Basic VHF Questions

[larryhorne]

My VHF radio works fine in the driveway. When I'm out in the Bay, the reception is broken. I'm thinking of just adding a second VHF radio and antenna then trouble shooting this one and using it as a backup.

I've been to West Marine to look at Shakespeare antennas and they carry 4 or 5. Their display has a cut-away so you can see the internals. I'm installing this on a 1987 24 Albe Express. I use the boat in the Chesapeake Bay and will be running 15-20 miles off shore. What do I look for in a VHF radio and antenna? Do VHF antenna's have to be "tuned" like a CB antenna and can you cut the cable?

Thanks for your help.

[Tom Powers]

You can cut the cable. The key is to make sure that you properly terminate the cable (i.e. mount the connector in accordance with the instructions). Usually that means soldering the center wire to the center pin of the connector and properly trimming back the outer conductor before tightening every thing down. If you do solder use flux designed for electronics not plumbing. Clean it when you are done with rubbing alcohol.

Also if you want to get the most efficiency make your cable as reasonably short as possible. Don't leave a coil of 10 feet of cable tucked away somewhere.

[larryhorne]

thanks. any suggestion re the old vhf....unit or antenna?

[night nurse]

It was my understanding that the length of cable had to relate to the wave length. So randomly cutting the cable would not be advised but rather a measured distance ie: the wave length of that antenna. I did have several feet of cable coiled up under the dash and it worked great on the old boat. Now the current boat has a short line to the antenna and the reception from a boat in shouting distance is rough at best using the SAME raido, just pulled out the old one. I will be replaceing the old antenna on WED. so I'll let you know what happens.

TED

[Tom Powers]

I am actually an electrical engineer that works in RF design. The deal with cable lengths being exact lengths has to do with the impedance match of the antenna. This is usually measured using a VSWR meter (voltage standing wave ratio) 1:1 Is best. Most antennas are spec'ed at 1.5:1. Part of that is the precise length of the antenna and the geometry of all of those metal spirals, etc. at the base of the antenna. Also if your antenna or a connection starts to fail you would expect a substantial change in the VSWR.

On a lot of the old CB antennas the length of the antenna was adjustable and you had to adjust it to minimize the VSWR. I brought in the Shakespeare $79 antenna for my boat a few years ago and took into work where I hooked it up to a $50,000 network analyzer. Low and behold it was a pretty darn good match. Much better than 1.5:1

As to the losses in the cable. Written on the cable is some type of label with a part number. For instance if you look at RG-58 cable at 150 MHz it has a loss of 6 dB/100 feet of cable (power down by a factor of 4) So if you have an extra 10 feet of cable tucked under your dash you are loosing 0.6 dB of your power or about 15% of your power.

Night Nurse, I what is the condition of the connector at the end of your cable. I would lay a wager that something is not soldered correctly or that something is corroded or dirty. The Shakespeare sight has directions on connector assembly. If you are not familiar with doing electronics assembly you can always ask on the board and someone will probably step up to attach a connector.

Back to the original question what to get for off shore. Look at antenna gain it is in dB generally the bigger the better. What that really means is the amount that the antenna focuses the signal towards the horizon and not towards the sky. The down side of a high gain antenna is that the direction that it is pointing is affected by wave action and the boat rolling. Most 8-foot antennas have a 6 dB gain. You have to go to the 15 or 20 foot antennas to get more gain.

Next is the quality of the antenna gold plating, etc. which effects reliability in the long run. For the same company this one is simple spend more money. Gold plating is good.

Next is where you mount the antenna. VHF is line of sight. That means that the higher up on the boat the better.

[LandingCrew]

come down to bass pro thurs. or friday, ill look at you r gear(comm bubba in marine corps) i dont work this weekend, but will get in the right track.

mark

[larryhorne]

I learned this weekend the source of the problem, its the RAYMARINE FF trasducer interferring with it.

Any way of shielding it?

[Tom Powers]

First thing to try is to run a piece of 12 Gauge two conductor marine grade wire directly from your battery terminals to your radio with an in line fuse rated for your radio. For the first test you can just string it along the deck in order to see if it works. For that matter for the first test you can just use lamp cord wire. If it works then you use the good stuff for the final installation. This will tell you if it is conducted interference that is getting into your wiring harness. You can even try hooking it up to an isolated battery that you borrow from a buddy which gives you complete isolation from a conducted problem.

Also you can experiment around with routing the power cable for your radio next to or not next to your fish finder transducer cable. This will tell you if it is radiated problem between the FF transducer cable and the radio power leads. If it is radiated then your best bet is to try keeping them separated.

[larryhorne]

thanks tom