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uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

No reflector on new TV antenna



 
 
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  #22  
Old May 17th 17, 12:41 PM posted to uk.tech.digital-tv
AnthonyL
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Posts: 183
Default No reflector on new TV antenna

On Wed, 17 May 2017 08:21:30 +0000 (UTC), d wrote:

On Tue, 16 May 2017 18:25:04 +0100
Robin wrote:
On 16/05/2017 16:41,
d wrote:
We just had a new antenna installed on the roof and it doesn't seem to have
a reflector at the base, just some extra elements instead. Is this normal
these days and is it something to do with digital vs analogue TV or just
cost cutting?

Does it look like this?

http://www.labgear.co.uk/wp-content/...original-1.jpg


Very similar except a bit longer.


Probably one of these, have a look at

http://aerialsandtv.com/ and specifically the

http://aerialsandtv.com/atvschoiceofaerials.html#DMlog

I've used these at two awkward locations and they've solved my
reception problems.


--
AnthonyL
  #23  
Old May 17th 17, 02:27 PM posted to uk.tech.digital-tv
[email protected]
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Default No reflector on new TV antenna

On Wed, 17 May 2017 12:23:15 +0100
"Brian Gaff" wrote:
Well of course they did.
I have a group B two stacked parabeam aray in my shed used originally for
hannington from Chessingt9on to obtain different film choices when the itv
regions did in fact screen other films to each other. I also hav an
Antiference log that is 21 to 68 channels but only has 89.5db gain on
average. It is after all just a string of different sized dipoles in anti
pase and those off resonance tend to act parasitically on those that are.
Brian


So in other words, if you go and buy an antenna yourself instead of getting a
professional to install one, you need to check its centre frequency to see if
it matches your local transmitter? Sounds like a faff. Why don't they just
build them with the centre freq in the middle of the band and add a few
more elements for extra db?

--
Spud

  #24  
Old May 17th 17, 02:44 PM posted to uk.tech.digital-tv
Andy Burns[_12_]
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Default No reflector on new TV antenna

d wrote:

Are you saying that rooftop antenna centre frequencies varied between regions?


And over time.


  #25  
Old May 17th 17, 03:19 PM posted to uk.tech.digital-tv
Mark Carver[_2_]
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Default No reflector on new TV antenna

On 17/05/2017 14:27, d wrote:

So in other words, if you go and buy an antenna yourself instead of getting a
professional to install one, you need to check its centre frequency to see if
it matches your local transmitter? Sounds like a faff.


It worked fine.

There were three main groups.

Group A that covered Ch 21-34

Group B that covered Ch 39-53

Group C/D that covered Ch 48-68 (replaced Groups C and D that covered
smaller ranges within 48-68)

The four channel analogue plan for a given transmitter was normally (90%
of cases) in one of the nine following clusters:-

21,24,27,31
22,25,28,32 Group A
23,26,29,33

39,42,45,49
40,43,46,50 Group B
41,44,47,51

53,57,60,63
54,58,61,64 Group C/D
55,59,62,65


Then there were other transmitters that used non standard channel
allocations, usually not anything that was out of one of the three
aerial groups.

For instance Dover 50, 53, 56, 66

In some exceptional cases, groups were straddled, the biggest affected
transmitter (in terms of coverage) was Hannington

39,42,45,66

In that case Group E was specified, Ch 39-68. Group E aerials
were rare to find, even in the affected areas. Often instead Widebands
had to be used (21-68)

Once C5 and then DTT came along, the whole plan fell apart in many
areas, as anything empty from 21 to 68 was used.

However, at DSO, in most cases three out of the original four analogue
assignments were used for PSB 1,2 and 3, to ensure anybody still with a
legacy grouped aerial received core services.

What fell out of use at DSO were UHF Ch 61-68.

Between this year and 2020, Ch 49-60 will be going (although there's
talk of Ch 55 and 56 being retianed until 2022 for COM 7 and 8).

Why don't they just
build them with the centre freq in the middle of the band and add a few
more elements for extra db?


If you add elements, you increase gain but at the expence of bandwidth.



--
Mark
Please replace invalid and invalid with gmx and net to reply.
  #26  
Old May 17th 17, 04:10 PM posted to uk.tech.digital-tv
David Woolley[_2_]
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Default No reflector on new TV antenna

On 17/05/17 15:19, Mark Carver wrote:
If you add elements, you increase gain but at the expence of bandwidth.



Also remember that the UHF TV allocation was very large. For good
resonance, you want element length to be within a few percent of the
theoretical value, but doing what you suggest would require the antenna
to work 25% off resonance. That is huge for a resonant system. Even
the original groupings were probably pushing things to the limit.
  #29  
Old May 18th 17, 01:24 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Default No reflector on new TV antenna

On 17/05/2017 23:24, Roderick Stewart wrote:
On Wed, 17 May 2017 13:27:29 +0000 (UTC), d wrote:

So in other words, if you go and buy an antenna yourself instead of getting a
professional to install one, you need to check its centre frequency to see if
it matches your local transmitter? Sounds like a faff. Why don't they just
build them with the centre freq in the middle of the band and add a few
more elements for extra db?


Adding more directors to a yagi tends to increase gain at the HF end but
has less effect at the LF end. The wider the design bandwidth the more
pronounced is this effect. The bandwidth necessary for the original UHF
TV band was approx 2:1. By the time that point is reached the effect
described above is pronounced. Adding directors does nothing for the LF
end, which becomes in effect nothing more than a badly tuned dipole with
a reflector. The result is that directional properties and gain are very
poor. Gain is typically 6 or 7dBd on chs 21, 22, 23. At the top of the
band (or more usually near it) the gain is at maximum, and might be 11
to 13dB. Directivity will also be reasonable. Such an aerial can find a
use where channels right across the band are in use, where there are few
CCI or multipath issues, and where the higher channels have lower field
strength (due often to poorer propagation at higher frequencies).
However a log-periodic (much smaller and lighter) will be a close
competitor, and because it can economically be mounted much higher might
well perform far better. A pair of correctly stacked or bayed logs will
outperform any high gain wideband yagi, except arguably at the top of
the band. It isn't just about gain: A reduction in noise achieved by
higher directivity is, dB for dB, worth as much as an increase in gain,
above a certain field strength level.

Adding a bigger reflector to a yagi is, beyond a certain modest point,
a sales rather than a performance issue.


They could have made the aerials cover the entire band, by using log
periodic designs for example, but in general wideband aerials are less
sensitive than aerials cut for the appropriate group.

I'm not sure that sensitivity can be increased without consequence
just by adding a few more elements, otherwise that's what would have
been done. I think it must have some effect on bandwidth too. I
daresay Mr Wright of this parish knows a bit more detail about this.


Bill

  #30  
Old May 18th 17, 09:58 AM posted to uk.tech.digital-tv
[email protected]
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Default No reflector on new TV antenna

On Wed, 17 May 2017 15:19:10 +0100
Mark Carver wrote:
Between this year and 2020, Ch 49-60 will be going (although there's
talk of Ch 55 and 56 being retianed until 2022 for COM 7 and 8).


One has to wonder about the wisdom of selling off parts of the TV band when
ever more bandwidth hungry HD channels are going on air and in the future
there will probably be a requirement for OTA 4K broadcasts. I know politicians
by nature are short sighted and techno-illiterate, but surely someone could
have advised them on this?

--
Spud

 




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