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-   -   Can NEC2 plot radiation resistance vs. frequency? (https://www.radiobanter.com/antenna/138069-can-nec2-plot-radiation-resistance-vs-frequency.html)

Scott Stephens October 31st 08 09:18 AM
Can NEC2 plot radiation resistance vs. frequency?
 
Can NEC2 plot radiation resistance vs. frequency?

Thanks

Jeff Liebermann[_2_] October 31st 08 01:03 PM
Can NEC2 plot radiation resistance vs. frequency?
 
On Fri, 31 Oct 2008 04:18:35 -0500, Scott Stephens
wrote:

Can NEC2 plot radiation resistance vs. frequency?

Yep. For example, 4NEC2
http://home.ict.nl/~arivoors/
can plot:
http://802.11junk.com/jeffl/antennas/gnd-plane-02/slides/impedance.html
for this ground plane antenna:
http://802.11junk.com/jeffl/antennas/gnd-plane-02/

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jim Lux October 31st 08 04:31 PM
Can NEC2 plot radiation resistance vs. frequency?
 
Jeff Liebermann wrote:
On Fri, 31 Oct 2008 04:18:35 -0500, Scott Stephens
wrote:

Can NEC2 plot radiation resistance vs. frequency?

Yep. For example, 4NEC2
http://home.ict.nl/~arivoors/
can plot:
http://802.11junk.com/jeffl/antennas/gnd-plane-02/slides/impedance.html
for this ground plane antenna:
http://802.11junk.com/jeffl/antennas/gnd-plane-02/


NEC2 doesn't plot anything.. purely text.

And while it can calculate feedpoint impedance vs frequency, which any
number of programs can plot, that's not radiation resistance.

Roy Lewallen November 1st 08 02:50 AM
Can NEC2 plot radiation resistance vs. frequency?
 
Jim Lux wrote:

. . .

And while it can calculate feedpoint impedance vs frequency, which any
number of programs can plot, that's not radiation resistance.

Unless you set all the losses in the model to zero.

Roy Lewallen, W7EL

[email protected] November 1st 08 03:55 AM
Can NEC2 plot radiation resistance vs. frequency?
 
On Oct 31, 7:50 pm, Roy Lewallen wrote:
Jim Lux wrote:

. . .

And while it can calculate feedpoint impedance vs frequency, which any
number of programs can plot, that's not radiation resistance.

Unless you set all the losses in the model to zero.

Roy Lewallen, W7EL

What about folded dipole vs non folded dipole? Rrad is the same, but
Zfeed is different, even in the lossless case. Or any antenna where
there's some built in impedance transformer (stubs, lumped components,
etc.)

Roy Lewallen November 1st 08 10:59 PM
Can NEC2 plot radiation resistance vs. frequency?
 
wrote:
On Oct 31, 7:50 pm, Roy Lewallen wrote:
Jim Lux wrote:

. . .
And while it can calculate feedpoint impedance vs frequency, which any
number of programs can plot, that's not radiation resistance.
Unless you set all the losses in the model to zero.

Roy Lewallen, W7EL

What about folded dipole vs non folded dipole? Rrad is the same, but
Zfeed is different, even in the lossless case. Or any antenna where
there's some built in impedance transformer (stubs, lumped components,
etc.)

This question comes up from time to time. The answer is that radiation
resistance is the resistive part of the antenna impedance when all
losses are zero, and this resistance can be defined at (or "referred
to") any point on the antenna, including the feed point. Some text
authors use a more strict definition, for example, the resistance
referred to a current maximum. But this isn't at all universal, and many
such as Kraus adopt the more general definition.

This allows us to call the feedpoint resistance of a lossless folded
dipole the "radiation resistance", a practice that's gotten quite a few
people into trouble when using the well known formula for efficiency

Eff = Rr / (Rr + Rl)

where Rr and Rl are the radiation and loss resistances respectively. The
fact is that this equation is just fine for folded dipoles (or
monopoles, where it more often causes trouble), but you have to refer
the loss resistance to the same point as the radiation resistance. (In
the case of a folded antenna, this means transforming it by the same
factor.) In a folded monopole where Rl is primarily ground system
resistance, you have to multiply the ground system resistance by 4 to
get Rl referred to the feedpoint. Then you can use the lossless
feedpoint resistance for Rr. Or, if you choose, you can use the
untransformed impedances, in which case you use ground system resistance
directly for Rl but divide the lossless feedpoint resistance by four for
Rr. Either method is equally valid and both give the same result, as
they should. But you have to be careful to specify the point to which Rr
is referred, so Rl can be referred to that same point.

A common mistake (made repeatedly by many people, some very well known)
is to use the lossless feedpoint resistance for Rr but the untransformed
ground resistance for Rl. This leads them to the incorrect conclusion
that "folding" a monopole improves the efficiency.

Roy Lewallen, W7EL


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