[time-nuts] GPS Antenna Feed Line Decision
jimlux at earthlink.net
Mon Sep 4 16:53:54 EDT 2017
On 9/4/17 1:18 PM, Wes wrote:
> 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.
Be careful, that's the "catalog spec" which means "what we can
It's like SMA connectors, which are specified at 1.05:1 or 1.1:1 and
In reality, they are a LOT better, it's just that measuring that in a
production environment is tough.
I'd not want to set up a manufacturing test set that measured loss with
an uncertainty of 0.01 dB.
I'll point folks to:
Jesch's paper in 1976
and then Maury
Someone at Maury did a paper which I can't find right now where they
measured a bunch of SMA connectors over hundreds if not thousands of
RF cafe has a nice summary
> 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"
But you could probably send an email to the author and ask. NIST, like
JPL, is one of those places where people work forever. Tom Otoshi, who
wrote a report on N connectors in 1963 cited by Maury, above, still
works at JPL (I think.. I confess I haven't seen him recently, he might
have retired, but he was certainly around in the last 10 years), and
given the span of years, that N connector work was probably when he was
a just out of school engineer.
> 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:
> https://www.belden.com/docs/upload/hdcarltp.pdf and
> 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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