VHF Aerial Length

[JB]

Does the length of the vhf aerial make a difference when transmitting, i.e. distance you can transmit etc? I have purchased an ICOM and i'm trying to work out which aerial is the best / most suitable. I have a 4.5 meter side console boat and the aerial will be mounted on side of the console if i can.

Jas

[Owen]

Aerial length does make a difference to a certain extent.
All aerials have to be a certain electrical length. This length is related to the wavelength on which you are transmitting. Usually they are an equal fraction of the full wavelength.
For example a full size 1/4 wave 27mhz aerial will be 9 feet long. That's too big for cars, so they make them physically shorter but wind wire around them to give the electrical length of 9 feet. This works electrically, but loses some performance
The higher the frequency, the shorter the physical length needs to be and still achieve a multiple of 1/4 wavelength.
I'm not sure exactly what length a 1/4 wave aerial is on VHF, but it's probably 7 feet or so.
Quite often the shortened versions have to be tuned to give an acceptable SWR (Standing Wave Ratio). This is the amount of signal that bounces back from the aerial to your transmitter due to the wavelength of the aerial not being exactly that of the frequency you are transmitting on. Too much SWR and your final transistors in the transceiver will blow.

Clear as mud eh

The short answer is bigger is better

cheers,

Owen

[Hagar]

G'day JB

I am no authority but have always been told bigger is better . But then that goes for a lot of things in this world - eh . Me personally I would go for the highest one that will fit without it being a problem when stowed down and protruding out past the boat front or rear .

Chris

[seatime]

JB,

Owen is correct, all transmitting radio aerials are tuned to length, even the length of cable is important for tuning on mf/hf . don't think it will matter a great deal with vhf in small boats if u use an off the shelf whip aerial.
I've had the top few inches broken of a vhf aerial and it still worked ok. it will only be when u get to the limit of range for your particular unit that u can't transmit, but it will still recieve for a few more miles. regards steve

[mackmauler]

jas, I go for a 6fter gives me all the range I need and importantly easy to lift a rod over it if one of those fast critters circle the boat, had a 10fter ripped off once lol.

[Owen]

Just had a quick look at a couple of specs on VHF aerials.

They show a 3.3 ft unit as having a 3db gain
An 8ft unit has a 6db gain

Each increase of 1db is an effective doubling of output (though not necessarily range).

Been a long time since my novice studies though so don't yell at me if the above is not 100% correct

cheers,
Owen

[25_ponies]

JB, I think you'll have problems trying to find a place to stow anything above a 1.8m, and as Rob said, it will be a PITA if you have to get your rod around it.

Owen, I think you'll find a 3db gain doubles the output, not 1db.

Cheers,

Ponies.

[Spaniard_King]

Look JB, don't worry about it. We are used to you not havin a radio

I think VMR jujst about have your hand signals down pat now [smiley=2vrolijk_08.gif]


I recon the 8 fter would be overkill on ya boat mate

Garry

[Owen]

Ponies,
I said it'd been a long time
Yes, you're right now that there's been a chance for those few brain cells to emerge from the fog.

cheers,
Owen

[seatime]

jb,
anything over 6' should be okay, vhf aerials works on line of sight anyway, and power output determines range for transeivers. i reckon u should always buy to the limit of your expenditure when it comes to marine gear, when the day comes that u need this gear in anger u will be glad u bought the most powerful unit u can afford. same goes for motors, how many complaints have u heard about overpowered boats compared to underpowered? (u don't always have to go flat strap). cheers steve

[rajawolf]

Where on the Gold Coast can you take your aerial to find out if its VHF?. I have one that came with the boat.. I have hooked up the VHF radio i own but will not transmit till i know for sure it is a VHF aerial. ..receives a good signal though

[JB]

Thanks all for the replies, will most likely get the 6 footer, but looking at the navman 2.5, they seem like a really good aerial.

Garry, .. hahah very funny. Yes everyone by now has gotten used to my hand signals i'm sure

Not much longer now has spoken