What in a conservation area? Luckily myself I don't need much of an aerial here, but some folk down in Kingston do, and its here that the feathered Karate Parakeets get them, or the overweight pigeons do. A friend told me Magpies have been trying to repossess his o f late as well. Brian
Or 50,000 volts between the boom and the elements?
Brian
If the tx is visible it is extremely unlikely that the aerial will need to point in any direction other than straight at it.
Bill
Google bird spikes
Bill
You have never seen ghosting as an aerial installer?
You missed the change to digital transmission? Ghosting is a thing of the past. With COFDM modulation, reflections within the guard interval contribute useful signal.
Blimey, Google make everything these days, don't they?
No, just multipath reflections;!...
Thinking of building myself one of these later on this year. Back to front depth is under 7cms and it can be mounted hard up under the eaves. Horizontal polarisation.
And just how far out does it need to be before it becomes noise?
Miles. For all UK DVB-T and -T2 implementations (8K mode with 1/32 guard interval fraction and 32K mode with 1/128 GI) the guard interval is 28 us, equivalent to 8.4 km path difference.
dennis@home wrote:
Dennis, you are clearly a fatuous pigheaded know-all fool who will learn nothing from the explanation below, but I give it for the benefit of others.
Leaving aside the fact that there is no ghosting with digital TV, the fact is that the idea of putting the aerial off beam to minimise multipath reception is largely a myth. It's a hangover from the VHF 405 line days when most aerial installation was done by trial and error. It very rarely worked, even then; the most it achieved usually being an adjustment to the phase relationship between the direct and indirect signals (because the dipole was moved across the two wavefronts as an accidental consequence of the rotation of the array), this changing the polarity of the second image. For UHF reception the situation is different for two major reasons. Firstly we are receiving four or more signals on different frequencies on the same aerial. This means that the technique of adjusting the aerial so a null faces the unwanted reflected signal is useless because the nulls are in different positions for different frequencies. Secondly, a UHF aerial is much more directional than a VHF one. This directivity means that if the aerial is off-beam the strength of the signal received directly from the tx is greatly reduced. Since multipath is caused by the ratio between the strengths of the direct and indirect signals being inadequate, and not simply by the strength of the reflected signal, an off-beam aerial is already highly unlikely to combat multipath without other factors being considered. I can honestly say that in 45 years the number of occasions when I have found that leaving an aerial off beam has been beneficial has been something like three, and these have been in places where reception has been so utterly hopeless and the direct path so totally obstructed that aerial alignment has been more or less arbitrary, with reflected and diffracted signals coming from a multiplicity of directions, many of them at closely similar strengths. In a case like that it is a matter of choosing which reflected signal to point the aerial at. Reception can never be satisfactory or reliable. Think in terms of a cottage in a deep valley, surrounded by trees and with very large buildings all around, or think of a low rise building in a city completely overwhelmed by the surrounding high rise. There is one situation that I exclude from my assertion that the aerial is always aligned on the direct signal. It's unusual, but I guess I've seen it about a dozen times. It's when the direct signal is completely obstructed, and thus isn't, in practical terms, there. There is only one significant refection (this condition is why the situation is unusual). The result is that the reflected signal is much stronger (infinitely stronger, possibly!) than the direct signal or any other reflected signals. In that case the aerial will be aligned on the reflection. Note that this is not a case of aligning a null on the reflected signal but of aligning the forward lobe on it. In these cases it is often possible to screen the aerial from directions other than the desired one by buildings (mount it low) because the reflection is coming from a high building not so far away, and this is visible perhaps even from the ground.
OK then Dennis, now go boil your head.
Bill
Should work quite well that unless your area id rather compromised for levels.
Least it's a bird unfriendly design;)...
In general these designs have a very broad forward lobe, which is bad if there's CCI from +/- 45deg of the tx. For instance, Rowridge reception from somewhere to the north of the tx.
I'm sceptical of the gain claim. Commercial products of that sort of design don't achieve that across the band.
Bill
Who makes them Bill?...
If it can "see" the satellite and the glass is RF transparent, and not all are, then yes it will work..
Now you're asking. There's a lot that are similar but have two or four stacked dipoles (nominally full wave). There are at least two makes that have the double diamond active element, both European. I have some pictures. I'll dig them out tonight.
Bill
Just a footnote, I've used double biquads centre connected to 1/4 wave sleeves, without a reflector, slipped over rubber duck aerials, to boost transmit and receive range of 2.4GHz systems. Changes polarisation from vertical to horizontal and improves directionality. A very simple mod to do.
Can also be used in the other orientation which retains vertical polarisation.
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