The GPS antenna that is specified for use with ground-based Rapco master clocks and GPS references is a fixed-pattern type which has a built-in preamplifier, powered via the co-axial RF downlead cable from a low-voltage d.c. source (< 5 volts) that is built into the unit.
The signal connector on the antenna head assembly, although sealed at its mounting face, is not a weatherproof connector. In order to provide weather protection for the interface between this connector and the antenna downlead, and to avoid mechanical strain on the connector when the downlead is connected, a waterproof housing with an entry conduit is provided.
Rapco Antenna-downleads, both the standard 20 metre version (E43) and the heavier 50 metre type (E44), are ready-fitted with a screw-on cable gland which allows the assembly to be weatherproofed at the point of cable exit. If longer cable lengths are required then consult Rapco for details.
In extreme environments the user should provide further protection for the co-axial cable itself from its point of exit on the antenna assembly.
Information regarding antenna installation is given in the pdf file, which depicts the Rapco antenna assembly (Type 4/4B).
If the unit is operated with an antenna not supplied by Rapco, the following points should be noted:
- There is a dc voltage ( <+5V )on the co-axial cable connector J4. This should not be shorted to ground, or connected to any dc load, other than an antenna with a dc current consumption of less than 30mA.
- If sharing an antenna with other equipment, any power splitter that is used must provide a dc block (capacitive coupling) in the feed to the master clock to prevent the dc source in the master clock becoming connected to another dc source in other equipment.
Failure to observe these precautions may result in permanent damage to the unit.
- The GPS L1 signal level provided to the master clock in these circumstances must be approximately equivalent to that from an isotropic antenna with a 20 - 25dB gain preamplifier connected to the unit via a loss-free cable.
- Poor signal-to-noise ratios in the GPS receiver can result in difficulties in maintaining lock. This can be caused by excessive signal attenuation, and in some circumstances, from an excess of signal. These situations may arise if the overall signal gain in the antenna circuit i.e.(antenna preamp gain) - (cables + splitter losses) is significantly greater than +25dB.
The 'signal level' readout from the RS232 port (RL Command) on the master clock is in fact a measure of the correlator SNR, and is a good guide to the RF behaviour of the 'front-end'.
In a typical scenario with a well-sited ground antenna there will normally be several satellites in view that are yielding signal-level readouts of above 10 and possibly up to 20. This range of levels will also apply with an airborne antenna situation, but there are sometimes more visible satellites at altitude, so the receiver (which favours the better signals) may have more choice and may be able to better optimise the situation.
If it is found that there are a large number of satellite signals that are (simultaneously) higher than (say) 18 on the signal-level readout, this may be an indication of excessive signal gain in the antenna system. This could give rise to problems in the presence of interference or multi-path reception.
The GPS antenna that is specified for use with ground-based Rapco master clocks and GPS references is a fixed-pattern type which has a built-in preamplifier, powered via the co-axial RF downlead cable from a low-voltage d.c. source (< 5 volts) that is built into the unit.
The signal connector on the antenna head assembly, although sealed at its mounting face, is not a weatherproof connector. In order to provide weather protection for the interface between this connector and the antenna downlead, and to avoid mechanical strain on the connector when the downlead is connected, a waterproof housing with an entry conduit is provided.
Rapco Antenna-downleads, both the standard 20 metre version (E43) and the heavier 50 metre type (E44), are ready-fitted with a screw-on cable gland which allows the assembly to be weatherproofed at the point of cable exit. If longer cable lengths are required then consult Rapco for details.
In extreme environments the user should provide further protection for the co-axial cable itself from its point of exit on the antenna assembly.
Information regarding antenna installation is given in the pdf file, which depicts the Rapco antenna assembly (Type 4/4B).
If the unit is operated with an antenna not supplied by Rapco, the following points should be noted:
- There is a dc voltage ( <+5V )on the co-axial cable connector J4. This should not be shorted to ground, or connected to any dc load, other than an antenna with a dc current consumption of less than 30mA.
- If sharing an antenna with other equipment, any power splitter that is used must provide a dc block (capacitive coupling) in the feed to the master clock to prevent the dc source in the master clock becoming connected to another dc source in other equipment.
Failure to observe these precautions may result in permanent damage to the unit.
- The GPS L1 signal level provided to the master clock in these circumstances must be approximately equivalent to that from an isotropic antenna with a 20 - 25dB gain preamplifier connected to the unit via a loss-free cable.
- Poor signal-to-noise ratios in the GPS receiver can result in difficulties in maintaining lock. This can be caused by excessive signal attenuation, and in some circumstances, from an excess of signal. These situations may arise if the overall signal gain in the antenna circuit i.e.(antenna preamp gain) - (cables + splitter losses) is significantly greater than +25dB.
The 'signal level' readout from the RS232 port (RL Command) on the master clock is in fact a measure of the correlator SNR, and is a good guide to the RF behaviour of the 'front-end'.
In a typical scenario with a well-sited ground antenna there will normally be several satellites in view that are yielding signal-level readouts of above 10 and possibly up to 20. This range of levels will also apply with an airborne antenna situation, but there are sometimes more visible satellites at altitude, so the receiver (which favours the better signals) may have more choice and may be able to better optimise the situation.
If it is found that there are a large number of satellite signals that are (simultaneously) higher than (say) 18 on the signal-level readout, this may be an indication of excessive signal gain in the antenna system. This could give rise to problems in the presence of interference or multi-path reception.