[time-nuts] Non-impedance matched antenna cables

paul at greenrover.demon.co.uk paul at greenrover.demon.co.uk
Sat Jun 14 06:18:46 EDT 2008


Magnus Danielson wrote:

> 
> Pointer to good phase stable cable with suitable connectors would be
> apprechiated.
> 


I've had good success with Andrew FSJ-1-50 in gps applications. I prefer 
  LDF-4-50 for tower runs, not because of the improved loss performance 
but the mechanical handling characteristics.

In terms of connectors, well I'd expect to be running something like the 
above cables into the equipment room and terminating into lightning 
surge protection - so N-type connectors are a fair bet - IMD suffers if 
you make many connect/disconnect cycles but in most cases I'd not 
anticipate any issues. As for tailing down to the GPS receiver using a 
none 50r connector (concerns about mismatch) I suspect the discontinuity 
at the input caused by "bodging" a nominally F-type connector onto a 50R 
ptfe cable will be no worse than the bump caused by the transition from 
cable to PCB in the receiver. remember the inner conductor of the cable 
is the actual transition to the connector so there are d/D dimension 
changes there anyway. I expect you could remove the connector and run a 
small  cable right to the receiver input if you felt the need AND you 
were sure the RX S11 really made it worth while. Evidence from another 
post here suggests it isn't.

> Oh. The electrical length change for many reasons. Physical length change is
> only part of it. The dielectrum change properties with temperature, and the
> changed wave-equation may change speed significantly larger than the length
> extention. I suspect humidity in cable to be part of it too.
> 

I agree. Two things I've experienced with flexible cables measured at 
install and subsequently as part of the ongoing maintenance regime:

First - even when very carefully installed, phase often changes slightly 
within a day or so of install. I suspect this is due to the cable being 
taken from the roll and placed into position. There is inevitable 
relative movement of the conductors as the dielectric changes shape. The 
evidence for this being that the *change* is more pronounced if the run 
has curves at the minimum bend radius for the cable. After the cable has 
gone through a few temperature cycles it settles and the cable only 
changes if the weatherproofing hasn't been done properly or someone 
subsequently damages the cable (rigger maybe stands on it or hits it 
with steelwork etc...)

Second - water ingression caused by pressure changes in the cable 
(caused by diurnal temperature cycling) is very likely even in dry 
environments. Condensation that forms on connectors and even on the 
waterproofing material itself can be drawn into joints as the pressure 
changes (hot cold) - each day a tiny bit of moisture ingresses until the 
cable fails, maybe it takes a week maybe a year but unless its properly 
waterproofed it will eventually fail. Connectors that have inherent 
waterproofing such as DIN 7:16 can be problematic if water is allowed to 
collect on the connector by incorrectly adding a tape cover. Shrouded 
link yagi antenna can literally fill up with litres of water even though 
they are carefully sealed with rubber gaskets.

Sorry I'm rambling on...

Regards Paul





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-- 
73 de Paul GW8IZR IO73TI
http://www.gw8izr.com



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