Antenna Selection and Fitting

[Bob F]

Here's some pointers on Antenna selection that I've picked up from various sources, especially the Marine Electronic Services Catalogue.

 

An inportant factor in the performance of your VHF radio will be the quality and positioning of the antenna. Many problems with VHF radio are related to poor antenna siting, faulty cabling, poor quality cable joints and low voltage supply.

 

As the range of VHF signals are governed by lne of sight, the antenna should be placed as high as possible, but be clear of any metallic objects that could influence the resonance of the antenna.

 

The most popular antennae for marine use are 1m long. On small boats they are usually positioned on top of the cuddy or cabin, either directly attached to the roof or mounted on a gantry for extra height.

 

Longer whip aerials are recommended only for larger boats. These radiate the same total power as the smaller antennae, but concentrate it into a narrower beam, which is advantageous on a tall mast at extreme range where concentrating the available power into a narrow horizontal beam becomes more important. However, if the antenna is not vertical when transmitting, the beam will be angled either too high or too low (e.g. when pitching or heeling in waves the narrow beam can be pointing up to the sky or pointing down into the sea).

 

Therefore, vessels with a large heel angle (like our small boats) would be better chosing the shorter 1m antenna, where the wider beam angle will be more universally effective, even though the signal will be weaker.

 

The antenna coaxial cable and any connectors used must be rated at 50 ohms. Under no circumstances should standard domestic TV cable and connectors be used. Incorrectly rated cable and connectors could result in power not reaching the antenna, but worse still, the power could be reflected back into the radio, damaging it in the process.

 

BF

 

 

[Paul J]

Nice one Bob,

 

Im just about to get mine in a couple of days time

Paul

[Coddy]

Hi Bob

 

Takes me back to CB days when everyone was desperate to get the most gain out of a set up to see how far you could transmit.

 

SWR meter at the ready and checking when needed for the optimum strength.

 

I did see a bit in a mag once about making an SWR meter with a dummy load so testing outputs could be checked for marine VHF sets.

 

Moden VHF marine antenna's are pre-set so unable to do much about it so I am told.

 

Coddy

 

 

[Bob F]

An SWR meter is still a handy bit of kit to have to help isolate problems (i.e. is it the VHF set or the antenna that's causing the problem).

 

BF

 

 

[Coddy]

Bob

Don't use a CB model as it will damage the set or antenna!

 

They have differant loads and frequencies

 

Dave

[Adam F]

I get a very slight 'buzzing' from my new VHF set...any ideas?

 

I didnt with the old set, and it is mounted in exactly the same place and in the same fashion.

 

Paul D mentioned it could be where I have the data, power and ariel cables all routed through the same trunking?? What can I do if this is the case, as I dont have many options re. running the cables else where.

 

Adam

[Bob F]

Adam,

 

First check that all the earths are tight, clean and making good contact. This will be the earth on the set (the negative), and the earth on your antenna.

 

Next, try turning off all your other equipment and see if the buzzing is still present.

If it disappears, turn on each piece of equipment in turn until the buzzing comes back. This will then be the culpret and we can the assess your next step.

 

Also, does it buzz only when the engine is running?

 

BF

 

[Bob F]

Good point Dave. Didn't realize CB had a different impedence.

[Newboy]

I get a very slight 'buzzing' from my new VHF set...

Keep it clean lads.....

[Coddy]

Hi Adam

 

I get the same sometimes on my set.

 

Traced it down to the GPS, not plotter.

 

Moved the power leads away as much as I could, if really bad might need a bit of shielding, could try some kitchen foil around the cable that is causing the probem, I think it will need earthing. Did not have to do it on my boat however just re-routed cables a bit.

 

Regards

 

Coddy