Re: Understanding CAI benchmarked aerial categories

On Jun 8, 1:43 am, Andy Wade <spambuc...@maxwell.myzen.co.uk> wrote:
> Basil wrote:
> > That PDF is what I read before I posted,
>
> OK, but your question "So, looking at the CAI aerial benchmarking
> document online, do the lower numbered standard aerials have better gain
> properties, particularly in wideband" suggested that you'd only been
> looking at the products list.  The table in the guidelines document
> clearly sets out minimum gains in dBd for each standard for each of the
> main bandwidth groups. For Group W products the minimum gains at each
> end of the band are:
>
> - standard 1: 10 - 12 dBd
> - standard 2:  7 - 10 dBd
> - standard 3:  5 -  8 dBd
> - standard 4:  7 dBd (flat).
>
> > hence the observation that many of the advertised aerials supposed
> > "high-gain" characteristics are not in line with the guidance in
that
> > document. My example is a Screwfix-advertised Labgear aerial with a
> > supposed 16.5 dB gain. It has a category "3" rating. Category
"3" is
> > appearently the minimum standard in the document and has the > LEAST
> > gain.
>
> Quite revealing, isn't it?  It's possible, though probably not very
> likely, that the product mentioned has enough gain to make it into
> standard 2, yet the advertised gain is 16.5 dB (note no reference
> stated).  Draw your own conclusions.  The benchmarking test provides t
he
> submitting manufacturer or vendor with detailed test results, but
> they're not under any obligation to publish them.  Everybody still seems

> to quote a single gain figure and if this bears any relationship to
> reality it will be at (or towards) the top of the band for a Yagi-type
> product.  Some surreptitiously quote the gain in dBi which of course
> gives an instant 2.15 dB boost for the unwary.  Others seem to use the
> dBWSCC scale (dB relative to wet string in coal cellar).
>
> > Thus is there any surprise that this appears contradictory? That's why I

> > asked for some guidance. The general feedback would seem to be to not fo
llow
> > that PDF's scheme. Isn't this rather poor that the industry can't agree 
with
> > it's own professional bodies? Shouldn't the advice be clearer?
>
> What advice?  The guidelines document is mostly explanatory, rather than

> advisory.  Advice is in the CAI codes, and in the DTG R-books - the
> latter available athttp://www.dtg.org.uk/retailer/publications.html.
>
> It's a shame if the scheme is being discredited.  A lot of effort went
> into setting it up (yes, I was involved), and into ensuring that
> accurate measurements were made.  Originally all the testing was done by

> Schaffner, on a specially-built 40 m test range at their Broadwood test
> site near Dorking.  Gain testing was by the substitution method, using a

> BBC log-periodic (itself calibrated at NPL) as the reference antenna.
> Testing has since transferred to NPL itself.
>
> Bear in mind that UHF coverage, for the most part, is
> interference-limited, rather than noise-limited.  Forward gain alone is
> not the only consideration and benchmarking also checks radiation
> pattern and cross-polar discrimination.
>
> Can a standard 3 product be called high-gain?  Certainly it can if
> compared with the wideband 'contract' aerials that were in common use
> before  DTT got started.  I remember attending a meeting at Crown Cast
le
> (Warwick) around 1997-98, at which they presented the results of gain
> measurements on various aerials available at the time.  From memory,
> typical gains of contract wideband Yagis (so-called) were around 0 to +3
> dBd at channel 21.  One specimen even had less than 0 dBd gain at that
> frequency.
>
> HTH
> --
> Andy

As someone who has only bought one aerial in his life, my only goal
was to buy a "branded" one which was the right group and had the
appropriate likelihood of good gain.

Bill Wright seemed to recommend Antiference, so that's the way I
went.  If I hadn't read here, I might have gone "fancy" and thought
that a DAT-45 or -75 "looked good", so I avoided them.

Other than Antiference, Blakes seem well recommended.  I guess once
they do the job, quality of build is essential as it is up there in
the elements for many years to come.

Matt
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