Gary

I have very little confidance in any calculations I do without outside
help. Maybe you can help me. I understand the 100 foot long center fed
dipole antenna is horizontal to the earth and is 35 feet up above it.
At the operating frequency of 3.8 MHz, its input impedance is 17 -j343.
Although I didnt realize the real part of the dipole fell that rapidly with
frequency and nearness to effective ground, I assume these numbers are
accurate.
I'll consider the 39 foot long shorted stub made from 300 ohm twin lead to
be about 0.166 wavelength long (if the velocity of propagation for that line
is 0.9). That makes the length of 300 ohm TV antenna twin lead from the
antenna to the place where the 50 ohm line gets connected, to be about 0.153
lambda. And the remaining 3 feet of 300 ohm line to be 0.012 lambda long.

Would the antenna's impedance plot at 0.057 -j1.14 on a 300 ohm Smith
Chart? Thats what I calculated.

If I go 0.153 lambda from the antenna along that 300 ohm line toward its
shorted end, the impedance looks like 0.02+j0.18, which might be something
like 6 ohms in series with 54 ohms of inductive reactance.
The three foot length of shorted twin lead seems to look like 21 ohms of
inductive reactance.

I figured the impedance at the location where the 50 ohm line connects to
the 300 ohm twin lead is something like 6 ohms resistive in series with 54
ohms of inductive reactance all in shunt with 21 ohms of inductive
reactance.

What am I doing wrong?? The way I'm looking at the problem, there will
be a serious mismatch to 50 ohms. at 3 feet from the shorted end of the
"matching stub".

If I had any confidance in my thinking ability, I'd suggest using a
shorter stub, so the impedance at the location where the 50 ohm line conects
is still R-jX. Then the shunt L will have a chance of providing a more
resistive match.

Jerry



"JGBOYLES" wrote in message
...
Just for practice, I'd like to know your numbers.
that three foot stub of 300 ohm line seems like it would impose alot of
low
impedance inductance in shunt with the feed

Sure Jerry, I was hoping someone would check it.

Eznec says at 3.8 MHZ a 100' dipole about 35' up over average ground is
17-j343. .126 lambda or 32.7' of 300 ohm line transforms to 7.5-j17.6.
..011
lambda shorted stub or 2.9' connected at the 7.5-j17.6 point gives 50-j0.
The
.011 lambda stub is .874 uh at 3.8 MHZ . This is according to Mr. Smith
and
his chart.

73 Gary N4AST