Chapter 4 — Antenna Hacks 77

Carrying a GPS Signal via Cable

You’ve already learned that cables that are too long or have unnecessary connectors cause signal loss — but how much signal loss? The following sections outline some of the variables that affect the answer to this question.

How Much Signal Do You Need?

Before we look at the losses, let’s look at how much signal you need to get to your GPS in order for it to work.

For a high-gain (high-sensitivity) antenna (roughly 35 dB), most receivers can function properly with 12 dB +/- 2 dB attenuation (signal loss) from the antenna to the receiver input.

For a lower-gain (low-sensitivity) antenna (those in the 26 dB region), most receivers can function normally with 6 dB +/- 2 dB attenuation from the antenna to the receiver input.

Cable Losses

There is significant signal loss through a cable. What we are interested in is the signal loss at 1575.2 MHz. The following list of four different cable types shows the average signal loss you’ll encounter (derived from data on www.belden.com):

RG8 (Belden 9913F), 100ft: 7.5 dB attenuation

RG213 (Belden 8267), 100ft: 12.0 dB attenuation

RG142 (Belden 84142), 10ft: 2.0 dB attenuation

RG58 (Belden 8919), 10ft: 2.7 dB attenuation

Therefore, putting the preceding information into context, the following table shows the maximum length of cable that you should be able to use.

Cable	Max Length Using High-Gain	Max Length Using Low-Gain
	Antenna (feet)	Antenna (feet)
RG8	150	80
RG213	100	50
RG142	60	30
RG58	40	20

The better quality cable (generally, “more expensive”) you use, the lower the attenuation and the less the signal loss. However, higher-quality cables cost more. If you want less attenuation over greater distances, the only option available to you is to spend the money.

You can also find rigid and semi-rigid cables that have better specification, but these will undoubtedly cost extra and the gains you get aren’t going to be massive.

78 Part I — Hardware Hacks

For more information on Belden cable, check out the Belden website at http://bwccat. belden.com/ecat/jsp/index.jsp.

Many people don’t realize that lower-loss cable usually means thicker cable. The thicker the cable, the harder it is to get it around bends.

Connector Losses

It’s not just the cables that attenuate; the connectors do too. Generally, each connector adds about 1.5 dB of signal loss into the system. However, if you are using cable with 75-ohm resistance on a 50-ohm system (these figures will be marked on most cables that you use, while connectors are sold with a particular resistance value stated), then this figure goes up to 3 dB. This combination is therefore considered to be a poor match and not recommended for GPS.

One example of a 75-ohm cable from Belden is the RG59.

Adding a connector at each end of the cable means you are looking at a minimum of 3 dB attenuation of signal, which alters the maximum cable lengths that you can have. Depending on whether your cable/antenna setup has one connector or two, you can figure this value into your calculations directly, as shown in the following table.

Cable (with connectors)	Max Length Using	Max Length Using
	High-Gain Antenna	Low-Gain Antenna
RG8	110	45
RG213	75	35
RG142	40	15
RG58	30	<10

Splitters don’t normally add to the total attenuation in the system because they generally contain signal boosters to compensate.

As you can see, even with the best cable, 100 feet (33 meters) is probably the best you can do if you want to ensure that you get a useable signal.

You can find numerous types of cable out there (many more than I have listed), so you might find the following website useful:

www.timesmicrowave.com/cgi-bin/calculate.pl

This site enables you to work out attenuation for a variety of cables at a variety of lengths and frequencies quickly and easily.