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GPS Antenna cable length

Old 04-23-2003, 12:46 PM
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Default GPS Antenna cable length

I'm mounting my GPS antenna soon. The guy at West Marine said that performance was affected on VHF antenna cables if cut too short.(under 3') Would this also be the case for GPS antenna cable? I'm planning on coming right through my pilothouse to the GPS. Even if I wound the cable up a little bit I'm talking 2'. The antenna I have has 30' of cable. If I cut it short am I affecting how it was designed to work?

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Old 04-23-2003, 01:55 PM
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Default GPS Antenna cable length

It may depend on whose gps it is. I have a raychart, and since it only passes digital signals and power, it doesnt care. Garmin uces a coax cable to pass power and an rf signal, so it might get bitchy. No clue what others use.
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Old 04-23-2003, 02:08 PM
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Default GPS Antenna cable length

I'm using a Garmin 162GPSmap.
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Old 04-23-2003, 02:23 PM
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Default GPS Antenna cable length

What GPS are you referring to? Garmin has both a coax type GPS antenna (182) and a self-contained GPS antenna that sends serial format signals (2006/2010). For these particular units, cable length is not critical and you can cut the cables as required. However, it is never a bad idea to coil up and ty-wrap 3 or 4 feet of excess cable.
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Old 04-23-2003, 02:41 PM
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Default GPS Antenna cable length

Here's an update. I called Garmin tech support. The guy said it would be OK to cut 6 to 8 inches off of the cable. Yikes! The cable is 30' long! He said it had to do with the impedance and you could make it longer but not much shorter for accuracy. He suggested rolling the excess cable up and wire tieing it.

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Old 04-23-2003, 03:43 PM
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Default GPS Antenna cable length

The 162 passes the rf signal itself on the coax, so it may very well be sensitive to length. But that doesnt stop you from trying it at least. If it doesnt work as well, you have plenty of cable left to make an extension cord for it.
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Old 04-23-2003, 03:51 PM
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Default GPS Antenna cable length

I shortened my cable last year with no ill effects. I also had the antenna itself repaired which Garmin told me couldn't be done. I figured it was broken anyway so I'd give it a shot. Try it and see if you notice a difference.
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Old 04-23-2003, 03:58 PM
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Default GPS Antenna cable length

With in reason I've never seen too many problems with different length antenna cables. If there is any difference then no recreational receiver will be able to display the difference.

As for Garmin well they make basically the same antenna with different length coax anyway. I would be more concerned with being too long than too short.

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Old 04-23-2003, 04:51 PM
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Default GPS Antenna cable length

The 162 uses the same antenna as the 182 and I cut my GA29 antenna from 30 feet to about 5 feet without any problem.

Garmin has great customer support, but some of these guys do not always know what they are talking about, trust me. The easy answer to your question was not to cut the cable, which is essentially what you were told.
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Old 04-23-2003, 05:36 PM
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That sounds good. I'd rather not have 30' of cable tied up. Maybe I talked to the janitor!


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Old 04-23-2003, 05:52 PM
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Default GPS Antenna cable length

Being a self-proclaimed electronics genius, DO NOT CUT IT, thats why it comes with the connector on it versus an antenna with NO connector!

JIK
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Old 04-24-2003, 04:05 AM
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Default GPS Antenna cable length

Some of youze guys crack me up. You can cut the cable as short as you want and it won't affect your accuracy or anything else. Make sure you put the connector on properly. The reason they put the connector on the cable already is 9 outta 10 people don't know how to do it right. Length is loss....so cutting it shorter is going to IMPROVE the level of signal getting to your gps.


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Old 04-25-2003, 03:37 AM
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Default GPS Antenna cable length

saltybugger, you need to crack the books open before making a statement like that. All RF circuits have tuned circuits for their frequency(s) of interest.

If you have a length of coax attached to a tuned circuit, it becomes part of the circuit and affects the resonant frequency.

Cutting a coax down from 30 meters to 1 meter, would give you 1/10 of the original coax's capacitance. That much change could easily throw off the center frequency of a tuned circuit.

It's not all about loss. It's not hard to retune a circuit so that it is never again resonant to the correct frequency.
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Old 04-25-2003, 04:06 AM
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Default GPS Antenna cable length

THAT is only true to an extent if it's a REGULAR antenna....and a GPS antenna has a LNA in it with a very tolerant output impedance....so YES you can cut the GARMIN cable ANYWHERE you want and NOT affect the frequency.
If you'd like you can come here and look at one taken apart and we'll put it on my 30K network analyzer and you can add or change the cable length to your hearts content?


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Old 04-25-2003, 05:21 AM
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Default GPS Antenna cable length

to the opinions stated above. Since I'm not an electrical engineer I cant speak to db, rf or necrotic sine-wave performance.

But...

You should keep one thing in mind when trimming cables. It is a lot easier to shorten one without affecting performance than it is to make it longer again.

I'd suggest you leave more slack than you think you need in case you have to relocate the unit later or need to work around the cable.

Good luck.

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Old 04-25-2003, 05:23 AM
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Default GPS Antenna cable length

My post was in response to your generalization of the coax/receiver, not to Garmin specifically. I am aware that they amplify the signal before running it down the coax, I did mention that they pass power on the coax as well.

However you cant make your post stick where the coax terminates into a tuned circuit. Not all gps's are garmins.
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Old 04-25-2003, 05:28 AM
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Default GPS Antenna cable length

I submit to you, it's obvious you have more Special High Intensity Training that I

Just cut the darn thing and get it over with, and if you cut it too short then go down to Boat US and buy a wire stretcher!


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Old 04-25-2003, 06:03 AM
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High intensity training? LOL no just a EE for 25 years and counting.


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Old 04-25-2003, 08:14 AM
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Default GPS Antenna cable length

SPECIAL High Intensity Training


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Old 04-28-2003, 01:56 PM
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Default GPS Antenna cable length

Well, I cut the cord. It works flawlessly!

Maybe the guy at Garmin who answered the phone really was the janitor.


Ed
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