I'm thinking of moving my ProMark Xcm into the house from now on.

This will require a distance of about 35-45 feet with a coaxial cable.

I wonder if, at this length, I should add an amplifier to the cable feed.

If so, I wonder what kind of amplifier would be best.

Oh . . . and . . . Right now the cables are RG58 cables 50 ohm. Would it be wrong to use an RG59 cable 75 ohm(a lot less signal loss)?

John:

I'm really NOT familiar with the ProMark, but the same "rules" should pretty hold true for any receiver.

If the factory cable is 50ohm, you NEED a 50ohm cable PERIOD.

My Novatel Receivers have 50ohm cables (I think, I would have to check the truck to be sure), and they were RG58 (pretty sure about that).

I looked around and found some RG59? (can't remember) cables (at Radio Shack) that were 50ohm and 50 feet long. I checked the actual resistance (it isn't 50 ohm by the way, that's a different subject) between the 10 meter factory cable, and the new 50 foot cable, and they were nearly identical.

Works like a champ, but the new cable is MUCH thicker and HEAVIER.

Loyal
Modified By Loyal Olson on 12/11/2004 at 9:11 PM

If you go long enough, an amp will be required, but length is your enemy at the GPS frequencies. I got a 50' cable built at my local radio shop, LMR400 is the stuff you need, big heavy cable. RG58 will have a lot of loss at 50 feet.

Here is an online calculator that lets you calculate the estimated line loss:

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

SHG

Your little experiment was what was in the back of my mind.

The cables(RG58) with the GPS are 50 ohms but have twice the "signal loss" of an RG59 with 75 ohms.

If the resistance of the cable is 1/2 agin higher, the cable is twice as long and the signal loss is about half . . . is the cable about the same?

I'm not sure HOW ohms relates to the matter of signal and loss. Does ohms mean a resistance that is computed into the cable by the manufacturer(that is will a 75 ohm cable not allow enough electrons to pass OR will a 50 ohm cable OVERLOAD the signal?). It's just curious that a Higher ohm cable actually has a considerably lower signal loss(RG58 to RG59).

MAYBE that's why the 30+ foot cable(RG58) compares closely to a 50' RG59 cable.

I haven't studied electronics since back in grade-school/Junior High school(back in my "radio years").

My understanding, which is very sketchy in this area, is that ground-plane antennas exhibit about 50 ohms of impedance at their resonant (design) frequency, and that any impedance mismatch in the chain between antenna and receiver will result in some of the signal being reflected back up the cable. Given the low power of GPS signals, any loss of signal strength along the chain is significant and to be avoided.

Impedance matching devices could be inserted at both the antenna and receiver ends of a 75-ohm cable to prevent the impedance mismatch loss, but I assume that some loss of power is involved in the conversion.

It seems to me that a better solution to extending cable length is to stick with 50-ohm cable and use an inline amplifier. I looked into this once, and found the cost of the amplifier to be around $250.

I have been using my Promark X CM's with forty feet of RG8/U for four years with no problems.
Rick

Wow . . . now that's GOOD news. Is that RG58/U?

But, just out of curiousity . . . 40 feet of cable?

I just looked up that cable. Boy it has some very impressive numbers.
Modified By John Francis on 12/12/2004 at 10:24 PM

It is Forty feet. And it is RG8/U. I had gotten this information from Ashteck teck support and I THINK the maximum footage was 75'?????
Rick

That information is a Godsend.

But, I'm curious about why you use 40 feet? Maybe the same reason?

Thats all I needed.
Rick

Rick,

Well now that's a rather plausible answer.

Dangerous Dave
Modified By Dave Huff on 12/13/2004 at 10:17 PM

From my ham radio days, you do not want to use the wrong impedence cable. However there are other 50 ohm cables that have lower loss. There are at least two ways that is done. One is larger diameter forms, and the other is other insulation types.

I think it is the deformation of the molecules in the insulation by the fields along the line that cause signal loss.

According to one vendor (http://www.isgsales.com/tei/ca_50c.htm)

There is RG8 with about half the loss for example. Here is a chart I found:

Scroll to Table 1

- jerry