[50mhz] W5ZF comments

[Bill VanAlstyne W5WVO]

Bud (mweisbergs at juno.com) wrote:

> ...It seems that antenna discussions bring out the exuberent
> imagination in folks; so much so that the ARRL has, for quite some
> time, prohibited gain claims in the antenna display ads in QST.

Your point about "exuberant imagination" is well-taken, but I'm afraid your
data is a little out of date.  :-)  The ARRL changed their advertising
policy with regard to antenna gain figures in April 1998 -- over seven years
ago. Here is a link to the "new" (i.e., current) policy:

http://www.arrl.org/members-only/tis/info/pdf/9804054.pdf

This link can only be accessed by ARRL members, but here's an unrestricted
link to the same article from April 1998 QST magazine:

http://www.arrl.org/antmodels/policyart.html

The article describes in some detail what the various forms of gain
measurement mean and how they are derived. Excellent reading for everyone
interested in antennas from ARRL's antenna guru, Dean Straw, N6BV.

In a nutshell: Gain figures in ads must be given in free-space dBi -- that
is, relative to an isotropic radiator in free space -- and must be backed up
with a pre-submitted computer model of the antenna. The ARRL Lab then
cross-checks the advertiser's computer model by modeling the antenna in NEC4
and, if it's a yagi, in K6STI's YO (Yago Optimizer). If the claims check
out, the ad is approved and allowed to run.

As long as gain relative to a free-space isotropic radiator is included in
the ad, the advertiser can also include other gain figures (e.g., dBd in
free space, dBi above "typical" ground at a certain height, etc). But any
gain figures not mathematically tied to free-space dBi have to be supported
by a computer model that can be replicated in NEC4.

So that's the story, guys. The times they are a-changin'.

Technically, it doesn't matter whether gain figures are reported as
free-space dBi or free-space dBd, since the relationship between the two is
merely an additive constant. But you have to use one or the other
consistently in order for antenna-A-to-antenna-B comparisons to have any
meaning. ARRL chose free-space dBi, for various technical reasons that I
think make sense. If you want to convert it to dBd, just subtract 2.15. I
think we all oughta be able to handle that.  ;-)

73,
Bill / W5WVO


>
> Growing old isn't so bad... at least when you refer to something in
> the dark past, there are few around to dispute it!
>
> Bud K2YOF
> Moderator: Ray Brown, KB0STN
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