[time-nuts] Re: 10 MHz Distribution Best Practices?

[Bob Camp]

HI

Ok, so 80’ is the max distance. That is well worth knowing. 

You still are past a half wave at 10 MHz / 30M. You can’t get around unterminated
cables acting as antennas at that sort of distance. If you can distribute the amplifiers
here and there, you *might* be able to reduce that a bit. Three amps spaced < 30’
apart (with good coax in between) would be a start. 

How many “endpoints” are there? 

If you have.a distribution amp that handles 8 to 12 loads, that factor into the “spread
them out” equation. 

Yes, this assumes that the good old Spectracom setup with “taps” on a common coax
is not the answer. They typically have a floor around 1x10^-11 or so. Yes, I *did* once
have a conversation that ended with “that sounds fine to me” about that fact. Times
change and requirements evolve. We’re talking about a 1970’s system …..

The same basic approach would work with fiber. Design a tap and then feed the device
from it. AFIK there are none of those on the market at this point. Maybe someday.

Bob

> On Mar 25, 2023, at 3:29 PM, David Bengtson via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> I only have 60' to 80' of separation, so not too far. WIth good cable,
> the cable losses are manageable. F/O cabling would be helpful to avoid
> ground loops but would add a fair amount of complexity
> 
> 
> Dave
> 
> On Sat, Mar 25, 2023 at 5:01 AM Dana Whitlow via time-nuts
> <time-nuts at lists.febo.com> wrote:
>> 
>> What's wrong with fiber optical distribution?
>> 
>> Dana
>> 
>> 
>> On Thu, Mar 23, 2023 at 9:54 PM John Miles via time-nuts <
>> time-nuts at lists.febo.com> wrote:
>> 
>>>> Has anyone run across any publications on best practices or examples
>>>> of 10MHz Lab wide distribution networks? I'm looking for a discussion
>>>> on how to physically locate oscillators/distribution amplifiers, cable
>>>> types and runs, RFI mitigation etc. I haven't come across any, and I'm
>>>> starting to build one at work. We've got a Cs oscillator and I'd like
>>>> to make sure we deliver that performance across our systems lab.
>>> 
>>> A lot of things can be said in favor of low-noise distribution amps with
>>> good VSWR, good channel isolation, and good PSRR (not so fast, HP 5087A.)
>>> But over the years, I've learned that the three most important factors when
>>> it comes to piping 10 MHz signals around are:
>>> 
>>> 1) Shield resistance
>>> 2) Shield resistance
>>> 3) Shield resistance
>>> 
>>> I've spent a lot of time recommending double-shielded coax in the TimeLab
>>> manual and elsewhere, and I still stand by that advice, but what I've come
>>> to realize is that this is really just a proxy for low shield resistance.
>>> Good grades of single-shielded cable are basically as effective at HF as
>>> double-shielded cable.  To the extent your cable ground shield exhibits
>>> resistance, it's not a shield, it's a resistor.
>>> 
>>> Avoiding ground loops is on the list too, but further down.  Never lift a
>>> ground to avoid a ground loop.  Use coax-to-coax baluns only when you can
>>> see a beneficial effect.  Focus instead on providing a shared
>>> low-resistance
>>> common ground  to your entire network -- ideally not the ground all the way
>>> back at the service entrance -- and rely on low shield resistance on the RF
>>> side to do the rest.
>>> 
>>> Every installation is different and your mileage will most certainly vary,
>>> but this is my take on it.
>>> 
>>> -- john
>>> 
>>> 
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