Fractal antennae

[Capitol]

Does anyone have any experience of these? The plots I've seen seem to have very variable gain/frequency responses.

[Martin Brown]

The devil is in the detail. Mostly it is all buzzword induced hype.

Unless you transmit is astonishing what you can get away with. (RF power amps take exception to mismatched antennae)

[tabbypurr]

check in the mirror that your antennae are matched

NT :)

[Brian Gaff]

Well its not a black art, all aerials still obey the same rules, its just the compromises made in the design. I recall these were used in millimetric radar systems and the main reason was that they needed to be able to steer them electronically. Not tried the scaled up versions, I have to say. Brian

[Martin Brown]

Although they obey the same rules the complex shape of a finite depth fractal does produce interesting resonance characteristics that may sometimes be useful - but there is a lot trendy snake oil being sold.

Some old TV aerials were (unrecognised at the time) fractal array designs. The Wiki article isn't bad for the OP as an intro.

Some of the deeper theory of serious fractal antenna designs has only been relatively recently understood (around 2000). Some very odd novel metamaterials are based on this sort of approach too.

My money would be on log periodic though for practicality.

[Capitol]

I'm not convinced that fractals are understood. Log periodic is a form of fractal. The problem as I see it is that there doesn't seem to be a calculating procedure for fractals which can optimise a design. You seem to have to pick a geometry, then calculate a possible set of results. Testing can be quite close to the calculation, but gain figures are never posted. Fractals seem to suffer from lots of notches in the response curves as a result of repeating one fixed shape. I was hoping someone here had some hands on experience of performance.

[jrwalliker]

I tried designing one a few years ago when I needed a wide bandwidth. In the end the results were disappointing and I gave up and used a more "conventional" structure where it is more obvious how any particular adjustment will influence the characteristics.

I used EM simulation followed by testing prototypes with a VNA.

One of the biggest problems is that an antenna design that needs to fit in an enclosure with nearby circuitry is difficult to simulate accurately and therefore requires trimming. If you can't predict what effect a particular adjustment will have, the process becomes painful.

Obtaining a good return loss for transmitter matching is easy if you use the procedure favoured by some manufacturers: just use several metres of lossy coax between the antenna and the analyzer.

John

[tony sayer]

Tried a "NEC" program at all?...

[tony sayer]

In article , Capitol scribeth thus

Whatever are you up to with that sort of thing?..

[Martin Brown]

A difficult to manufacture DIY aerial?

[Capitol]

I am currently building a pigeon proof TV aerial, a double biquad. I can mount this under the eaves so the bloody pigeons can't perch on it and fight. Our pigeons IMO resemble small turkeys which I blame on the deranged neighbour who feeds them. They destroyed the last new log periodic TV aerial in 3 months and next doors a few months later. I give next doors weedy looking replacement 6 months at the outside. I pigeon proofed the original Yagi with spikes, this worked within limits until they destroyed the feeder cable at Xmas. They also crap all over the roof causing more moss growth and the patio, which she complains about. Preliminary tests on the double biquad are promising. I haven't attempted to measure the gain bandwidth yet but may get round to it this week in a very crude manner. I came across fractal antennae so investigated the theory and discovered there really wasn't a predictive one. As this group has a wide range if interests, I thought I'd ask here to see if there was any hands on experience. NEC programs require that you start with an empirical geometry and then you can calculate the performance. On fractals the geometric limits are infinite, so you end up doing the calculations for ever. Aerials always end up as cut and try IME, it's just like trying to lay out frequency synthesisers in UHF radios.

[Martin Brown]

Unless you are a big fan of Dave why not install FreeSat? I doubt if pigeons can do much harm to a dish beyond adding "dielectric coating".

How well it works depends on local signal levels. Where I live a foot of random wire will do almost as well as a real Yagi antenna.

[Capitol]

Doesn't really suit >8 TV receiver locations & 6 recording locations.

[tony sayer]

Umm .. wouldn't a decent aerial in the loft achieve what you want?.

Or perhaps an air gun;-)...

[tony sayer]

Well using a "multiswitch" does that...

[Capitol]

Can't see how that helps.

[tony sayer]

Right U use a certain type of LNB which has four outputs. IIRC its called a Quad or Quattro. Each one of the outputs which receives signals is on either the upper or lower part of the band and vertical or horizontal polarisation.

Each outlet then can take whichever one of the four inputs it needs and by such means a lot of receivers etc can get a feed..

[Capitol]

So, you are limited to 4 channels at a time? Incidentally, loft aerials don't work very well in this location, the additional problems of a low pitched concrete roof and rain/snow etc rather rules one out.

[Martin Brown]

It does require a satellite tuner on the end of each though.

It is hard to get a dish these days with less than two LNB outputs. Mine has four of which only one is actually used. Splitters do exist. No idea if they are any good since I only have one satellite tuner.

[Martin Brown]

BTW you might want to pose this question in s.e.design where there are some folk who do RF & UHF stuff. Although specify the centre frequency and bandwidth you want to obtain if you don't want to be flamed.

They are more than a little intolerant of ill specified problems.