On 6/28/2011 11:02 AM, Richard Clark wrote:

One curiosity about this - the 4:1 BalUns.

As with many of these "matching devices at the base of a vertical"
they're not actually a broadband 4:1 ideal transformer. they tend to
have a fair amount of leakage inductance and capacitance, and (by
empiricism) they have a design which generally improves the match over a
wide band. At low frequencies, where the feedpoint Z is low, the
shunting effect of the interwinding C isn't as big, so it acts more like
a transformer. At higher frequencies where the feedpoint Z is higher,
the interwinding C tends to "bypass" the transformer, so it works less
like a transformer, and you see more of the Z of the antenna.

You can fool around with some test data and a lumped model (with lots of
parasitics) and come up with some numbers for the parasitics that works
out pretty well and combined with some loss in the feedline, makes the
typical match at the transmitter end not too bad (within the range of a
tuner that can handle, say, 2.5:1 or 3:1, mostly reactive)

It is almost impossible to *design* such a thing from that data. I am
quite certain that these things were designed by empiricism. Spreading
the windings or squishing them, choosing various winding ratios, etc.
until it works "good enough".


The underlying problem is, as you pointed out, that a 43 foot radiator
on higher bands has a pattern that is somewhat less than ideal.

A trapped vertical might be a better solution, although mechanically a
bit more complex and definitely more "antenna looking"..