VHF Antenna

[sailaway78]

Here's a question for you radio/electronics whizzes out there. I know someone on this board knows the correct answer. I need a new mast top antenna for my VHF, as the old one had a close encounter with a low bridge. ( Another story.) In looking at various models I find that there are basically 3db gain antennas and 6db gain antennas. Seems like somewhere in the past I have read that for a sailboat, the 3db is better, as the heeling of the boat makes the 6db less effective. To my sometimes feeble mind, more always seems better, and a 6db gain should be better than 3. Any of you have some real info. concerning this.

 

[Don S/V ILLusion]

some info to ponder

Antennas don't produce gain per-se. What they do is create a wave form off the antenna to direct more of the signal in a specific direction. In the case of a vertical, the gain refers to how flat the donut shaped wave becomes - the more gain, the flatter the form such that more signal is radiated closer to the horizon thereby appearing to create gain.So, in general, you want more "gain". The problem arises, as you might imagine, when the boat heels and the donut wave form is now pointing either up or down away from the horizon which is the argument for less gain or a more omni-directional wave form.As a practical matter, a 6 dB gain antenna not perfectly perpendicular with the horizon will approach the performance of a 3 dB gain antenna as it heels so the difference becomes questionable.Another consideration is length of the antenna - higher gain antennas are usually longer (or in this case higher above the mast).Long way of saying it likely makes very little difference which you choose.

 

[paulj]

re

Pick a omni 3db antenna and a omni 6db antenna that you think you would want then poat it then we can help you in a comparison.You also have to discribe the location where you sail and what you use the radio the most for..paulj

 

[RAD]

My 2 cents

If your antenna is at the mast a 3db will work fine,I have a 3db sailboat antenna on the top of my mast and was heard on the south shore (Great South Bay)while sailing on the north shore (long island sound) see link

 

[paulj]

RE

With the best antenna the worst time for radio transmissions is when it's hot and no clouds.Sometimes when you transmit from the boat the radio signals bounce off clouds, cold air bands, water and the good old refraction thingy and travel quite a distance.paulj

 

[Ken Palmer]

Another way of putting it

Tropospheric ducting can occur in front of and parallel to an advancing cold weather front, especially if there is a marked difference in humidities between the cold and warm air masses. A duct can form approximately 250 km (155 mi) in advance of the cold front, much like a ventilation duct in a building, and VHF radio frequencies can travel along inside the duct, bending or refracting, for hundreds of kilometers.I have had the "ducting" affect happen on Lake Ontario, where I could communicate from the Rochester area to points on the norther shore, a distance of over 40 nautical miles. This happened with a 1/4 wave 3 dB whip antenna atop my mast at 54 feet above water level.

 

[George H260]

Gary: What a great graphic! A picture is worth a 1000 words.

 

[MKirk51]

Antenna Tests

Good explanations about the 3db vs 6db question. I'm in the process of replacing my radio and figured I might as well replace antenna and coax while I'm at it. There's a review of the 3db models that was done recently and the Digital VM-222 looks pretty good.

 

[BarryL]

anteanna height - power boat vs sailboat

one comment,VHF performance is directly related to the height of the antenna.Since power boats can't get the antenna very high, they usuallyuse a 6db. Sailboats have the advantage of putting the antennaon top of the mast. So you only need the 3db. If you are goingto put the antenna low, the 6 db would be a better choice butbe aware that heeling will decrease performance.Barry

 

[Brian D]

Build your own...

nm