Jerry Martes wrote:

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.

Nope, 29.5 feet (0.127 lambda) around the 300 ohm SWR circle from the 17-j343
antenna, the impedance will be 7-j17 without the stub. In parallel terms, that
is 48 ohms in parallel with 2000 pf (~21 ohms). Neutralize the 2000 pf with an
inductive stub and you have a feedpoint impedance of 48 ohms. You seem to need
about one microhenry for that. Matching can be accomplished with a coil instead
of a shorted stub.

IMO, a better way would be to extend the feedline to 33.8 feet (0.145 lambda)
where the impedance is the conjugate of the above, about 7+j17, and a capacitive
stub will cause the match to 48 ohms. Or a parallel cap of about 0.002 uf will
accomplish the same thing.

These matching methods make use of the 1/50 conductance circles on the Smith
Chart. These easy matching methods will work for any SWR greater than Z0/50,
i.e. for Z0=300, for any SWR above 6:1. At the point where the SWR circle
crosses the 1/50 conductance circle, simply install an appropriate parallel
inductance or parallel capacitance and it twists the impedance at that point
to 50 ohms (48 ohms above). Since that is also a voltage minimum point,
relatively cheap low voltage capacitors can be used, e.g. 600v micas.
--
73, Cecil, W5DXP



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Matching can be accomplished with a coil instead
of a shorted stub.

Thanks for explaining this matching technique in slightly different terms that
people may be more comfy with. I was trying to keep the parts count down since
this is a camping antenna. Using one 300 ohm line of correct length, shorting
the end, and tapping at the 50 ohm spot seemed pretty simple. Or as you state,
use the conjugate method and leave the line open and tap for 50 ohms.
These matching methods make use of the 1/50 conductance circles on the Smith
Chart. These easy matching methods will work for any SWR greater than Z0/50,
i.e. for Z0=300, for any SWR above 6:1. At the point where the SWR circle
crosses the 1/50 conductance circle, simply install an appropriate parallel
inductance or parallel capacitance and it twists the impedance at that point
to 50 ohms (48 ohms above).

I didn't know I was doing all that! Thanks for the info.

73 Gary N4AST

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

Nope, 29.5 feet (0.127 lambda) around the 300 ohm SWR circle from the
17-j343 antenna, the impedance will be 7-j17 without the stub.

Hi Jerry, I need to apologize to you. I realize now that your 0.057
-j1.14 figure is a value *normalized* to 300 ohms. 17-j343 is indeed
0.057-j1.14 when normalized to 300 ohms. Sorry about the misunderstanding.
Guess I'm getting senile.
--
73, Cecil, W5DXP



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Hi Jerry, I need to apologize to you. I realize now that your 0.057
-j1.14 figure is a value *normalized* to 300 ohms. 17-j343 is indeed
0.057-j1.14 when normalized to 300 ohms. Sorry about the misunderstanding.

Hi Jerry and Cecil, When I did my plots it was with a 50 ohm chart. I must
confess I use a Smith Chart program that lets you plot any impedance on the
normalized 50 ohm chart, with correct results. The program prompts when using
Lines, Impedance=? Having been spoiled by the computer, I never really
considered plotting transmission line Impedances on a Smith Chart that were
different from the normalized Impedance.
This is a neat little free demo program I use. Maybe I need to get back to
basics.
73 Gary N4AST

"JGBOYLES" wrote in message
...
Hi Jerry, I need to apologize to you. I realize now that your 0.057
-j1.14 figure is a value *normalized* to 300 ohms. 17-j343 is indeed
0.057-j1.14 when normalized to 300 ohms. Sorry about the
misunderstanding.

Hi Jerry and Cecil, When I did my plots it was with a 50 ohm chart. I
must
confess I use a Smith Chart program that lets you plot any impedance on
the
normalized 50 ohm chart, with correct results. The program prompts when
using
Lines, Impedance=? Having been spoiled by the computer, I never really
considered plotting transmission line Impedances on a Smith Chart that
were
different from the normalized Impedance.
This is a neat little free demo program I use. Maybe I need to get back
to
basics.
73 Gary N4AST

Gary

Theres a very good article on use of Smith Chart for impedance matching in
the July 1966 isue of Electronic Design. I like it because I wrote it.
Other than that, it is of little value or interest.

Jerry