Antenna wire for AIS transciever

[KASHMIR]

I am pulling my mast at the next slip sometime soon and one of the jobs to do on it is fit a VHF antenna to my top spreader for a future AIS installation.
I wonder if anybody can help me with what kind of antenna wire I should fit.
The price here varies dramatically from about $1.20 per metre to over $4 per metre.

Does the higher price cable offer any advantage?

Cheers

Peter

[denverd0n]

This is a transmitter, so you need a good quality coaxial cable. In general, if you buy from reputable dealers, you get what you pay for. So, yes, the more expensive cables will provide a better signal.

There are a wide variety of cables available, most with fairly standard names (at least I assume the names are the same in Oz as they are in America). Tell us more specifically what you are looking at and we can give you more specific feedback.

[continuouswave]

Here is some general advice: don't trust advice you get from anyone who calls the 50-Ohm coaxial transmission line "antenna wire."

Originally Posted by KASHMIR

Does the higher price cable offer any advantage?

In any transmission line, the best outcome is generally to have the lowest loss in the transmission line. To reduce loss in a transmission line the method is generally to increase the size of the conductors and to alter the dielectric of the insulation. You will find that the cost of coaxial transmission line is generally in inverse proportion to the loss in the line, that is, less loss means higher price.

For use in an outdoor environment, the outer insulation of the coaxial transmission line must be resistant to ultra-violet degrading.

Some coaxial transmission lines are made with a semi-rigid outer conductor and cannot be easily fitted to running inside a mast.

Loss is related to length of the transmission line. Typically if the transmission line is more than 20-feet long, you should change to a transmission line cable that is about 0.5-inch diameter if you want to prevent excessive loss. I don't know what sort of coaxial cable is sold in Australia, but something like LMR-400-Flex might be appropriate.

http://www.timesmicrowave.com/downlo...LMR-400-UF.pdf

[El Rubio]

How long of a cable run are you considering? I disagree with the previous posters on something being more important if it transmits and that runs over 20' should be larger lower loss cable. The fraction of power you save/ lose will be small and undetectable in real world communications. The losses are the same on transmit and receive and the little bit of improvement going from good 1/4 inch cable to 1/2 inch is minimal and unnecessary expense. Give a list of cables/ cost and i'll tell you what I would go with.

[chuckr]

i am an novice here -- but we put our ais antenna on the top of our bimini between the solar panels and not an issue -- you can put it at the top and not sure what you get but a bit longer range and more cable expense -- i mean how far out do you want to see the other boat??

we use rg8x cable from west marine i think -- and works great

[KASHMIR]

Originally Posted by El Rubio

How long of a cable run are you considering? Give a list of cables/ cost and i'll tell you what I would go with.

The length of run would be less than 20m, probably around 17m.
Would something like this:
Radio Parts - Electronics Components - RG6 Stranded Cable Reel

Do the job? Its just cheap stuff but if it does the job?

Peter

[btrayfors]

NO, don't use RG-6. It's nominal impedence is 75 ohms and the materials are not suitable for the marine environment.

You want a good quality 50-ohm coax cable.

RG213 is widely available and would be a better choice. It's 1/2" in diameter.

If you must use the smaller diameter stuff, look for a good quality RG-8X (quality varies considerably) or, better, LMR-240.

You must also choose a VHF antenna which, sitting atop your spreader, won't bounce around and bang into the mast or the shrouds.

Be sure all connections are correctly made, and do your best to waterproof them with tape or other material.

Bill

[St. Elsewhere]

If you are confused by conflicting information here, resolve it by reading the AIS equipment manufacturer's installation instructions. They should cover coax spec's and length issues that meet the needs of your arrangement.

[jolly jack tar]

KASHMIR,

Whichever cable you choose, make sure your antenna is balanced to your cable and transponder. If both of those are 50 Ohms then your antenna needs to be 50 ohms. Do not coil over-length cable and do not bend it through any sharp angles and do not over-tighten cable ties. Once you are all set up you can check the performance with an SWR meter. The Ideal ratio is 1:1 but that's in a perfect world. In reality if you get 1:1.3 or 1.4 you are doing good.
Keep your cable clean and dry and avoid UV where you can. If you can, use a quality cable gland and heat shrinkable boot at your antenna connection.

cheers,
JJT

[continuouswave]

There really is no standard for a cable called RG-8X. As a result, there are many inferior products labeled with RG-8X. Be careful in buying any cable marked as RG-8X as that marking does not really designate any official cable specification.

If your transmission line length is 17-meters (56-feet), and you use a small-diameter transmission line like RG-58C/U (a Mil-Spec cable version of what is found attached to most boat antennas), at 150-MHz the loss will be more than 3-dB, or half of your transmitter power will be lost in the transmission line.

Designing an antenna and transmission line system in which more than half of the power is lost in the transmission line is not an acceptable practice for most installation, but it may be just great for some boaters. It seems to me that the purpose of installing an AIS transceiver is to let other boats receive your signal. The notion that it does not matter how little signal you emit seems inappropriate. If you are installing a Class-B AIS transceiver, you will already be operating at reduced power (2-watts) relative to the other Class-A devices.