[time-nuts] pin-wheel antenna

[Magnus Danielson]

Hi Dave,

On 04/21/2013 10:32 AM, David Kirkby wrote:
> On 20 April 2013 20:52, Tom Van Baak<tvb at leapsecond.com>  wrote:
>
>> For the rest of you:
>>
>> http://www.leapsecond.com/images/gps-pinwheel-1.jpg
>> http://www.leapsecond.com/images/gps-pinwheel-2.jpg
>>
>> It's a thing of mysterious beauty. And the GPS World photo saves me from the temptation to break open my own pinwheel antenna just to see what's hidden inside.
>>
>> /tvb
>
> Does the antenna work better than other types?

You should get close to choke-ring antenna performance at a much smaller 
size. For many purposes its good enough.

Check out:
http://www.novatel.com/products/gnss-antennas/high-performance-gnss-antennas/
http://www.novatel.com/assets/Documents/Papers/GPS-702L.pdf

As you look at this, realize that antenna gain does not really fit this 
description as you need to receive signal from about the full upper 
hemisphere of the antenna, the directivity comes in as the ability to 
surpresss the other hemisphere where the majority of multipath is 
expected to be.

In addtion, these antennas are by necessity active (you can get passive, 
but you need to have your own LNA very close by).

> As someone who used to
> design antennas for a living, I'm well aware there are a lot of
> antenna "designs" which are either badly understood by their
> "designers" or are just put together to look impressive. One company I
> used to work for based their specifications on that of their
> competitors. This seems to be pretty common practise in the antennae
> industry.

In the precission side of things, they regularly calibrate the antennas, 
it would show for that market.

> I once offered my boss at a company I worked for a bet. I would pay to
> get one of our antennas tested at NPL, and if it met the specification
> we claimed, then he owed me nothing. But if it failed, he had to pay
> the cost of the testing. I could possibly not win any money with this
> bet  - the best I could hope for was to break even, but I was
> sufficiently convinced it did not meet the specification that I could
> take that chance. Needless to say he would not accept the bet!!!
>
> There are lies, damn lies, and antenna specifications.
>
> This particular antenna design
>
> http://www.rason.org/Projects/collant/collant.htm
>
> has a claimed gain of 9 dB. We don't know if that is 9 dBi, dBd, or
> dB_wet_string. Both myself and someone else have modelled this based
> on the use of perfect conductors and we get a gain of about 7 dBi. I
> used the dimensions given in that article, him a rescaled version for
> 2.4 GHz.
>
> We used different software - me the exceedingly expensive HFSS 3D
> electromagnetic simulator and him another very expensive EM simulator.
> These both solve Maxwell's equations, although one uses  the integral
> form and the other the differential form. I've contacted the author
> and got no response.

How good is NEC2?

> One obviously way to convince yourself that antenna can't work as
> described, is what would happen if the coaxial cable had a very high
> permittivity lossless dielectric - say Er=10^6. All dimensions would
> scale down by 10^3 from free space, and you end up with a very high
> gain antenna 1.4 mm long.
>
> Hence I tend to take antenna specifications with a pinch of salt.

Well, I think any specification should be taken with a pinch of salt, 
but I think you should look at this particular application and see if 
they are really that insane. Your rant seems more relevant to Yagi and 
similar directed antennas.

Cheers,
Magnus