'DAT45' type aerials springing up in Bedfordshire

On Thu, 26 Mar 2009 08:07:42 +0000, Ian Jackson
wrote:

In message , Bill Wright
writes

"Paul S" [email protected] wrote in message
...

"Tanuki" wrote in message
...
In , Paul Ratcliffe [email protected]
clara34.co56.uk78 said
What is the function of the two outer booms? Seems like F all to me.

The two outer booms are there to provide an extra 5.7dB marketing-gain.

s'obvious innit - it's to catch the signal reflections from the ground and
clouds. Also if it's a bit breezy as it waggles back and forth (what with
such a big aerial on the undersized pole) either the top or bottom ones
will be pointing at the transmitter so no probs there!

The justification will be that they increase the capture area.

There are a lot of aerials with 'X-shaped' directors, each of which
consists of four insulated approximately halfwave elements. At the
tail end, the active element is usually still only a halfwave folded
dipole. How much more effective are the X-shaped directors compared with
single elements?

None whatsoever.

Marky P.

On Tue, 24 Mar 2009 00:52:02 -0000, "Bill Wright"
wrote:


"Alan" wrote in message
...
In message , Bill Wright

The difference would be small beer compared to the total invoice.


small beer? Surely you mean Red Bull?

Yes, of course.

Bill

I've found a DAT 75 in a small village in cambridgeshire that's
attached to the wall using a small 2"x2" bracket. It is currently
falling off. I will take a pic sometime, if it's still there.

Marky P.

On Thu, 26 Mar 2009 23:50:08 +0000, Marky P wrote:
I've found a DAT 75 in a small village in cambridgeshire that's attached
to the wall using a small 2"x2" bracket. It is currently falling off.

Is that not a danger to those passing by underneath?

"Ian Jackson" wrote in message
...
In message , Bill Wright
writes

"Paul S" [email protected] wrote in message
...

"Tanuki" wrote in message
...
In , Paul Ratcliffe
[email protected]
clara34.co56.uk78 said
What is the function of the two outer booms? Seems like F all to me.

The two outer booms are there to provide an extra 5.7dB marketing-gain.

s'obvious innit - it's to catch the signal reflections from the ground
and
clouds. Also if it's a bit breezy as it waggles back and forth (what
with
such a big aerial on the undersized pole) either the top or bottom ones
will be pointing at the transmitter so no probs there!

The justification will be that they increase the capture area.

There are a lot of aerials with 'X-shaped' directors, each of which
consists of four insulated approximately halfwave elements. At the tail
end, the active element is usually still only a halfwave folded dipole.
How much more effective are the X-shaped directors compared with single
elements?

I'd say that for the same boom length the difference is about 1dB. If you
accept the extra boom length of a big high gain aerial versus a normal 18
element it is about 1.5dB.

I've always said that a major problem with aerials that are basically a yagi
but with elements of a more complex shape than a simple straight rod, is
that they tend to have less polarisation discrimination and often they have
polar response diagrams that show large unwanted lobes. These factors can
result in a c/n ratio that is worse than that from a straight 18 element,
despite the signal level being perhaps 1.5dB better.

Bill

In message , Bill Wright
writes

"Ian Jackson" wrote in message
...
In message , Bill Wright
writes

"Paul S" [email protected] wrote in message
...

"Tanuki" wrote in message
...
In , Paul Ratcliffe
[email protected]
clara34.co56.uk78 said
What is the function of the two outer booms? Seems like F all to me.

The two outer booms are there to provide an extra 5.7dB marketing-gain.

s'obvious innit - it's to catch the signal reflections from the ground
and
clouds. Also if it's a bit breezy as it waggles back and forth (what
with
such a big aerial on the undersized pole) either the top or bottom ones
will be pointing at the transmitter so no probs there!

The justification will be that they increase the capture area.

There are a lot of aerials with 'X-shaped' directors, each of which
consists of four insulated approximately halfwave elements. At the tail
end, the active element is usually still only a halfwave folded dipole.
How much more effective are the X-shaped directors compared with single
elements?

I'd say that for the same boom length the difference is about 1dB. If you
accept the extra boom length of a big high gain aerial versus a normal 18
element it is about 1.5dB.

I've always said that a major problem with aerials that are basically a yagi
but with elements of a more complex shape than a simple straight rod, is
that they tend to have less polarisation discrimination and often they have
polar response diagrams that show large unwanted lobes. These factors can
result in a c/n ratio that is worse than that from a straight 18 element,
despite the signal level being perhaps 1.5dB better.

That's interesting about the gain, and an interesting observation about
the polar diagram etc. If there's so little advantage with this
configuration, I'm surprised that so many aerials use it (in its many
variants). It's possibly simply a case of 'styling' (ie 'that's what the
customer expects').
--
Ian

"Ian Jackson" wrote in message
...
I've always said that a major problem with aerials that are basically a
yagi
but with elements of a more complex shape than a simple straight rod, is
that they tend to have less polarisation discrimination and often they
have
polar response diagrams that show large unwanted lobes. These factors can
result in a c/n ratio that is worse than that from a straight 18 element,
despite the signal level being perhaps 1.5dB better.

That's interesting about the gain, and an interesting observation about
the polar diagram etc. If there's so little advantage with this
configuration, I'm surprised that so many aerials use it (in its many
variants). It's possibly simply a case of 'styling' (ie 'that's what the
customer expects').

It's a fact that large multi-element aerials are very often used because
they impress the customer, and allow the installer to extend the price scale
upwards. It's all about establishing 'product difference'.

Of course there are occasions when 'every microvolt counts', but given that
most urban locations are quite noisy, masthead amplifier noise (and thus
aerial gain) are often not as significant as might at first be thought. All
too often the installer gains a couple of dB that should in theory greatly
reduce pixellation, only to find that it doesn't, much. There are occasions
when a really big high gain aerial is called for, but they are few and far
between.

In any case, I can only repeat what I've been saying since 1978, that two 18
element aerials stacked or bayed will normally beat a high gain aerial that
has the same weight and windage, as long as they are installed with skill. I
see that Vision have finally agreed with me -- see their new product range.
Look forward to the Antiference response in a few months. Eventually we will
be back to the aerial that had more gain than any other, the quad MBM88 from
J Beam.

At the start of DTT I wrote
http://www.wrightsaerials.tv/topics/...g-aerials.html
I don't agree with all of it now, but it gives an idea of what I'm on about.

A very import -- nay crucial -- factor is the present trend to use wideband
aerials for everything. Wherever possible use a grouped aerial. The
difference is considerable.

Bill