I made one of these this morning:-
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I was wondering if those versed in the Dark Arts of RF know of a better way of coupling it to the set. It's currently connected to the ends of a bobbin of 24 SWG (I think) ECW and obviously needs something which is a closer fit to the telescopic job at least.
Also, does anybody know the dimensions for a Band II version? I know Radio Two broadcasts on about four feet but that's the extent of my knowledge.
Cheers
Joe
Hi Joe, interesting, I'm intrigued by the one you show, why is the braiding exposed, what are the 2 turns for.
That aside I would say that the coupling method you have used is for all intents capacative.
You might be able to connect direct to the RF input to the tuner, you ideally need a circuit diagram for that. I know zilch about DAB.
Lawrence.
I couldn't see why stripping 9" of sheathing off would make any odds either. If it's meant to form a dipole surely it would need to be the same length as the de-braided bit.
As for the coupling coil, I was wondering if a short fat one or a long thin one would be better.
I found somebody selling these on e-Bay and a bit more digging found the design.
- Joe
Hi Joe, that design just looks like a screened short whip (assuming it could be coaxially coupled to the receiver).
For best reception I would say a dipole or Yagi is best.
Is there a web link to the design you have?
Lawrence.
Here's the link,
https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CDEQFjAA&url=http%3A%2F%2Fwww.fatcat-photography.co.uk%2Fworkshop%2Fdabaerial%2Findex.htm&ei=-UN5UZ-RDoilO6-VgIAF&usg=AFQjCNEwYk-ClwWdsJoH3SjFrRX2DBIB3A&sig2=O2TwpsCbbNcib2R-5gK-mw&bvm=bv.45645796,d.ZWU I got inturupted earlier.
I've also found it in other places but this was the clearest.
I don't suppose it matters what sort of signal it is, DAB or analogue, so long as it works at around 200Mc/s.
- Joe
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.
Is there any balun function in that aerial?
The reason I ask is that I'm getting conflicting ideas (like yer do) when doing a web search eg: "The coil is a choke balun" and is doing both...."It's choke therfore it can't be a balun"
Lawrence.
(25-04-2013, 08:18 PM)pwdrive Wrote: [ -> ]Is there any balun function in that aerial?
The reason I ask is that I'm getting conflicting ideas (like yer do) when doing a web search eg: "The coil is a choke balun" and is doing both...."It's choke therfore it can't be a balun"
Lawrence.
As you probably know Lawrence, a dipole is a balanced antenna, but the antenna connection on the radio is unbalanced, in that one side is connected to earth. The role of a balun (Bal-Un - balanced to unbalanced) is to match the antenna to the radio, much more important in a transmitter to ensure that the antenna - not the feeder, radiates the RF power. Coiled coax 'choke baluns' as they've come to be known feature in many antenna systems.
This might be of interest:
http://www.hamuniverse.com/balun.html
Receiving antennas don't need quite the same careful considerations that transmitting antennas do, which is why many modern solid state amateur radio transceivers progressively reduce the output power if connected to a mismatched antenna and often have a built-in automatic antenna tuning unit to try to make as good a match as can be attained between the antenna and transmitted output.
For VHF or DAB, the most important aspect is height, given that VHF/UHF is line of sight, so the higher up the antenna, the better the signal received will be. True, in strong signal strength areas a set-top TV aerial will be fine. However, to get all Freeview TV channels without any pixilation we have a proper Labgear digital TV antenna on a 3M pole on the chimney, with a mast-head amplifier and low loss feeder. (I've only got one DAB radio - a 'Pure Evoke' which works well enough on its telescopic antenna). Don't want to spark a debate about the merits or otherwise of DAB/FM, but DAB sounds fine to my ears).
Waffled and dribble Yorkie, back home readjusting after a week in London with three granddaughters, a dog, and trying to get to sleep under the flight path of Heathrow. The silence is very spooky - no planes, no police/ambulance sirens, no dogs, no kids practising the flute, saxophone and piano, no traffic noise. I just hope I've not gone to heaven!
Ever since the analogue TV switch-off I've had a FreeView box in use purely to listen to 6 Music on.
I know the TV aerial has blown a bit off its correct alignment and at the w/end the reception seemed to go off a bit and today I couldn't get a thing on it - I suspect it's down to a tree coming into leaf.
Sadly I don't do rooves so can't sort it out myself and am loath to pay goodness knows what to somebody else to do it.
DAB reception also seems to be leaving something to be desired, though given how bad VHF is I'm surprised it's as good as it is.
I'll see what I can rig-up in the loft tomorrow, I can even stick the receiver up there it it comes to it. I'll also see if I can bodge an aerial socket onto it. The design under discussion does seem better than anything else I've tried. Sadly I have no way of objectively measuring it.
- Joe