[time-nuts] Best replacement for Trimble Bullet antenna

[Bob Camp]

Hi

Regardless of material the gotcha is that the plastic has a dielectric constant. That turns it into a lens and moves the phase center. 
If the phase center moves with sat angle, you get a different path length based on angle. That messes up your solution, even
for timing. Do you care about the amount it messes things up? This is Time Nuts …. There are indeed documents out there that
discuss the problems people have had putting radomes over GPS antennas. 

Bob

> On Nov 23, 2016, at 10:25 AM, Nick Sayer <nsayer at kfu.com> wrote:
> 
> At work I have access to an industrial one that (I am told) uses lasers to harden a liquid... something. I'm not a plastic expert. We use that stuff for outdoor enclosures and it is UHF transparent, at least to some degree (it would have to be given our use case). I used it once to make a couple of wall mount J1772 fake inlets to hold our car charging plugs. Those have survived outdoors without any issues. At some point in need to repaint them, as the native color is battleship grey, and the white paint is starting to wear off.
> 
> If Tupperware made something conical, I'd chose that in a hurry. It may not have a great deal of UV staying power, but "microwave safe" implies good RF performance. I've used that solution to quickly weatherproof home brew stuff before. 
> 
> Even if whatever the cap is made of isn't perfectly RF clear, I've certainly got S/N margin for some attenuation, even if it isn't uniform. I would posit that as long as it's performance doesn't alter over time scales shorter than a few minutes that the impact would be immeasurable - certainly compared to the ionosphere's.
> 
> I probably won't bother, though, as we don't seem to have bird trouble and it doesn't snow here in Silicon Valley. If it were an unattended installation, that might change things. 
> 
> 
> Sent from my iPhone
> 
>> On Nov 23, 2016, at 5:07 AM, Bob Camp <kb8tq at n1k.org> wrote:
>> 
>> Hi
>> 
>> Ummm …. errrr …. radio transparent plastic. You want to avoid making a 
>> lens that distorts the paths over the antenna so it’s not just an issue of transparent. 
>> You also want it to be “clear” (non distorting). You need it to be stable in terms of UV,
>> humidity and temperature. The guys who made the antenna didn’t have it quite so tough, 
>> they could design the cover as part of what determined the pattern of the antenna.  
>> 
>> The short list of what your low cost printer will handle:
>> 
>> PLA
>> ABS
>> PETG
>> 
>> If it is really low cost, it will only handle the first one. All are a bit of a 
>> disaster at 1.5 GHz. PLA likely will not hold up outdoors, let alone do well at RF.
>> The other two are poor at RF. 
>> 
>> The longer list on a higher priced printer:
>> 
>> Nylon
>> Polycarbonate
>> HIPS
>> Flexable’s 
>> 
>> The last two are out right from the start. One dissolves in water, the other has no structural integrity. Nylon is hydroscopic 
>> and not all that great at 1.5 GHz. For what ever reason the polycarbonate that they sell also is a bit hydroscopic. It also is 
>> quite challenging to print unless you have a very fancy printer. 
>> 
>> Certainly not an easy thing to do.
>> 
>> Bob
>> 
>>> On Nov 23, 2016, at 1:05 AM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
>>> 
>>> 
>>>> On Nov 22, 2016, at 9:39 PM, Dennis Lloyd <dlloyd at musi-tronics.com> wrote:
>>>> 
>>>> Your problem will be Birds and Debris, it is flat and you will have interruptions and phase shifts.
>>>> 
>>>> Did you not understand why we use pointed antennas for timing and fixed installations.
>>> 
>>> No, I guess I didn’t.
>>> 
>>> That said, our cats seem to be doing a good job discouraging any birds, and I can see the antenna mornings when I go out to the car and there’s been no debris problem I’ve been able to detect.
>>> 
>>> If it were required, I’d suspect it would be fairly easy to 3D print a pointy hat from radio-transparent plastic.
>>> 
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>> 
>