[time-nuts] Cable delay correction for Tbolt Cs substitude
John Ackermann N8UR
jra at febo.com
Sun Oct 16 18:34:22 UTC 2011
I wouldn't think the cable type will make an order-of-magnitude difference. Referenced in the Haystack note is another paper that goes through the theoretical derivation that produced the "expected results" column. I think it's the same URL but 067.pdf as the file name.
On Oct 16, 2011, at 2:21 PM, "WarrenS" <warrensjmail-one at yahoo.com> wrote:
> Thanks John
> Any chance using 75 ohm cable (as suggested in the Tbolt manual) like RG6U, when used in a 50 Ohm system could be orders of magnitude worse than LMR-400?
> Sounds like may be time to do some controlled cable experiments comparing different cables.
> I do know that Cheapie GPS timing antenna's can have a large Phase variation when the Sun hits them.
> I had one antenna that changed 25 ns every day around Noon time.
> That is when I changed over to a Symmetricom 58532A antenna and things improved 10 fold.
> With the new antenna the phase error change is now down at least near the GPS noise level,
> but it seems to still have some "antenna system" temperature effects.
> Maybe a silly question but how can a 1.5GHz preamp and filter change the phase over so many cycles?
> Does anyone ever add a temperature controller on the antenna? Maybe that should be my next test.
> from John Ackermann N8UR
> I did some very rough measurements last summer with. Run of LMR-400 that was laying on the roof in the hot Georgia sun.
> Using a network analyzer to "ping" the cable I found the day vs. night delay difference was pretty much in the noise. I'll see if I can find the details and if so will post them.
> I found via google a brief paper from Haystack that measured LMR-400 and LMR-240 and found in the range of -11 to +17 ppm/K of the total cable delay.
> They note that 9 ppm/K is about 3ps/degree in 100M of cable:
> However, there's another possible tempo contributor that I suspect could be a significant contributor, and that's the preamp up in the antenna, particularly if it has a bandpass filter. It wouldn't surprise me at all if preamp/BPF tempo was noticeable.
> On Oct 16, 2011, at 1:32 PM, "WarrenS" wrote:
>> Anyone know what the propagation delay temperature coefficient is for RG6U coax and how much it varies between different brands of cable?
>> In my efforts to improve the Tbolt's performance to make it into a better Cs substitute,
>> test suggest that the temperature coefficient of the antenna lead-in cable's propagation delay is contributing to diurnal errors.
>> Anyone have a idea for a SIMPLE & cheap voltage controlled delay line that can be changed by a few ns as a function of the outside air temperature?
>> As an alternative, Mark, want to consider adding another LadyHeather feature that tweaks the Tbolt's cable delay value as a function of the outside temperature?
>> If interested, I have a couple ideas of how to get the outside temperature to LadyHeather.
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