At 14:00 9/10/98 -0400, you wrote:
>NOTE: This winding method has slightly more loss, but has MUCH better
>feedline to antenna isolation. Isolation is much more critical with
>receiving antennas than loss, so I avoid trifilar and other windings that
>provide direct common mode paths. This makes the transformer more "core
>critical", so substitute at your own risk.
Tom, my experiences with RX antennas over the last few days supports the
isolation theory. My new Pennant was showing reasonable directivity on
in-band signals, but seemed to be overly susceptible to computer birdies
and power line leaks. I finally decided that the feedline was the problem.
The transformer I started with was a 9:1 trifilar on a FT50-43 core. Looked
great on the bench for match, etc. I wound a new one with seperate windings
with a 4 to 1 turns ratio. It looked pretty bad on the bench, but what a
difference on the Pennant! It really made the pennant come alive all the
way up to 30 meters. What a difference. Flushed with the excitement of a
great discovery, I immediately made a similar transformer for my
"underground beverages". There was a noticeable improvement on them as
well, even though the feed line is buried.
It sure seems like feedline isolation is a critical parameter of any high
feedpoint impedance RX antenna. Because of the very high impedance of the
pennant and flag antennas. I suspect that the usuall 1:1 "current" balun or
choke might not be as effective as you would want.
The alternative is the type of transformer that you described. In my
experience, albeit limited, I have never had any luck with the binocular
core transformers at HF. I am begining to believe that the cores I have,
which were spoda be old Indiana General "H" material, are really Q2. Looks
like I need to get some new ones.
Please keep us posted on any other transformer designs you come up for this
Larry - W7IUV
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