OK, so what is this ?

Channel Hopper

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I assue it's some FM aerial, as part of the TV/DAB/FM in the block, but why is it s shaped ?
 

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rolfw

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Seen loads up in London, it's a VHf vertically polarised dipole, a circular omni with one of the sides reversed.
 

PaulR

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I think it's supposed to be a little bit H polarised and a little bit V polarised.
 

2cvbloke

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It's scrap metal, cos it's not going to work... :-rofl2
 

rolfw

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There are hundreds, if not thousands of them in certain parts of London, so I can only assume that they do work.
 

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As they're such a compromise they'll have a gain significantly less than 1. They could surely only work where there are multiple transmitters in various directions at high signal level.
 

2cvbloke

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I'd bet real money that they perform worse than proper polarised dipoles, as that half-swostika thing is probably picking up loads of rubbish that the user doesn't want, but they bought it anyway cos they actually believed the description of them being a universal aerial for FM or DAB, bit like how people buy wideband high-gain TV aerials from argos to pick up a transmitter 2 miles away like they do round here... :-rofl2

They're probably picking up a tiny fraction of what a straight dipole in the same place would be, as half of the antenna is in the wrong polarity, so if you're after vertical signals, then only the outer tips are picking a little bit of it up, and for horizontal, again, a tiny bit is picked up, and if there's a radio station on one polarisation, and another on the opposite polarisation (such as a pirate radio station, seeing as they're common in that there london), then you're going to get a lot of problems with discriminating between the two... :)

And just cos there's lots of them doesn't mean they're any good, there's loads of $ky minidishes on the sides of houses, and they're crap in slightly wet weather... :p
 

rolfw

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There are many hundreds of commercially fitted units and if they didn't work companies wouldn't fit them, yes, they probably have neutral gain, but in a large area surrounding London transmitter sites, they are obviously more than adequate.
 

Channel Hopper

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Can anyone provide the manufacturers technical specifications of such a compromise ?
 

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There are many hundreds of commercially fitted units and if they didn't work companies wouldn't fit them

:-rofl2

You're forgetting the cowboys, they'll fit anything so long as they can over-charge for it claiming they're better than the more expensive (and properly designed) alternatives, just look at the average chimney stack with various forms of cheap & nasty aerials that have fallen apart within a couple of years of being fitted from being made of thin metal, all fitted by companies who insist "they're the best you can get and anything else is junk, and it's all we'll supply and fit", from wideband high-gain aerials in strong signal areas to circular dipole antennas for FM & DAB that actually have negative gain... :-rofl2

Can anyone provide the manufacturers technical specifications of such a compromise ?

Funnily enough, nope, not from the manufacturers (but then the chinese are very secretive about their manufacturing techniques!!), but looking about, there's some data from people who have tried them, and looking and the numbers, I'd say you'd be better off sticking a coathanger in the aerial socket on a radio rather than waste money on one of those S-curve things...:-rofl2

http://imageevent.com/holl_ands/omni;jsessionid=v2dfreznx6.eagle_s
(Scroll down to test no. 13, waste of money and better off sent to a scrapman after purchase!!)
 

rolfw

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2cvbloke

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Well, for all round performance, they could have used a much better halfwave dipole fitted diagonally, mostly because you'll get much higher decibel gain over one of those s-curve things, the s-curve is just one of those "Hey!! Let's make it look different so people think it's special" things that performs worse than a coathanger...

And while it may have been a "professional" job, don't forget that often even the pros install junk, especially if it's for contractual work as the people wanting the work done will go for the cheapest option as it's the cheapest option in the short-term (long term though, not so much!!)...
 

rolfw

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You're missing the point 2cv, if the aerial does the job (provides good solid reception for the location), is well installed, is good quality and stands the test of time, then the product is suitable and the installer has done a good job for the customer.
 

Channel Hopper

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2cvbloke

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You're missing the point 2cv, if the aerial does the job (provides good solid reception for the location), is well installed, is good quality and stands the test of time, then the product is suitable and the installer has done a good job for the customer.

No, the point I'm making is they've installed a piece of rubbish, and they won't be getting the best reception they could be with something that looks like someone came up with when they accidentally sat on a folded dipole, that thing won't pick up a decent amount of FM to reduce hiss to an acceptable point (never mind attaining a stereo signal), as it'll be pulling in signal from all over, causing crossover and interference, and if it's used for DAB, then it won't be any good as it'll keep cutting out and garbling the way it does...

Yeah, it's fitted properly, looks different, and may be up there for years, but providing a good signal, I highly doubt it, if I hired a professional to install an FM or DAB aerial (not needed here given the proximity to Pontop Pike and Burnhope transmitters) and they tried to fob me off with one of those claiming it's a great product, I'd tell them to get stuffed, the problem is that a lot of the people who have had them fitted don't know their Yagi from their Dipole, so they just take the installer's word for it, and hey presto, there's a lot of them about, but like I said earlier, just because there's a lot of them doesn't mean they're any good...
 

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As they're such a compromise they'll have a gain significantly less than 1. They could surely only work where there are multiple transmitters in various directions at high signal level.

These folded omni dipoles for mixed polarity reception are common on the continent. A lot of companies sell them but few of the companies sell in GB due to local companies like Antiference and Blake having most of the market.

Gain is often given as minus 1dB but they are used in strong signal areas and the loss in the cable is so low it doesn't matter.
 

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These folded omni dipoles for mixed polarity reception are common on the continent. A lot of companies sell them but few of the companies sell in GB due to local companies like Antiference and Blake having most of the market.

Gain is often given as minus 1dB but they are used in strong signal areas and the loss in the cable is so low it doesn't matter.
Sounds legit. Many FM radio transmissions in mainland Europe still only have horizontal polarisation, whereas in the UK and also Ireland they are mostly mixed polarisation or otherwise vertical. In a few cases however the transmitter aerial might be configured where by in certain directions the horizontal signal component may be a good bit stronger than the vertical component.
 

Channel Hopper

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No, the point I'm making is they've installed a piece of rubbish, and they won't be getting the best reception they could be with something that looks like someone came up with when they accidentally sat on a folded dipole, that thing won't pick up a decent amount of FM to reduce hiss to an acceptable point (never mind attaining a stereo signal), as it'll be pulling in signal from all over, causing crossover and interference, and if it's used for DAB, then it won't be any good as it'll keep cutting out and garbling the way it does...

Yeah, it's fitted properly, looks different, and may be up there for years, but providing a good signal, I highly doubt it, if I hired a professional to install an FM or DAB aerial (not needed here given the proximity to Pontop Pike and Burnhope transmitters) and they tried to fob me off with one of those claiming it's a great product, I'd tell them to get stuffed,...

From the specifications provided earlier, this is single band only, and Hexah has already confirmed , the shape is such for dual polarity reception, though why this is installed in South London is anyone's guess
 
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