Re: NEC-LIST: NEC2 circularity printout question

Yes, the key is the axial ratio.
An axial ratio=1.0 is perfect circularity; you can see that by the fact
that E(theta) magnitude and E(phi) magnitude are equal (or very nearly so),
and the phase difference between them is 90 degrees (or very nearly so).
If you run a linear polarized antenna, the axial ratio will be zero (or
very nearly so).
When you hear people talk of axial ratio in dB, that's -20log of the axial
ratio printed here. It's 20log because it's the ratio of two voltages.
The total directive gain column gives "dBic" for an elliptically polarized
antenna. If axial ratio is nearly one, you'll note that the total gain is
3dB higher than both the major and minor axis gains. If the axial ratio is
less than one, the calculation of total directive gain is rather
complicated, but NEC does it automatically.

At 14:35 2002-12-05 -0800, John Mock wrote:
>I've been working with NEC2 for a long time and one thing still puzzles me.
>I don't understand NEC2 output for circular polarized antennas. So rather
>than looking at some complex example, i've made simple 2 el yagi (see text
>attachment below) to try to understand what NEC2 is trying to tell me. If
>only one of the sample yagi is excited, it shows a gain of 5.41 dBi with
>77 ohm impedence (or at least, that's what's printed as the "MAJOR AXIS
>GAIN NORMALIZATION FACTOR).
>
>If i feed a second yagi rotated by 90 degrees and delayed by a 1/4 wave-
>length, i see 2.41 dBi with half of the impedence (38.5 ohms) of a single
>yagi. So is that the 3 dB penalty people talk about for using a circularly
>polarized antenna against an linearily polarized signal (or visa versa)?
>
>If i look at this table from NEC2:
>
> - - - RADIATION PATTERNS
> - - -
>
> - - ANGLES - - - DIRECTIVE GAINS - - - - POLARIZATION - -
> - - - - E(THETA) - - - - - - E(PHI) - - -
> THETA PHI MAJOR MINOR TOTAL AXIAL TILT SENSE
> MAGNITUDE PHASE MAGNITUDE PHASE
> DEGREES DEGREES DB DB DB RATIO DEG.
> VOLTS/M DEGREES VOLTS/M DEGREES
> 90.00 0.00 -1.61
> -999.99 -1.61 0.00000 90.00 LINEAR 2.99921E-12 -147.64
> 7.33103E-01 -60.34
> 89.00 0.00 -1.47 -38.57 -1.47 0.01397 -89.96 RIGHT
> 1.04148E-02 32.15 7.44717E-01 -60.51
> 88.00 0.00 -1.34 -32.42 -1.34 0.02793 -89.93 RIGHT
> 2.11439E-02 31.91 7.56250E-01 -60.70
> ...
>
> 3.00 0.00 2.40 2.37 5.40 0.99646 64.46 RIGHT
> 1.16035E+00 5.69 1.16293E+00 -84.15
> 2.00 0.00 2.41 2.38 5.40 0.99698 55.89 RIGHT
> 1.16172E+00 5.66 1.16302E+00 -84.18
> 1.00 0.00 2.41 2.38 5.41 0.99713 49.60 RIGHT
> 1.16254E+00 5.65 1.16307E+00 -84.19
> 0.00 0.00 2.41 2.38 5.41 0.99714 47.40 RIGHT
> 1.16281E+00 5.64 1.16309E+00 -84.19
> -1.00 0.00 2.41 2.38 5.41 0.99713 49.60 RIGHT
> 1.16254E+00 5.65 1.16307E+00 -84.19
> -2.00 0.00 2.41 2.38 5.40 0.99698 55.89 RIGHT
> 1.16172E+00 5.66 1.16302E+00 -84.18
>
>So, in this case, is the "AXIS RATIO" functioning as some kind of measure of
>circularity?
>
>Finally, we see "TOTAL DB" being 5.41 dB (which is the same as just exciting
>half of this antenna). Does that correspond to "dBc" (or "dBic"??) assuming
>the circularity is good?
>
>In short, what do these "dB" numbers mean and how do i tell how good a
>circularly polarized antenna is?
> -- JM
>
>-------------------------------------------------------------------------------
>CM Specious 300MHz yagi to try to understand circular polarization printouts
>CM Single yagi reports gain of 5.4 dBi at 77 ohms
>CE
>GW1 15 .25 0 -.26 -.25 0 -.26 .001
>GW2 15 0.231 0 0 -.231 0 0 .001
>GM10 1 0 0 90 0 0 .01 0
>GW99 1 .005 0 -.51 -.005 0 -.51 .001
>GE
>TL99 1 2 8 77 .24
>TL99 1 12 8 77 .50
>EX0 99 1 0 1
>FR 0 5 0 0 250. 25
>XQ
>FR 0 1 0 0 300 0
>RP 0 1 361 111 90 0 90 1
>RP 0 361 1 111 90 0 -1 90
>EN
>===============================================================================
>--
>The NEC-List mailing list <nec-list_at_gweep.ca>
>http://www.gweep.ca/mailman/listinfo.cgi/nec-list

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