[time-nuts] GPS Antenna Cable Splicing

[John Franke]

First; how long does the total length need to be?  With lower loss cables, 
10m is no problem.

Second; if you go to the trouble of adding a length of cable, it would be 
best if you replace as much of the 5m cable as possible with lower loss 
cable rather than just adding to the 5m cable.

John  WA4WDL

----- Original Message ----- 
From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Saturday, March 01, 2008 8:25 AM
Subject: Re: [time-nuts] GPS Antenna Cable Splicing


> Matthew Smith wrote:
>> Hi Folks
>>
>> I'm not so hot on RF stuff (actually not so hot on anything above
>> 50/60Hz - only studied power stuff) so thought I should ask:
>>
>> I have a few GPS antennae here, all with 5m (15ft-ish) cables on them.
>> For me, that won't get the antenna anywhere near the highest ridgeline
>> of the house.  (Australian "Federation Style" house - lots of weird bits
>> of roof all over the place.)  So, I need to extend the cable.
>>
>> 1) Can I actually extend the cables, or is 5m as far as I can go - all
>> these are active antennae.
>>
>> 2) My normal way of extending a bit of coax would be to splice in an
>> appropriate length of similar cable.  To do this, I would strip the
>> outer insulation for a couple of inches on both pieces, pulling back the
>> braid.  I would then strip enough of the central insulation so that I
>> can solder the two cores together.  I insulate this with tape or
>> heatshrink, then solder the two braids together, wrap the whole thing
>> with aluminium foil and use a glued heatshrink to case the whole lot.
>>
>> I know that this method works for 10-Base2 Ethernet and UHF Television,
>>  but have no idea if is suitable for SHF stuff like 1.5GHz...
>>
>> 3) How far apart do I need to space my GPS antennae, or can they be
>> nearly touching?  I suppose it would be easier if I used a distribution
>> amp (assuming such things exist at this frequency), but that sounds
>> expensive.
>>
>> Cheers
>>
>> M
>>
>>
> Matthew
>
> GPS distribution amplifiers are readily available, they appear from time
> to time on ebay.
> Symmetricom 58537 4 port active smart splitters can be quite useful.
> HP/Agilent/Symmetricom 58537A, 58536A are also useful.
>
> The small diameter captive cable used by some GPS antennas tends to be
> somewhat lossy and it may be better to extend the length by using a
> lower loss cable.
> Your splicing technique will create a significant discontinuity in the
> cable impedance at 1.57542 GHz.
> The maximum cable length is determined by the maximum allowable cable
> attenuation between the antenna and the GPS receiver.
> A high gain active antenna allows greater cable loss than a low gain
> active antenna or a passive antenna.
>
> The better active antennas use a TNC, N or similar connector so you can
> use a custom length cable.
>
> Bruce
>
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