Hi all,
I am simulating a helical antenna with planar, square ground plane for 162 MHz operation in HFSS (see "helical_antenna_with_GP"). When looking at the 3D polar plot results for total gain at 162 MHz, which would you recommend using: GainTotal or dB(GainTotal) (see attached figures)?
In my work, I am measuring the the peak gain (via 3D polar plot) and half-power beam width (measuring the theta between the peak gain, which is orientated along the Z-axis in the 3D polar plots, and -3 dB from that value via radiation plots) vs number of helical turns. I have used GainTotal thus far and believe the units are dBi in HFSS having read this in other forums. However, you can see a difference between the two 3D polar plots for the peak gain, 14.64 and 11.66, using GainTotal and dB(GainTotal), respectively.
The greatest disparity between the two approaches is in the half-power beam width, 28 and 40 deg, using GainTotal and dB(GainTotal), respectively.
So which approach do you think would be the most suitable for these purposes?
Thanks,
Geoff.
Geoff
Gain total and dB(gain total) are what they say. Your peak gain is about 14.64 times the gain of an omnidirectional antenna. 10*log10(14.64) is about 11.66 which is the gain in dBi. Either take the half-power (gain) widths from the gain values or the -3 dB widths from the dB(Gain) values - they are equivalent. Plotting in dB allows you to see very small features which do not show up in linear plots, that is why there are lumps and bumps in the dB plot that look quite big - they are all right at the bottom of the linear plot
So to help clarify Malcom's answer, you're looking at linear gain (power ratio) vs. logarithmic gain in dB (dBi). So the half power beamwidth is at 50% of the peak in the linear gain plot and is the same beamwidth as the -3 dB beamwidth that is 3 dB from the peak (half the power) in the logarithmic plot. "3" has no particular meaning in the unitless linear plot, and (14.64 - 3) is not half of 14.64. However, 11.66 dBm (for the sake of argument) - 3 dB = 8.66 dBm is half the power (to within about 0.01 dB, since 10 log10(0.5) is not exactly -3).
Malcolm and Michael,
Thanks so much for your answers, you've cleared up a lot of my understanding, very much appreciated!
Thanks,
Geoff.
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