[time-nuts] GPS Antenna Feed Line Decision

jimlux 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 
inexpensively measure"..

It's like SMA connectors, which are specified at 1.05:1 or 1.1:1 and 
<0.3dB loss.

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 
mate/demate cycles.

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"
> cable.

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
> http://www.keysight.com/upload/cmc_upload/All/E206COMPTEST_METHOD.pdf.
> At least the authors admit, "Thus far we have seen little difference in
> the data."
> Wes
> 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:
>> http://tf.boulder.nist.gov/general/pdf/1384.pdf
>> Absolute Calibration of a Geodetic Time Transfer System:
>> http://xenon.colorado.edu/paperIrevise2.pdf
>> --
>> Bill Byrom N5BB
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