No, Jim, absolutely no attempt to ever consider measuring the "gain" of my
450-foot doublet!!! At best, I model it using 4NEC2 and even that with a
large crystal of salt.
Measuring the gain (directivity?) of an antenna applies to VHF and above!
HF? Do the modeling. But, then, the NEC code does a miserable job of
handling "ground" (earth).
Dave - W?LEV
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On Tue, Aug 23, 2022 at 7:08 PM Jim Lux <jimlux@...> wrote:
On 8/23/22 11:00 AM, W0LEV wrote:
A dipole, by definition, is resonant: +jX = -jX = 0.
A doublet is a dipole, but not resonant where you require.
My 450-foot doublet exhibits its first 1/2-wavelength resonance at
roughly
950 kHz, but does not show resonance in any of the HF ham bands.
Yes, measurements need to be made to reduce / minimize unwanted
reflections. That's the devil in the details.
You're not going to be measuring the gain of a 450 foot antenna with a
NanoVNA.
The OP was for a printed circuit antenna, presumably well above 100 MHz.
The fundamental problem will be the free space loss. Let's assume
isotropes for the moment. Let's further assume that the probe/reference
antenna is 5 wavelengths away.
At 300 MHz, that's 5 meters.
Loss = 32 + 20*log10(0.005) + 20 *log10(300) = 32-46+50 dB = 36 dB loss.
So that's plausibly ok - your S21 readings will still be big enough that
the uncertainty due to SNR won't be too bad.
And you can measure that in a backyard range. You need to move the
probe up and down so that you can quantify the "ground bounce"
contribution.
By the way, the usual guideline for antenna test distance of
2*D^2/lambda isn't really valid for low gain antennas. But if you take D
as 1/2lambda, you wind up with 2*0.5^2 * lambda^2/lambda which is 1.4
lambda.
--
*Dave - W?LEV*
*Just Let Darwin Work*
--
Dave - W?LEV
