25-04-2013, 07:02 PM
It's an interesting - but not a novel - idea.
Overall design concept.
To get a 'feel' for the concept, it is helpful to think about how it is designed to work as a transmitting aerial. The end of the inner that is 'up in the air' constitutes an open-circuit as far as a current wave travelling up that length is concerned. Consequently, it constitutes a high impedance. Since that length is a quarter-wave (electrical length), one-quarter-wave down from the top will be a low impedance. By the same reasoning, at another quarter-wave below that low impedance point, another high impedance wave will occur - or won't, in this case, unless we can make it a high impedance - and that's the purpose of the choke.
So, overall, we have a low impedance point in the centre and a high impedance at both 'ends': in essence, the same situation as with a traditional half-wave dipole. And with such a dipole, the centre feed impedance will be about 75 Ω resistive. Hence, 75 Ω coax and a source designed to deliver maximum power into a 75Ω load. Now reverse that power flow (transmitter - to - aerial) and think of received power at the aerial flowing to a load - the receiver - which needs to have a 75Ω input impedance.
Comments on the design and concept.
First, the idea of stripping off the cover over the braid on the lower half of the coax is a complete waste of time: doing that will not affect the performance of the aerial one iota. Secondly, for this aerial to give any sort of reasonable performance, the effectiveness of the choke is critical: I would be tempted to use a few ferrite beads. Thirdly, since this aerial is, in essence, a vertical half-wave dipole, it will only give anything like its designed performance if it is located well away from earthed objects and other screening items. And that means using it outdoors and at least three wavelengths away from any obstructing items - metallic or otherwise.
How would I expect this aerial to compare to other types.
To the best of my knowledge, the polarisation of DAB signals are same as that used on FM: vertical and horizontal mixed. Hence, I would simply put up a horizontal half-wave dipole as close to the ceiling as can be arranged; a lot easier to manufacture and to suspend. And that is the arrangement I have in my bedroom for FM - and considering that that is a simple dipole and is indoors, it works fairly well.
HTH,
Skywave.
Overall design concept.
To get a 'feel' for the concept, it is helpful to think about how it is designed to work as a transmitting aerial. The end of the inner that is 'up in the air' constitutes an open-circuit as far as a current wave travelling up that length is concerned. Consequently, it constitutes a high impedance. Since that length is a quarter-wave (electrical length), one-quarter-wave down from the top will be a low impedance. By the same reasoning, at another quarter-wave below that low impedance point, another high impedance wave will occur - or won't, in this case, unless we can make it a high impedance - and that's the purpose of the choke.
So, overall, we have a low impedance point in the centre and a high impedance at both 'ends': in essence, the same situation as with a traditional half-wave dipole. And with such a dipole, the centre feed impedance will be about 75 Ω resistive. Hence, 75 Ω coax and a source designed to deliver maximum power into a 75Ω load. Now reverse that power flow (transmitter - to - aerial) and think of received power at the aerial flowing to a load - the receiver - which needs to have a 75Ω input impedance.
Comments on the design and concept.
First, the idea of stripping off the cover over the braid on the lower half of the coax is a complete waste of time: doing that will not affect the performance of the aerial one iota. Secondly, for this aerial to give any sort of reasonable performance, the effectiveness of the choke is critical: I would be tempted to use a few ferrite beads. Thirdly, since this aerial is, in essence, a vertical half-wave dipole, it will only give anything like its designed performance if it is located well away from earthed objects and other screening items. And that means using it outdoors and at least three wavelengths away from any obstructing items - metallic or otherwise.
How would I expect this aerial to compare to other types.
To the best of my knowledge, the polarisation of DAB signals are same as that used on FM: vertical and horizontal mixed. Hence, I would simply put up a horizontal half-wave dipole as close to the ceiling as can be arranged; a lot easier to manufacture and to suspend. And that is the arrangement I have in my bedroom for FM - and considering that that is a simple dipole and is indoors, it works fairly well.
HTH,
Skywave.
