[time-nuts] GPS backup for the stationary time and frequencyuser

Jim Lux jimlux at earthlink.net
Fri Oct 8 20:25:53 EDT 2010


48dBi is way way too big

18 I might go for

Look at aperture.  Say it's six square wavelengths(40x60 cm). A dipole is about 1/8th square wave lengths..so the gain is 48 times that of a dipole.  Say about 17dB +2dB or 19 dBi 

On Oct 8, 2010, at 3:05 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> On 10/08/2010 08:22 PM, jimlux wrote:
>> Brooke Clarke wrote:
>>> Hi Jim:
>>> 
>>> I've got a spare Ku band satellite dish and would like to use it for GPS.
>>> In an ideal application the GPS antenna would be mounted in the normal
>>> manner and above it would be a sub-reflector aimed at the Ku dish.
>>> That way the antenna might pickup sats near the horizon directly and
>>> from a narrow part of the sky by means of the dish.
>>> The dish might be aimed at a WAAS GPS sat.
>>> I've heard that you can just use the TV dish with a normal GPS
>>> antenna, and it gas gain even though the polarization is reversed.
>>> 
>>> 
>> 
>> Give it a shot. The other thing is that if you have your GPS antenna
>> facing straight up, at the focus of the dish, you're looking at the side
>> of the gps antenna, where the polarization might be less circular.
>> 
>> But one thing to think about here... a standard Ku dish isn't very big..
>> 
>> At GPS frequencies, you're looking at 20 cm wavelength. The dish is
>> perhaps 2, maybe 3 wavelengths across. That's not a huge amount of gain.
>> 
>> You might do just as well with a flat cookie sheet.
> 
> Well, a 1 m dish gives you 48 dB gain at L1 if I calculate correctly. The normal antenna is at 6 db of antenna gain?
> 
> Even if less than perfect, not too big size is needed to get meaningful antenna gain.
> 
> Cheers,
> Magnus
> 
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