MDS 05-2806A01, Rev. A MDS iNET 900 User’s Guide 101
Feedlines
The choice of feedline used with the antenna should be carefully consid-
ered. Poor-quality coaxial cables should be avoided, as they will
degrade system performance for both transmission and reception. The
cable should be kept as short as possible to minimize signal loss.
For cable runs of less than 20 feet (6 meters), or for short range trans-
mission, an inexpensive type such as Type RG-8A/U may be acceptable.
Otherwise, we recommend using a low-loss cable type suited for 900
MHz, such as Heliax
®
.
Table 5-1 lists several types of popular feedlines and indicates the signal
losses (in dB) that result when using various lengths of cable at
900 MHz. The choice of cable will depend on the required length, cost
considerations, and the amount of signal loss that can be tolerated.
5.1.5 Conducting a Site Survey
If you are in doubt about the suitability of the radio sites in your system,
it is best to evaluate them before a permanent installation is underway.
This can be done with an on-the-air test (preferred method); or indi-
rectly, using path-study software.
An on-the-air test is preferred because it allows you to see firsthand the
factors involved at an installation site and to directly observe the quality
of system operation. Even if a computer path study was conducted ear-
lier, this test should be done to verify the predicted results.
The test can be performed by first installing a radio and antenna at the
proposed Access Point (AP) station site (one-per-system). Then visit the
Remote site(s) with a iNET 900 unit and a hand-held antenna. (A PC
with a network adapter can be connected to each radio in the network to
simulate data during this test using the PING command.)
With the hand-held antenna positioned near the proposed mounting
spot, a technician can check for synchronization with the Access Point
station (shown by a lit
LINK LED on the front panel) and measure the
reported RSSI value. (See “Antenna Direction Optimization” on
Page 93 for details.) If adequate signal strength cannot be obtained, it
Table 5-1. Length vs. loss in coaxial cables at 900 MHz
Cable Type
10 Feet
(3.05 m)
50 Feet
(15.24 m)
100 Feet
(30.48 m)
500 Feet
(152.4 m)
LMR 0.39 dB 1.95 dB 3.90 dB Unacceptable
Losses
1/2 inch HELIAX 0.23 dB 1.15 dB 2.29 dB 11.45 dB
7/8 inch HELIAX 0.13 dB 0.64 dB 1.28 dB 6.40 dB
1-1/4 inch HELIAX 0.10 dB 0.48 dB 0.95 dB 4.75 dB
1-5/8 inch HELIAX 0.08 dB 0.40 dB 0.80 dB 4.00 dB
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