Hi, Everybody,

A short vertical such as for mobile use is capacitive and this Xc is usually
tuned out with a shunt coil at the feedpoint. Yet, when I model this with
EZNEC I cannot simulate this effect properly. I must be doing something
wrong. Does anyone have an EZNEC example showing a shunt matching coil
inserted as an EZNEC Load at the feedpoint that they can send me?

Thanks very much.

Regards,

Al W6LX

"Al Lorona" wrote in
:

Hi, Everybody,

A short vertical such as for mobile use is capacitive and this Xc is
usually tuned out with a shunt coil at the feedpoint. Yet, when I
model this with EZNEC I cannot simulate this effect properly. I must
be doing something wrong. Does anyone have an EZNEC example showing a
shunt matching coil inserted as an EZNEC Load at the feedpoint that
they can send me?

In my experience, the capacitive reactance of a short vertical is usually
"tuned out" with a series inductor (inductance and resistance).

If you do offset all of the reactance, you may have a very low feedpoint
resistance, and that may drive you to consider some form of L match at
the base, the L match formed by a radiator that is not resonated and some
shunt L or C element.

Owen


Thanks very much.

Regards,

Al W6LX

A source and load placed on the same segment are always connected in
series. The only ways in EZNEC to simulate a coil in parallel with a
source a

1. Construct a physical model of the coil using wires (if it's an air
core coil), and connect it appropriately using wires.

2. Make a small loop of three or four one-segment wires. Put the source
in one leg of the loop and a load in another. This is a bit tricky, and
doesn't always work -- NEC doesn't like small loops, so there's a
minimum practical size. But if the loop is too large, the wires it's
made from add reactance and radiate. You might or might not be able to
find a size which works. This is one of the very few situations where
the double precision calculating engine of EZNEC+ might be an advantage,
since it'll tolerate loops which are a bit smaller than the standard
engine. You should run an Average Gain test on the antenna to make sure
the NEC calculating engine isn't getting unhappy with the small loop.

But as some others have commented, it's much more common to take care of
the capacitive reactance of a short whip with a series, rather than
shunt, inductor.

Roy Lewallen, W7EL

Al Lorona wrote:
Hi, Everybody,

A short vertical such as for mobile use is capacitive and this Xc is usually
tuned out with a shunt coil at the feedpoint. Yet, when I model this with
EZNEC I cannot simulate this effect properly. I must be doing something
wrong. Does anyone have an EZNEC example showing a shunt matching coil
inserted as an EZNEC Load at the feedpoint that they can send me?

Thanks very much.

Regards,

Al W6LX

Owen Duffy wrote:
In my experience, the capacitive reactance of a short vertical is usually
"tuned out" with a series inductor (inductance and resistance).

Owen, he is probably trying to achieve a feedpoint
impedance of 50 ohms instead of a resonant feedpoint
of 20 ohms. If one makes the feedpoint 25-j25, for
instance, and installs a base shunt coil, one can
twist the feedpoint impedance to 50+j0 ohms using
a +j50 ohm shunt coil.

In like manner, a -j50 base shunt capacitor will
twist 25+j25 to 50+j0 ohms.

The ARRL Handbook describes such shunt matching
techniques in the mobile antenna section.
--
73, Cecil http://www.w5dxp.com

On Sun, 29 Apr 2007 23:41:13 -0700, "Al Lorona"
wrote:

Hi, Everybody,

A short vertical such as for mobile use is capacitive and this Xc is usually
tuned out with a shunt coil at the feedpoint. Yet, when I model this with
EZNEC I cannot simulate this effect properly. I must be doing something
wrong. Does anyone have an EZNEC example showing a shunt matching coil
inserted as an EZNEC Load at the feedpoint that they can send me?

Thanks very much.

Regards,

Al W6LX



FWIW, I use the information provided by EZNEC as input to the TLW
program (ARRL Antenna Book) for this set of conditions.

Note that radiation from the matching device is ignored under this
circumstance.

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"

Roy Lewallen wrote:
But as some others have commented, it's much more common to take care of
the capacitive reactance of a short whip with a series, rather than
shunt, inductor.

Quoting the ARRL Antenna Book: "The input impedance of
short, high-Q coil-loaded antennas is quite low. For
example, an 8-foot antenna optimized for 3.9 MHz with
a coil Q of 300 and a ground loss resistance of two
ohms has a base input impedance of about 13 ohms."

It is common practice to adjust the feedpoint impedance
to about 13-j22 and install a base shunt inductive
reactance of ~+j30 to twist the feedpoint impedance
to a near perfect 50+j0 ohms.
--
73, Cecil http://www.w5dxp.com

"Al Lorona" wrote in message
...
Hi, Everybody,

A short vertical such as for mobile use is capacitive and this Xc is
usually
tuned out with a shunt coil at the feedpoint. Yet, when I model this with
EZNEC I cannot simulate this effect properly. I must be doing something
wrong. Does anyone have an EZNEC example showing a shunt matching coil
inserted as an EZNEC Load at the feedpoint that they can send me?

Thanks very much.

Regards,

Al W6LX

In any case, whatever you do, it is unlikely that a single element will
achieve
a match. Unless you are very lucky you will need a two element network.
For example a shunt L, series C, or Series L, shunt L, or series L, shunt
C will achieve a match for virtually any capacitive load. A Smith Chart
is ideal for such analysis.

Frank

"Frank" wrote in
news:0hqZh.14503$JF6.1620@edtnps90:

....
In any case, whatever you do, it is unlikely that a single element
will achieve
a match. Unless you are very lucky you will need a two element
network. For example a shunt L, series C, or Series L, shunt L, or
series L, shunt C will achieve a match for virtually any capacitive
load. A Smith Chart is ideal for such analysis.

But one of those reactances can be supplied by detuning the loaded antenna,
the second typically by a shunt coil.

For example, if the R component at the base at resonance is say 20 ohms
(radiation resistance, equivalent coil resistance and equivalent ground
resistance), detuning the radiator + loading for about 25 ohms of capacitive
reactance will allow a shunt coil of about 2uH to form an L match at 3.6MHz.

Owen

"Owen Duffy" wrote in message
...
"Frank" wrote in
news:0hqZh.14503$JF6.1620@edtnps90:

...
In any case, whatever you do, it is unlikely that a single element
will achieve
a match. Unless you are very lucky you will need a two element
network. For example a shunt L, series C, or Series L, shunt L, or
series L, shunt C will achieve a match for virtually any capacitive
load. A Smith Chart is ideal for such analysis.

But one of those reactances can be supplied by detuning the loaded
antenna,
the second typically by a shunt coil.

For example, if the R component at the base at resonance is say 20 ohms
(radiation resistance, equivalent coil resistance and equivalent ground
resistance), detuning the radiator + loading for about 25 ohms of
capacitive
reactance will allow a shunt coil of about 2uH to form an L match at
3.6MHz.

Owen

True, I was forgetting the loading coil.

Frank

"Roy Lewallen" wrote in message
...

A source and load placed on the same segment are always connected in
series.

Aha! Thanks, Roy, for clearing this up. No wonder I couldn't get it to work
right. I had placed the source and the load on the same segment and thought,
'Hey, cool, maybe they'll be in parallel,' but obviously I was wrong about
that.

And thanks to everybody else for their helpful suggestions.

Regards,

Al W6LX