[time-nuts] Aerial coax downlead placement

[Dave B]

On 05/07/2019 06:01, Gerhard Hoffmann wrote:
>> 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 :-)
> I cannot think of a single good reason against putting the coax line
>
> into the pole. In fact, my rocket-shaped Datum antenna leaves me no
>
> other choice. Its plastic case has a ~ 1 1/4" thread that happily fits on a
>
> water pipe and the N connector is concentric to that; so the cable MUST
>
> go through the pole, at least initially. I think that is good also in 
> case of
>
> a thunderbolt hit; the tube will carry most of the current and lead it 
> to earth.
>
> That won't help the antenna, but maybe me in the lab.
>
> And it is the only true symmetrical solution.
>
> That rocket-shaped antenna spawns a lot of interest amongst my neighbors.
>
> :-) Gerhard
>
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Well, it shouldn't "mess up" the cable characteristics, unless the cable
is bad anyway, but...  All the weight of the free hanging cable inside
the pole will be supported by the connector on the antenna.

Plus as the pole moves in the wind (they all do) the cable can move
about inside, and if repeatedly impacting the inside of the pole over
time, that will damage the cable.

Best run it down the outside, clipped or tied at regular intervals to
the pole, but not so tight as to distort the cable.

In fact, if it's a long pole, you can wrap the cable around the pole as
a very wide spaced spiral, that will help prevent vortex shedding as air
passes over the pole, reducing wind powered oscillations.  The pole will
still bend a bit but should not sway about anyway near as much.

Have Fun.

Regards.

Dave B G0WBX.





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