External Antenna Cord Length

January 12, 2005

I have a compact flash GPS for my PocketPc. It came with an external antenna that greatly improves accuracy. The only problem is the cord for the external is 20' long and that makes it pretty bulky. I like to put it on the roof of my car when driving, but I end up with a big bundle of wire laying around which is cumbersome.

Would splicing the cable to make it shorter affect the performance of the antenna in this situation? I know in regular radio frequency applications, the length of the cord to antenna makes a difference, but is it the same in a GPS situation?

Edited January 12, 2005 by nejohnson

January 12, 2005

I'll add my two cents here being a Ham (KQ4MM). In theory with any RF application being receving or transmitting the length of the feed from the antenna to the receiver/transmitter matters. The higher the freq (GPS is up there) the more it matters. Adding splices with connectors adds futher loss. Best situation is to have feed line just long enougth for application. If I remember correctly there is a approx 3Db loss per connector at 2m (144Mhz) lilely worse at the freqs GPS operates at.

I could be wrong and will bow to any corrections

Brian

January 12, 2005

The splice at GPS frequencies would result in significant signal loss. Also in addition to splicing the center conductor you will need to re-establish the shielding (outer conductor as some people will call it), if you do not do this you will be injecting noise into the system, effectively reducing signal again. Finally, if it is an active antenna you have to be really careful that the splice does not act as a capacitor those reducing DC power to the antenna that the GPS was providing.

I would recommend that instead of attempting this you stop by rat shack rrrrr radio shack and pick up a set of velcro style cable ties to keep the cable neat.

Also brian_41, I just double checked and inserting a connector pair (N, BNC, or ISO style) at 2-meter (144 MHz) adds a lose of 0.3db. Not good but not the approximate 50% loss of 3db.

January 12, 2005

Doesn't matter. I have an external patch antenna that I use with my Map76. The stock cable was way too long so I shortened it by cutting to length and installing an 90° MCX connector. I also made up an extension to use when I'm using it in the vehicle. The loss per connector at 2 meters is about .01db and at GPS freqs of around 1.5ghz, it's about .1db As an electronics tech., Iv'e installed many of these connectors and have the proper crimp tools and soldering skills to do it. The crimp tools are very expensive and the job isn't easy for a first timer. I don't recommend a simple "splice" at these frequencies, you need to use the proper connectors.

5.3k · January 12, 2005

You are not transmitting so don't would not worry about the length of the cable.

Edited January 12, 2005 by JohnnyVegas

January 12, 2005

You are not transmitting so I would not worry about length.

Signal degradation works both ways (transmitting or receiving) due to cable loss, connector loss, radiation loss, impedance mismatch...etc.

January 12, 2005

Thanks NiclL, forgot the . Here is a great site for calculating coax loss

http://www.ocarc.ca/coax.htm

At GPS freqs you need to keep it as short as possible with quality connections.

Brian

January 12, 2005

Iv'e installed many of these connectors and have the proper crimp tools and soldering skills to do it. The crimp tools are very expensive and the job isn't easy for a first timer. I don't recommend a simple "splice" at these frequencies, you need to use the proper connectors.

I'm experienced with soldering (High School Electrionics Class and many modded Playstations) and wouldn't even consider something like this without soldering. I was planning on doing the splice using MMCX connectors since that is what the original plug is. Do they require solder and crimping? I've never worked with these before. I really shouldn't have used the word "splice", I was actually just planning a cut at 5' and a new connector.

Edited January 12, 2005 by nejohnson

January 12, 2005

MMCX, that's even smaller! With a straight connector there is a center pin that gets soldered on to the coax and then there is a sleeve that gets crimped over the outer braid. With the right angle connector there is an opening in the back of the connector that you'll need a long thin conical soldering iron tip to be able to solder the center conductor down in that opening and then there is a round cover that is pressed into the opening to seal it up and the sleeve again gets crimped over the outer braid. A lighted magnifier will be a great help. Buy yourself a couple extra connectors so if you screw up, just chop it off and try again. Good luck.

Poindexter

By the way, they do make these connectors where the outer braid is soldered instead of crimped, Iv'e always used the crimp type and I always use Amphenol connectors and usually get them from www.mouser.com

Edited January 12, 2005 by Poindexter

January 12, 2005

I was planning on doing the splice using MMCX connectors since that is what the original plug is. Do they require solder and crimping? I've never worked with these before. I really shouldn't have used the word "splice", I was actually just planning a cut at 5' and a new connector.

Did a quick search on MMCX connectors and they come in both varaitions (crimp and solder). Cutting down to length you desire and properly installing a new connector is the best way to go IMHO.

Cheers

brian

January 12, 2005

Well, I'm going to give it a try. Worst case scenario, a new antenna off of ebay is like $10. Thanks for all the advice!

January 13, 2005

Once again I feel I [ ] MUST [/u] stress the best solution to you problem is controlling the cable. I do some (not enough but I only have so many free hours) work in this freq rang (ham call KB9WWS) and for what you are looking to gain verses the cost and possible loses it is best to go with the Velcro tie wraps. Face it one MCX connector will probably cost you as much as 5 wraps and unless you are use to working with these small in size and tolerance components you will need many more than one to get a good connector.

that is the pure and simple version of what I staed above.

BTW: THNX Poindexter I had one reference to 0.3db but that seemed a bit high from experience. I will add your number to my notes.

5.3k · January 13, 2005

You are not transmitting so I would not worry about length.

Signal degradation works both ways (transmitting or receiving) due to cable loss, connector loss, radiation loss, impedance mismatch...etc.

On a cable the is only a few feet long it is not going to make a defferance what a waste of time cutting a cable to a GPS antenna feed line.

Edited January 13, 2005 by JohnnyVegas

January 13, 2005

You are not transmitting so I would not worry about length.

Signal degradation works both ways (transmitting or receiving) due to cable loss, connector loss, radiation loss, impedance mismatch...etc.

On a cable the is only a few feet long it is not going to make a defferance what a waste of time cutting a cable to a GPS antenna feed line.

Johny if it didn't make a difference then I have spent a whole bunch of time and energy making sure I can hear them to work them in the general vicinity of the GPS bands. Granted the ham bands are do not have guard frequencies to help protect them and they are in sections of the spectrum were atmospheric attenuation and noise are big. But if the goal of adding an antenna is to increase signal strength and therefore accuracy why attempt to change the engineering specifics without the proper equipment (minimally a phase angle analyzer), supplies (electronic silver solder and not the stuff that rat shack sells) or knowledge. Yes you may and most likely will get lucky.

BUT the OP was really about control of the cable and one Velcro cable wrap will do the same thing. No mess, no burned fingers, no possible screw-ups.

5.3k · January 13, 2005

I stand by what I have stated, the cord is not long enough for there to be any significant lose. How much would you expect to gain. it is not worth all the effort to the small amount of gain that is invloved. Using your claim that the lenght of the cable needs to be shortend, then just putting an external antenna on a GPS degrades the signal because of the added cord and conectors.

January 13, 2005

nejohnson: Don't worry about all this talk about signal loss. Part of my job is installing, maintaining and repairing GPS receivers on the Naval Academys training vessels. We have over 40 of them! On our sailboats, the antenna cable is about 30' of RG58 coax with bnc connectors at about 6' from the antenna below deck and another connector at the receiver. While observing the satellite screen on the receiver, there is no perceivable difference in signal strength while watching the signal strength bars when switching between the boats antenna (30' cable and 3 connectors) and a test antenna with 6' of cable and 1 connector. Yes, the longer the cable and the more connectors there are in the line, the more loss there is, transmitting or receiving, but you need to do some real world tests to determine if it's going to be a problem. In your situation, your only trouble is getting the connectors on properly. If your confident, go for it, and let us know how it worked out.

Poindexter

January 13, 2005

FWIW, I agree, but go here http://www.ocarc.ca/coax.htm and plug in rg-58, 30 feet at 1500MHz and you will see the resulting loss is 6.476dB.. In the example, 100 watts is put into a 1:1 SWR Load and the resulting power output is only 22.5 watts... If I understand that correctly one would have to have a antenna with more than 6.5dB gain to negate the rg58 coax loss.

Bottom line is it really doesnt matter much in our application

January 13, 2005

These antennas come in two flavors, active and passive. We use both types but the manufacturer recommends that the passive antennas use a max of 25' of cable. The active antennas have built in amplification that more than compensates for the cable losses.

January 13, 2005

I've got 5 CCMX solder-on connectors ordered. This should be fun. And I'm guessing the soldering won't be nearly as hard as the 20+ tiny solders on a PS2 modchip. I'll try to do as much of a before-after comparison as possible and report back.

Edited January 13, 2005 by nejohnson

2.7k · January 13, 2005

I've got 5 CCMX solder-on connectors ordered. This should be fun. And I'm guessing the soldering won't be nearly as hard as the 20+ tiny solders on a PS2 modchip.

I hope you MMCX! <_<

ACtually, the soldering can be more difficult (at least on right angle units), as you have to have 8 hands to hold everything steady and a really fine point iron. I had to use a magnifying desk lamp, a vise, clamps for the wires, and one hand on solder and the other on the iron. Hated doing those, but I got OK at it.

The fun part is hammering the little brass 'manhole cover' on. Yeesh.

January 13, 2005

I hope you MMCX!

DOH!

1.2k · January 13, 2005

Putting tiny connectors on is best left to nimble-fingered girls working in far-east factories. I can do it myself but my yield is only about 80%. :rolleyes: You might find that its easier to dismantle the antenna itself and cut & re-solder the connection to the circuit board inside. On my GPS antenna here that's all there is to it.

I concur that for an active or passive GPS antenna shortening the coax is perfectly fine. FWIW, you can almost always tell a passive vs. active GPS antenna by the choice of coax. If it is RG-174/LMR100 I guarantee you the antenna is active.

Hope these nuggets of wisdome are of some benefit.

Cheers!

Coupar-Angus

Radio Amatuer (& Professional)

External Antenna Cord Length

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