timing antennas

[Chris Caudle]

On Sun, August 5, 2018 10:55 pm, Paul Theodoropoulos wrote:
> On ebay, and on Amazon, there are a few instances of 'timing antennas'

The biggest distinction of a timing antenna is that it is made for fixed
mounting, and typically has a pointed cover so that snow or leaves will
slide off, and birds will not stand or roost on the antenna.

> for sale. Some appear to be specifically for cell sites, and some are
> advertised as up to 40dB gain

Higher gain may be needed for a long cable run (e.g. the GPS antenna is
mounted on top of a tall antenna tower, or at the top of a building
feeding a receiver at the bottom of a building).
Also many of the older timing receivers had very little gain in the GPS
receiver itself compared to modern designs.
You need to match the antenna amplifier gain to the gain required by your
particular receiver.  If you try to use a 40dB antenna with a modern
receiver you would likely need an attenuator in the cable path to keep
from overdriving the front end of a modern receiver.
If you look at the manual for your receiver it should indicate how much
gain is required and/or optimal.  If you have a long cable run then look
up the cable attenuation at 1500MHz and add that to the optimal signal
gain at the receiver to figure out the best antenna amplifier gain for
your use.

>unlike the little puck antennas which
> tend to be in the 20-28 dB range.

The little puck antennas are made for use  with modern receivers, and
typically within 1m or at most a few meters of the receiver.  I have used
a puck antenna with an older Trimble timing receiver and 10m of RG6 cable
with no problem at all.

> Here's a current example - http://r.ebay.com/2QlAm2

You did not say which receiver you have, but just based on average
requirements that is unlikely to be a good fit for what you need.  You
could have a particularly low sensitivity receiver, or maybe you want to
run a cable for 50m or more from the antenna to your receiver, but you
would definitely be an outlier if that is the case.  In the more typical
case that you have no more than about 10m of cable from antenna to
receiver, and your receiver was made sometime in the last 20 years, an
antenna with 20dB to 25dB gain will be what you need.

I am using one of these Symmetricom 58532A antennas with good results:
https://www.ebay.com/itm/Symmetricom-58532A-GPS-L1-Reference-Antenna-/262251051626

> that yes, a more sensitive antenna means better signal and potentially
> more satellites usable at any given time.

Good line of sight is usually much more important than antenna gain.  Get
the antenna high enough that there are not a lot of buildings, poles,
trees, etc. blocking the view from the antenna to the sky.  Especially the
southern view if you are in the northern hemisphere, or the northern view
if you are in the southern hemisphere.  The GPS orbits do not cover the
poles, so depending on your latitude you may be able to ignore blockages
between you and the closest pole.  If you are relatively nearer the
equator then having a view of the entire sky can provide an advantage.

-- 
Chris Caudle