Understanding CAI benchmarked aerial categories

How much stock do the professional installers place in the CAI aerial
benchmarking scheme? I receive DTT from Reigate 6 miles away, which seems to
be powered by an AA battery, plus I'm in a dip behind some trees. Installers
hate my site and won't guarantee anything when quoting.

I'm willing to stick up my own aerial. I've installed my own dish, used
reasonable PF100 cables etc, carefully cut/ crimped/ fed around careful
radii etc on corners, and kept away from noisy sources. I'm about to migrate
to a multiswitch and a quattro LNB etc for better satellite distribution
with triplexing plates.

So, looking at the CAI aerial benchmarking document online, do the lower
numbered standard aerials have better gain properties, particularly in
wideband (needed for Reigate)? I see the cheapo Screwfix stuff have plenty
of "category 3" aerials, supposedly high-gain, but I note these are the
minimum standard and have less gain really than "category 1" aerials. Or
have I musunderstood?

I do appreciate there's much more to installing aerials than this - I've
followed this group for a while - but as no-one seems to want to come near
my site with any guarantee I'm prepared to take the risk of doing this
myself.

Advice would be gratefully received.

Thanks,

Basil.

"Basil" wrote in message
et...
How much stock do the professional installers place in the CAI aerial
benchmarking scheme?

The score must depend on how much a company pays them! They certainly don't
go inspecting a lot of the installations.
Too many cowboys fitting aerials now, even those apparently approved.

"Basil" wrote in message
et...
How much stock do the professional installers place in the CAI aerial
benchmarking scheme? I receive DTT from Reigate 6 miles away, which seems
to be powered by an AA battery, plus I'm in a dip behind some trees.
Installers hate my site and won't guarantee anything when quoting.

I'm willing to stick up my own aerial. I've installed my own dish, used
reasonable PF100 cables etc, carefully cut/ crimped/ fed around careful
radii etc on corners, and kept away from noisy sources. I'm about to
migrate to a multiswitch and a quattro LNB etc for better satellite
distribution with triplexing plates.

So, looking at the CAI aerial benchmarking document online, do the lower
numbered standard aerials have better gain properties, particularly in
wideband (needed for Reigate)? I see the cheapo Screwfix stuff have plenty
of "category 3" aerials, supposedly high-gain, but I note these are the
minimum standard and have less gain really than "category 1" aerials. Or
have I musunderstood?

I do appreciate there's much more to installing aerials than this - I've
followed this group for a while - but as no-one seems to want to come near
my site with any guarantee I'm prepared to take the risk of doing this
myself.

Advice would be gratefully received.

Thanks,

Basil.




The only aerials I use which are CAI benchmarked is the log periodics from
Blake. I wouldn't take too much notice of the benchmark scheme for aerials.
I rate it highly for cables though.

Rob wrote:
"Basil" wrote in message
et...
How much stock do the professional installers place in the CAI aerial
benchmarking scheme?

The score must depend on how much a company pays them! They certainly don't
go inspecting a lot of the installations.
Too many cowboys fitting aerials now, even those apparently approved.


The DAT 75 is a Standard 1 Wideband aerial, but the Mast passes through
the aerial between the elements. This us usually frowned upon by any
good installer with any other aerial. How this passes is strange...

Glenn...

Basil wrote:

So, looking at the CAI aerial benchmarking document online, do the lower
numbered standard aerials have better gain properties,[...]

Advice would be gratefully received.

You need to read the Benchmarked Aerial 'Guidelines' Document:
http://www.cai.org.uk/downloads/Guid...0 Aerials.pdf

--
Andy

You need to read the Benchmarked Aerial 'Guidelines' Document:
http://www.cai.org.uk/downloads/Guid...0 Aerials.pdf

--
Andy

That PDF is what I read before I posted, 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.

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 follow
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?

Basil

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 a

- 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 the
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 follow
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 at
http://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 Castle
(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

On Jun 8, 1:43*am, Andy Wade 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 a

- 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 the
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 follow
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 Castle
(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