Andy, I don't know how you arrived at that conclusion. It simply isn't
true. Dbd "could" be referenced to a dipole in free space if so noted,
but there is no convention that says it always is, or that it should
be. If you believe that dbd by definition should always reference a
dipole in free space, could you please provide a single industry
reference that states that?
In any case, your rationale is flawed. You state:
"Using your definition would be marketing hype, because all they need
to do is find the right environmental conditions that make their
antenna look good in comparison to the dipole. Using free space is
the only good way to make the comparisons fair and unbiased."
It is far easier to generate marketing hype, with much more inflated values, by
finding the right environmental conditions that make a manufacturer's real-life
antenna look good in comparison to a reference dipole in free space than it is
to find the right environmental conditions that make their real-life antenna
look good in comparison to a reference dipole in the exact same location.
And where did comparing a vertical antenna six feet off the ground to a dipole
in free space come from?
Dave AB7E
Andy wrote:
>> It's no fair trying to claim that dBd means "gain over a dipole in free
>> space"
>>
>
> But that is EXACTLY what it means!!!
>
> Anything else is inexact science, not even science, and would require
> a disclaimer that all numbers have an error tolerance of +/- 10 dB or
> so because we don't really know what they are.
>
> A dipole in free space is relatively achievable (with reasonably good
> approximation) at VHF and above. Not at HF, of course.
>
> But that is the standard for comparison.
>
> Just like the isotropic antenna is unachievable. But it is our other
> standard for comparison.
>
> When we say a tuned halfwave dipole has a gain of 2.16 dBi (or 2.15 or
> 2.2 or whatever number you round it to), that number MEANS, and ONLY
> APPLIES TO, a dipole in free space ... or close to it. It DOESN'T
> mean a dipole in YOUR back yard, or over a salt marsh, or lying in the
> grass, etc., nor does it account for the differences between a V or H
> dipole over any ground. It means in free space. Period. All those
> other choices would have a very different gain with respect to the
> isotropic antenna.
>
> This is not marketing hype. It is the science (or engineering, if you
> prefer) of antenna theory.
>
> Using your definition would be marketing hype, because all they need
> to do is find the right environmental conditions that make their
> antenna look good in comparison to the dipole. Using free space is
> the only good way to make the comparisons fair and unbiased.
>
>
>> The clear, misdirection free use of dBd is "gain over a dipole in
>> the same location." That has meaning and is a useful gain measurement,
>> because it automatically subtracts out any ground gain advantage.
>>
>
> Hardly.
>
> Take a halfwave dipole, and some new whiz-bang antenna. Mount them 50
> feet up in your backyard and measure their difference. Now mount them
> 6 feet off the ground somewhere else. Their difference will not be
> the same! (Especially if one is a vertical and the other horizontal.)
> It is not "clear, misdirection free".
>
> The only comparisons that are clear, misdirection free, are relative
> to an isotropic antenna in free space = dBi, or a halfwave dipole in
> free space = dBd.
>
> Andy
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