[time-nuts] Non-impedance matched antenna cables
paul at greenrover.demon.co.uk
paul at greenrover.demon.co.uk
Sat Jun 14 10:18:46 UTC 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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