[time-nuts] More antenna stuff
randy at synergy-gps.com
Fri Oct 7 11:37:28 EDT 2005
I thought I would throw in my two cents on the two antennas I know well: the Timing2000 and the VIC-100.
Both have proven to be excellent antennas, but there are differences in their basic specifications.
1. The Timing2000 has a two-stage ceramic filter, while the VIC-100 (HP58532A) has a three stage filter. While I agree
that the filters add some delay, this really shouldn't be too bothersome since they operate in a pretty narrow band of frequencies.
2. The Timing2000 filtering is 40dB at +/-50MHz from L1, while the VIC-100 (3-stage) is 60dB at +/-50MHz. Basically, all of the telecom
companies stick with the VIC-100 with the extra filter because of co-location RFI problems with cell towers, etc., but for most other uses the
Timing2000 works just fine and is half the price.
3. Another thing in the VIC's favor is that it includes tranzorbs on the supply line which help protect things during a near miss lightning strike. You might suffer
a fried antenna, but hopefully the rest of your installation will be unaffected. Fried antennas smell REALLY bad......
4. Probably the last consideration is gain. The Timing2000 is rated at +25dB, while the VIC is rated at +30dB minimum and usually produces +35-40dB.
They had to spec it at +30dB as an ABSOLUTE MINIMUM to keep HP happy, but I have never seen one below +35dB. What this normally means that an installation
that would require LMR-195 coax (basically an RG-58 equivalent with better microwave properties) with a VIC-100 might require LMR-240 coax (a microwave version of RG-8X) with a Timing2000. It is just about always cheaper to bump up the coax size rather than add an inline amplifier to the antenna system.
For those old timers out there, the old Motorola "Hockey Puck" is an excellent antenna for timing if you can find them. Hard as it is to believe, these
antennas were basically hand built by a ceramics facility on the East coast that Motorola used to own, and carried a ridiculous price tag. If you have an old dead one
it's a lot of fun to remove the radome (a hammer works well) and see how many discrete components went into a GPS antenna back in 1995....... How far we have come.
Senior Applications Engineer
Synergy Systems, LLC
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Doug Hogarth
Sent: Thursday, October 06, 2005 11:09 PM
To: 'Discussion of precise time and frequency measurement'
Subject: [time-nuts] Precision GPS timing
I wanted to comment on a few items in the most recent digest mail.
Regarding GPS antennas, of course there can be multipath issues which are
typically reduced with choke-ring or large groundplane, and site-dependant
multipath issues can still occur. Of course there are many other issues
such as elevation mask and accuracy of coordinates assigned to the antenna
if your receiver is operating in some 0D/timing mode.
Regarding antenna filters, I'd trust whatever Dr Clark says but my simple
understanding is that filters can add delay which may not be flat across
frequencies and maybe have tempco issues.
Regarding single- vs dual-frequency, the first enhancement typically
achieved by dual-frequency is removing the daily ionosphere signal (rather
than just modeling it which might only achive around 50% success). However,
dual-frequency receivers often have some other feature such as
anti-spoofable P-code for authorized government users and/or
geodetic-quality carrier phase (where the receiver is driven by the
frequency from your own clock).
That carrier-phase method involves fancy post-processing of multiple
stations and results in a very precise difference between your clock and
some reference clock (typically an H-maser or better timescale) at the
processing epoch (typically every five minutes or so). The most specialized
receivers can also be calibrated and get an initial time sync so that you
are comparing time rather than just frequency - also you don't need to be
using an external Time Interval Counter. To be clear, those receivers are
typically >$10K and often use ~$5K antenna, and have some PC constantly
logging data to RINEX files (for post-processing).
If you can view or print a .ps file, an example of such dual-frequency
carrier-phase method measuring my 5071A high-performance cesium clock near
the end of August can be found at the location below (note that the chart
excludes a phase offset which was almost 50ns). The chart is dominated by
my cesium's typical wandering around +/- couple nanoseconds; the method
itself is definitely sub-nanosecond. In other words chart would look 10x
better if I had an H-maser (or could somehow steer my clock to reduce the
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