[time-nuts] Aerial coax downlead placement

[Peter Vince]

Hello all,

     A new contact, whose background is in computer programming rather than
RF, is getting into accurate GPS positioning, and has been tapping me for
any knowledge I might have.  I persuaded him to get the new Ublox F9P
receiver and also a "proper" dual-band antenna - albeit from China, so
affordable!  This seems to be going quite well, and he sent a photo today.
He has mounted said aerial on what looks like a six-foot pole which is
good, but currently the coax down-lead is just hanging - flopping about in
the breeze.  Now that is bad for so many reasons :-)  However, I am not
sure of the best advice to give him.  I'm sure I heard that you should
never drop the coax down the middle of your support-pole, as the conducting
pole will mess up the characteristics of the cable by affecting the
currents in the outer braid.  And so similarly, you presumably don't really
want to tie-wrap it tightly to the outside of the pole for pretty much the
same reason.  So my gut instinct would be to put some (half-inch??)
insulating spacers between the coax and the pole where the tie-wraps go.
Now when I worked in a TV shop in my youth, they didn't bother with any of
that - a) they probably didn't know about any such effects, b) they
certainly couldn't have been bothered, and c) TV signals are strong, so it
wouldn't have made a significant difference.  GPS signals, however, are NOT
strong, and need to be treated with much more respect - especially if you
are after some seriously good positional accuracy.  Is my idea right, or am
I barking up completely the wrong tree?

     Thanks in advance,

          Peter  (London, England)