in line with the Pennants about 50 feet away and the
other tower is only about 20 feet to the side of the Pennants.

When direct-feeding with coax (prior to installing the xfmr), the
flourescent lamp I use in the shack induced an S-4 noise into the
Pennants on 160m.  I also heard a lot of birdies across the band.  After
installing the xfmr all of this garbage was gone.

My next step is to take Peter, VK3APN's suggestion and try an FT-140-43
toroid with 8 turns in one winding and 28 turns in the other (12.25
impedance transformation to closely match my 75-ohm line), and see if
that improves things on 160m.

Many thanks again to all for their help and suggestions, which hopefully
will make the Flag and Pennant perform as well as the modeling software
says they should.

Will keep y'all informed on the results of the new transformer when I
install it.

73, de Earl, K6SE

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