Jeffdeham wrote:

If you could answer yet another question. How does a autotransformer
work exactly? I can see how you can tap up and down the coil to hit a
certain point where you can a 50 ohm match. But when I look at the
schematic all I see is a coil in parallel with a variable cap. How
does the autotransformer in essence become a transformer? Or rather
how does it become the above schematic?

That amounts to two questions that aren't necessarily related. One is how
an autotransformer works, the other is how inductance and capacitance can
be combined to match an antenna to a feed line.

I'll take a stab at the first question. Think about how a transformer
works. If you place two inductances in close proximity, a magnetic field
induced in one will induce a current or voltage in the other. By selecting
the proper turns ratio between the two inductances, you can transform an AC
voltage or current up or down. You can transform high voltage/low current
to low voltage/high current or vice versa. Which is another way to say you
can transform impedance.

In an autotransformer the same principles apply, but a portion of the
inductance is common to both circuits. A voltage applied across the tapped
portion of the coil will produce a higher voltage across the whole coil. A
voltage applied across the entire coil will produce a proportionately lower
voltage across the tapped portion. The current in the tapped portion will
be the sum of the currents flowing in the two circuits. Not good if you
need isolation, but not an issue if all you want is to transform voltage,
current, or impedance. Does that make sense?

On the second question you might be better off to read the antenna matching
chapter in any Radio Amateurs Handbook. :}


73 de KD5SZ