[time-nuts] Cable delay correction for Tbolt Cs substitude
John Ackermann N8UR
jra at febo.com
Sun Oct 16 14:43:21 EDT 2011
I should have noted that the Haystack theoretical paper does indicate that SWR could increase the tempco, so 50 vs. 75 ohm might have some effect -- but that would be relative to the load (i.e., RX) end so if the Tbolt is designed for 75 ohm input (and its front end is actually optimized for return loss, which I wouldn't bet on) you'd actually be better off with 75 ohm cable. But in either case, I think the effect is still well into the noise.
On Oct 16, 2011, at 2:34 PM, John Ackermann N8UR <jra at febo.com> wrote:
> 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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