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Digidome / omni direction TV aerials - are they any good?

Discussion in 'Audio Visual' started by gwernybwch, 20 Jul 2020.

  1. gwernybwch

    gwernybwch

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    Been asked by a friend about omni-directional TV aerials.
    The situation is that their TV aerial is attached to the chimney and the redundant chimney stack is being taken down.
    There is no obvious other high place to put an aerial and putting it at the front of the house would mean a lot of cabling to the back of the house (where they have their TV).

    I've read about digidome aerials and have read good reviews on them. Anybody have experience with them.

    It is in Cardiff and an area generally with good signal, although you can often pick up two transmitters - Wenvoe (South Wales) and Mendip (Somerset). No sure if this is a good thing for omni-directional or not?

    Thanks in advance for any replies.
     
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  3. sircerebus666

    sircerebus666

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    There's the loft , on a cranked pole at the back of the house nice and high , you could also get one of the slate brackets and attach it to your roof , you could also fit it to the front of your house and run the cable to the back of the house through the loft

    Basically there's lots of options

    Don't forget the transmitter are on a hill nice and high so you should get a signal at the back of your house , plus with two transmitters to choose from , you should have no problems getting a signal no matter what you decide to do
     
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  4. bernardgreen

    bernardgreen

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    Another option is to have a short mast fitted to roof or wall when the chimney stack is removed.
     
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  5. winston1

    winston1

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    In answer to the original question omnidirectional TV aerials are useless except in extremely strong signal areas with no alternative transmissions nearby to cause interference e.g. 4G and 5G. Avoid them like the plague. Not even sure if they can be adjusted for polarity.
     
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  6. Lucid

    Lucid

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    Like any amplified portable aerial, if the signal is strong generally then they'll work, and so the purchaser will have their expectations met by the "wonder aerial" when in reality a simple small unamplified Log Periodic would probably work just as well or maybe better.

    The unamplified aerial will generally give a better level of margin between signal level and background noise, and so be more resilient to changes in reception conditions.

    Omnis are generally a bad idea. I can see the appeal of an aerial that doesn't need to be adjusted for direction, but the problem is that so little of the metal ends up pointing in the correct direction that the results are always compromised. To give you an idea, FM radio aerials - the Omni "halo" style - are popular for the same reason (and radio does pick up from transmitters in different locations, so there's some justification). However, when the signal level is measured and compared to a simple 'vertical stick' dipole, the omni performs significantly worse. It actually loses signal. That's because so little metal points in the direction of each transmitter.
     
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  8. sircerebus666

    sircerebus666

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    Don't forget a quality screened aerial cable with as few connections as possible too , combine that with a decent log periodic like lucid says and you should have no problem getting a signal not matter where your friend decides to site the aerial

    https://www.aerialsandtv.com/

    This is quite an informative site
     
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  9. gwernybwch

    gwernybwch

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    Thanks all for the replies. The message of 'use a log periodic aerial' is coming through loud and clear!

    With regard to cabling to the aerial, is Webro still a good make of cable? And WF100 a good choice?

    Thanks in advance for any replies.
     
  10. Lucid

    Lucid

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    Webro WF100..... 100% thumbs up.

    Regarding other aerials, there are alternative choices, but you really need to know a bit about your local signal conditions now and what's likely to happen in the future to make a good choice.

    In my part of the county (N.W. England), the local transmitter is Winter Hill. For years it had all the transmission channels bunched up in the upper third of the old ch 21-68 range. That was a perfect match for what's commonly referred to as wideband high-gain aerials. They were wide band in that they received all the ch21-68 range, but only got to be high gain in the upper third of that range. About halfway down the range the gain was only average, and in the bottom third it was distinctly poor.

    For transmitters where the channels were grouped in the lower third or middle section of the 21-68 band then there were better aerial choices. This is where an aerial tuned to a narrower range would give a much stronger signal. These are the Grouped Aerials.

    A Group A aerial picks up really well at the bottom third of the channel range. However, take that aerial to somewhere within the Winter Hill transmission area and you'd find that the reception sucks.

    That's an example of using a good aerial in the wrong place; and this was always the problem with wide band high-gain aerial sold as a universal solution. They never worked equally for all transmitter areas because they were never both wideband AND high gain at the same time. The challenge for manufacturers, retailers and the general public is that there's no simple one-line way of communicating this complex situation.

    Consumers often just want an "easy" solution. Trying to educate every prospective purchaser at the point where they're making a purchasing decision is a fools errand. They'll look at the seller with suspicion because it doesn't fit with what they expect. "But this one is wideband and high gain. You're telling me it won't work as well for me as this other aerial? Er... okay. I'll think about it." That's called confirmation bias.

    It's a similar story with mass merchandise retailers. They want a simple sell off the shelf or off the page. The aerial with the biggest numbers wins.

    After a while, the message gets back to the manufacturers through their sales figures. Join the herd or fall by the way side. So everyone sings the wrong tune.

    Logs aren't "high gain". Nor are they a true universal solution. However, in the abscence of detailed knowledge about the signal now and in the future, theyre a decent safe bet.
     
    Last edited: 25 Jul 2020
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  11. gwernybwch

    gwernybwch

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    @Lucid thanks for the reply.

    I (also) didn't realise that TV aerials were so technical!

    I fitted what I thought was a 'good quality' aerial to my house in Mid Wales, but didn't really get a great signal. Maybe it was because it was the wrong type of aerial. The channels are on 22 to 28.

    My friend will try and pick up the Wenvoe transmitter. The channels are on 41 to 55. The alternative is the Mendip transmitter which has channels on 32 to 55.
    Based on that is there any particular type of aerial that you would recommend?
     
  12. Lucid

    Lucid

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    BIB - yeah, this is what happens when you scratch the surface.

    Channels 22 and 28 are in the lower part of the channel range, and so they'll fall where wideband high-gain (WB H-G) aerials aren't that efficient. There's also the difference between various qualities of WB H-G. Not all aerials are equal despite the advertising blurb. It's this numbers thing again; counting (or really over-counting) the number of aerial elements so you end up with claims of 48-element or 52-element aerials because each director has been counted four times rather than once because there are four legs. It's a massive con. Then there's the construction itself. There are a lot of 'tin foil' aerials sold which are little more than bent bits of sheet aluminium. Some of the better aerials have aluminium bar as thick as a pencil for the elements.


    I'm short of time at the moment to go in to detail, but start with this graph from the guys at Aerials and TV. These are the response curves for good aerials. The good bit is important. Lesser quality aerials may have a similar profile, but sit lower down on the graph because they aren't as efficient due to thinner metal or not such good design.

    Another trick is to measure in a way that artificially boosts the figures (moving the goal posts). The graph here shows dBd which is a measure of deciBels against a reference dipole - hence dBd. The dipole already has some gain (2.15dB). The other way of measuring is dBi - this is dB with reference to something called an isotropic radiator. This is a theoretical aerial where there's no gain (0 dB). To get a bigger looking gain figure, some manufacturers quote dBi and thus inflate their real world claimed figures by 2.15dB. The graph here doesn't use that sort of flattery.


    For your home, the red curve is a Group A aerial. This gives the best reception strength for channels in the lower 1/3rd of the channel range.


    Grouped-and-wideband-aerial-gain-curves-500H-L10.jpg


    You can read more on the aerialsandTV web page. They're good guys and I'd recommend purchasing from them for you and for your friend.
     
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