[time-nuts] RG 6 U couplings

[Bob Camp]

Hi

Most of these antennas have 25 to 35 db of gain in their preamps. The
noise figure of most receive modules is in the ~ 1 db range. You do not
need much net gain to keep the system running. Anything over about 
8 db or so is “gravy”. If you have one of the 35 db antennas, you can have
a *lot* of coax loss. 

Indeed there *are* receivers out there that have much worse noise figures
on the front end. One quick hint that you have one of these is the 50 db gain 
antenna that comes with it. 

The bigger issue is not “what works today” but what works a few years down
the road. None of these parts get better with age. Each of them have  
degradation mechanism. Having some surplus gain at the start is a rational way
to design a system that will work for a while. 

Bob

> On Dec 6, 2015, at 10:07 AM, Rob Sherwood. <rob at nc0b.com> wrote:
> 
> I initially had 50 feet of RG-58/U going to my GPS antenna.  The loss was likely around 12 dB.  That cable eventually failed, and all I had was a 100 foot length of the same type of cable, and surprisingly the GPSDO still worked ok.  Recently I purchased a 35 foot length of 3/8th inch hardline with factory-installed N connectors on eBay for $40.  The loss should be less then 2 dB.  Obviously it works, but I cannot tell that the 10 dB reduced loss from the original cable made any difference.  Rob, NC0B
> 
> Sent from my iPad
> 
>> On Dec 6, 2015, at 7:00 AM, "David J Taylor" <david-taylor at blueyonder.co.uk> wrote:
>> 
>> 
>> 
>> Bert,
>> 
>> Extending the RG-6U using N-connectors should not be giving you more
>> than 0.20 dB, probably less. Considering that you have have 6.12 dB per
>> 100 feet in RG-6U at 1 GHz (should be about 8 dB @ 1.575 GHz), so it
>> will be more. Using N-connectors to extend the cable-stretch isn't going
>> to be a major issue, it may be the cable length that could present an issue.
>> []
>> Cheers,
>> Magnus
>> =============================
>> 
>> From: Bert Kehren via time-nuts
>> 
>> Thank you all for the information, worse case I look at 70 feet with two
>> couplings all in an inaccessible attic. The cable is quality but vintage
>> 1989.  Part of an extensive cable TV wiring. So far nine with two I am not able
>> to  locate the other end.The way they are installed I am not able yet to use
>> one of  them to pull a new one in.
>> Twenty years ago I probably would have risked crawling in there. Don't want
>> to fall 16 feet at age 73.
>> Bert Kehren Palm City .
>> ============================
>> 
>> Magnus,
>> 
>> RG-6U is 75-ohm cable and uses type F connectors (horrible!).  Any F-N adapters and the impedance mismatch would surely introduce more loss than a simple Female-Female F connector.
>> 
>> Bert,
>> 
>> If the cable is that old, 1989, I would consider replacing it with a more recently purchased cable, which will not have deteriorated, and may have lower loss.  I appreciate that may not be possible in your case.
>> 
>> Cheers,
>> David
>> -- 
>> SatSignal Software - Quality software written to your requirements
>> Web: http://www.satsignal.eu
>> Email: david-taylor at blueyonder.co.uk
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