[time-nuts] GPS Antenna Feed Line Decision

Attila Kinali attila at kinali.ch
Sun Sep 3 08:42:02 EDT 2017


On Sat, 2 Sep 2017 18:48:52 -0500
Clay Autery <cautery at montac.com> wrote:


> Not sure why you and the other guy both recommended RG-6 75-Ohm cable
> and F-connectors, when the nominal impedance of literally everything
> else in the system is 50 Ohm, including the antenna and the HP GPS
> Distribution Amp....  And then adding N to F adapters?

Because it doesn't matter in a receive only application.
The transmission coefficient for a 75R<->50R step is 0.8, or -1.9dB.
In return, you can get a much better cable for half the price with
less damping. Not to talk about all the other bits and pieces that
are readily available for 75R F connectors. Like lightning arrestors
and grounding blocks. And please, include at least a lightning arrestor
at the point where the cable enters the building!


> Doesn't make any sense unless one has $$ as a top priority, already has
> a spool of RG-6 quad shield, etc...  but I specifically stated that $$
> is not a top priority....  Not really even in the top 5 or 10...


If you have more than enough money, then by all means go for the 50R
solution :-)
 
> Strain relief solution:  Really depends upon which cable stock I end up
> using....  Obviously, the larger the cable, the more weight will be
> suspended under the antenna (approx. 38 feet, depending on where I pull
> the cable out of the mast at the bottom.  (The mast is on a tilt-base of
> my own construction....  which brings up a valid consideration.  it is
> already a chore to walk the mast up manually.  The more weight added at
> the top and inside the mast, the more difficult a manual walk-up will
> be.  I'd prefer to keep this a manual tilt for now...  at least until I
> add the tri-band vertical dipole)
> 
> LMR-400 would be less than 4 lbs total weight...  I'd likely suspend the
> cable by the connector alone. (although, it wouldn't be JUST the
> crimp...  The connector would have at a minimum, 1 layer of
> adhesive-lined shrink tube... probably 2, with the second, overlapping. 
> Then a nice wrap of self-fusing tape and then electrical tape over that.
> Even LMR-600 could be suspended by the connector alone, at 5 lbs max in
> the 38 foot max vertical section.
> I do have several methods of secondary suspension within the top (and/or
> second) mast section though if I decide I need it.

You do not want to have 2kg of weight on a RF connector.
Even if it will hold now, over time material creep will lead to
intermediate faults. Better use a dedicated strain relieve. This
can be something as simple as a wire wound around the cable that
holds the weight.

 
			Attila Kinali

-- 
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor


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