[time-nuts] GPS Antenna Feed Line Decision
wes at triconet.org
Mon Sep 4 16:18:43 EDT 2017
Not being as nutty as many on this list I only skimmed the papers you provided.
Interesting and I will stand corrected regarding the use of 75 ohm cable in 50
ohm systems in critical situations. In situations like my own, I'm not going to
fuss about it.
I realize this is NIST, the 1384 paper was probably peer-reviewed, and I
shouldn't question, nevertheless fools rush in.
In the block diagram of Figure 3 and the accompanying text, is described
installing attenuators to increase return loss (RL) to what is claimed to be 40
dB. Forty dB RL is in the realm of precision calibration standards.
(https://www.maurymw.com/Precision/Precision_Fixed_Terminations.php) and it's
not trivial to measure on a single device. Here we have a cascaded of two
attenuators, two bias tees and a length of cable.
If these are COTS attenuators, their own return loss is unlikely to be 40 dB.
In fact grabbing an old HP catalog off my bookshelf (I'm dating myself) I see a
typical type N attenuator specified as 1.2 VSWR (~21 dB RL). I went on a quick
"shopping" trip looking for an L-band, type N bias tee. I'll spare you the
links, but typically they are also rated at 1.2 VSWR.
Perhaps NIST, with an unlimited supply of tax money, splurged and manufactured
bias tees with >40 dB RL. Maybe they did the same with the attenuators. We'll
never know because they didn't provide an equipment list or a measurement
procedure. They said nothing about the cable either, other than they started
with RG-58 and replaced it with "better" cable.
A few words about cable, since that is what this discussion is all about.
Cable, regardless of type and manufacturer, has its own RL, also known in that
business as Structural Return Loss (SRL) See:
At least the authors admit, "Thus far we have seen little difference in the data."
On 9/3/2017 3:02 PM, Bill Byrom wrote:
> For precision timing measurements, I would think that there would be
> concern about the double reflections of a badly mismatched low loss
> transmission line (such as using 75 ohm line in a 50 ohm environment).
> The re-reflected signal will act similar to multipath (as a delayed
> aggressor) on all satellite signals equally. The impedance mismatch
> delayed reflection aggressor could aggravate timing errors due to
> changes in temperature or stress in the cable. Whether this is important
> for you depends on how time-nutty you want to get.
> See these papers:
> Effects of Antenna Cables on GPS Timing Receivers:
> Absolute Calibration of a Geodetic Time Transfer System:
> Bill Byrom N5BB
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