Application Notes

Application Notes


GPS ANTENNA CABLE TESTS & ALTERNATIVES

The antenna supplied with ESE GPS Master Clocks is designed to provide excellent reception under a variety of conditions. For best results, the top of the antenna should have at least a partial view (unobstructed line of sight) of the sky. However, the view of the sky may be through a window and the antenna may be mounted indoors if the roof or upper floors do not shield the signal. Bench testing is recommended before the permanent location for the antenna is decided.

One of the main concerns when using a GPS receiver (Master Clock) is the distance between the antenna and the receiver. The antenna is supplied with 16' of coax cable which can be connected directly to the clock. If more cable is required an in-line amplifier may be required. When extra cable is required, several options exist. The most simple and least expensive methods are discussed below. It is, however, the ultimate responsibility of the end-user to decide which option will best satisfy the specific situation. Please feel free to contact the ESE factory for assistance. Also... please read the section below which discusses the tests performed at ESE.

CABLING SUGGESTIONS/ALTERNATIVES

Method 1 (low loss cable):

Several types of "low loss" coax cable are available including RG-8, RG-213, and Belden 9913. Using any of these types of cable "may" allow up to 300' of additional cable to be added to the 16' supplied with the antenna (perhaps more in the case of 9913). These cables are expensive (relative to RG-58) and are cumbersome to work with due to their larger diameter and/or stiffness.

Method 2 (in-line amplifier):

The use of an in-line amplifier such as the LA-12F (manufactured by Raven and available from ESE) may be more convenient than low loss cable. Installing one of these amps "may" allow up to 150' of RG-58 (or 300' of RG-8/RG-213/9913) to be added to the 16' supplied. The LA-12FN has 'N' connectors which allow for convenient interconnections with the low-loss cable. See Figure 1.

Method 3 (in-line amplifiers with power supply):

Due to the power limitations of the receiver and antenna, only one in-line amplifier can be used. However, if a power supply such as the ES-AB1A is used, up to five (5) in-line amplifiers may be added. The more amplifiers used, the shorter the cable run between amps. See Figure 2.

ESE TEST RESULTS

The methods mentioned above are conclusions based upon actual tests performed by ESE and on information provided by various manufacturers. The performance of your unit may differ due to antenna position and obstructions to its line of sight, weather/atmospheric conditions, cable length or signal reflections. Listed below are the "best" consistent performances. That is, they were repeatable performances on a consistent basis... not just fluke observations. Caution is recommended: as the GPS Satellites age, their signal strength may decrease and today's cable length could cause undesirable results at a later date.

The ESE factory conducted several tests which demonstrate how the Master Clock/GPS Antenna can be expected to operate for given sets of circumstances. All tests were conducted at ESE (in El Segundo, CA) in 1998 and due to the architecture of the GPS Satellite Constellation, can be considered applicable most anywhere in the world.

Test 1:

The first test shows that up to 112' of RG-58 cable can be added to the 16' without any significant loss in signal. Adding 125' impaired the clock's performance.)

Test 2:

When using a single LA-12F, in-line amplifier with 150' of RG-58 cable (in addition to the 16' supplied), the clock would "lock-on" in a nominal amount of time. (Adding 175' impaired the clock's performance.)

Test 3:

Using an ES-AB1A (Antenna Power Supply) and two (2) LA-12F with 100' of RG-58 cable attached to each (219' total) permitted the clock to "lock-on" in a nominal amount of time. Adding a third LA-12F with 75' of RG-58 cable did not impair the clock's performance, however, increasing the 75' to 100' did impair the clock's performance.

Test 4:

Using an ES-AB1A (Antenna Power Supply) and four (4) LA-12F with 75' of RG-58 cable attached to each (319' total) permitted the clock to "lock-on" in a nominal amount of time. Adding a fifth LA-12F with 25' of RG-58 cable also permitted the clock to "lock-on" in a nominal amount of time. Increasing the 25' to 50' impaired the clock's performance.

All of the tests mentioned above were conducted twice; first with the antenna indoors and second with the antenna outdoors. (Indoors refers to the ESE factory which is a single story building with a wood ceiling and asphalt composite roofing. And outdoors, the antenna had a very narrow look at the sky with approximately six feet of clearing between buildings.) In all cases, the unit "Locked-on" within fifteen minutes, and in less time when the antenna was outside. ("Locked-on" refers to starting the clock from a completely "powered down" mode and the nine-digit display "catching" real-time with the "GPS Lock" LED lit.) Various tests conducted away from the factory show that the exact unit which took five minutes to lock-on at the factory, may take up to 45 minutes at a different location. Possible explanations for this phenomenon lead us to believe that atmospheric conditions or poor antenna locations may be responsible. Please note that once the unit has "Locked-on", and then removing AC power while the battery is "On", the unit will typically re-"Lock-on" within ten seconds after AC is reapplied.

ESE has noticed that the manufacturers' cable length specifications for the Motorola GPS Receiver and the Raven LA-12F differ from the test results of ESE. Please be aware that Motorola specs the maximum cable distance between the antenna and the receiver is 16' (5 meters) without amplification. Also, please be aware that Raven specs the maximum RG-58 cable that the LA-12F can drive is 50 feet. ESE's test results may indicate that longer lengths of cable may be used. We are not inferring that the longer cable lengths should be used. We are only reporting the results of our tests and repeat the caution mentioned earlier. As the GPS Satellites age, their signal strength may decrease and excessive cable length may cause undesirable results. All ESE tests were conducted using RG-58 coax cable. According to several manufacturers of low loss cable, the signal loss attributable to cable length can be reduced with the use of "low loss" cable. According to the RG-8 and RG-213 cable specifications, when compared to RG-58, cable lengths may be doubled (or more in the case of 9913) with "equal to" or "better than" results. Once the unit has "Locked-on", the receiver creates a semi-permanent "Library" of where it expects to "see" a satellite(s) at a specific point in time. However, if after "Locking-on", the unit is relocated (for instance, from the ESE factory in California to an end-user's site in Europe), the unit's 1 PPS (and other outputs') accuracy may appear erratic for up to three hours. When the receiver has created an updated Library, all outputs will then comply with the specifications discussed in the product manual.