Spare VHF aerial 3db or 6db?

[NPMR]

I have plans for a spare/emergency VHF aerial mounted on a small mast at the back of our sailing boat. (We have whip aerial at top of mast).

Would this be better with another whip at 3db gain (whatever that is), or 6db which seems to be a chunky GRP MOBO looking thing.

Is there a technical superiority for one over the other in the lower, nearer to sea-level position? Price seems to be similar, so why?

 

[twisterkai]

Might get shot down for this one, but, IIRC, your vhf manual will recommend the level of gain required from the aerial, so in the first instance I'd be inclined to check the destruction manual.

 

[Benbow]

Gain is the apparent increase in power your antenna gives as a result of 'focussing' the signal it transmits. A gain of 3db means you are doubling the apparent power; 6db quadrupling it. If all the power is focussed into a flat ‘disc’ around the antenna you have a high gain and see a big signal within the beam, but if you tilt the antenna, as is usually the case on a sailing boat, then the power is all going up into space or down into the sea. That is why sailing boats are usually advised to use lower gain more omnidirectional antenna.

Anyway, in practice, the sheer power you are delivering is rarely the limiting factor. VHF is pretty much line of sight, so you are far better to double the height than to double the power.

So get the 3db whip!

 

[NPMR]

Damn Good Idea but having now read the book, from front to back, and apart from some nice illustrations re heeling angles and a discussion about 1m whip aerials being popular, but wider beam from a shorter aerial being more universally effective (I quote) it implies that a smaller vessel i.e.sailboat would be better off with a shorter aerial but nothing about 3db or 6db 'gain'.

Still confused.

 

[Oldhand]

Increased gain is achieved by using increased directionality in the vertical plane. As mobo's don't normally steam with any permanent heel, they can use an antenna with a narrower vertical beam width than a sailing boat. Thus the large GRP whips mostly seen on mobos are 6db gain antennas, wheras the smaller spindly whips seen on the masthead of yachts are 3db gain antennas.

So the answer to your question is if you think you will need your low mounted emergency antenna only if you have lost your mast or will not be under canvas for some reason, a 6db antenna would be best. I suspect you already know VHF range is commonly little more than line of sight, so a low level emergency antenna will never have the range of a primary masthead unit, whatever their relative gains are.

 

[gjeffery]

Sailing boats normally use 3dB antenna because the vertical beamwidth is greater. The radiation pattern is wider in the vertical plane, so enabling the antenna to radiate usefully way when the boat and the antenna is heeled. If the antenna on a sailing boat had a tight vertical beamwidth, eg like a flat disc, when the boat heeled, all the power would be radiated into the sea, or up into the sky.

Motor boats tend to use 6dB antenna because they heel less, and the tighter vertical bandwidth is not so critical. The higher gain may compensate in some degree for the lower antenna position.

Whether 3 dB or 6dB, gain at the antenna should not matter to the vhf set - it will see the same load.

 

[Danny]

You need to think in terms of your likely situation when using it. You don't get ought for nought and the difference is in the shape of the beam emitted by the antenna. The 6db is 'shallower' - most energy is emitted at 90 degrees from the antenna. The 3db is 'deeper', emitting more energy at, say, 45 and 135 degrees than the 6db. So, if you're heeled over then the 3db may well send more energy in the right direction - IE towards the horizon.

 

[Talbot]

If you are looking at a small aerial to site as an emergency system - have a look at the vtronix helical antenna. For best results the boat needs to be upright, but it is a flexible antenna that is also short. I am using mine as the feed to AIS engine, with appropriate plugging to enable it to be an emergency antenna for my VHF.

 

[steverow]

Explanation of Antenna Gain..for those interested

An explanation of antenna gain for those interested....
The term dB stands for decibel. This is most commonly associated with the measurement of sound, but can be applied to any logarithmic entity, in either a voltage or power format.
It is used for the measurement of radio frequency gain...but to be useful the dB has to referenced to something.
By itself it is meaningless.
For the measurement of antenna gain, the test antenna can either be 0dB referenced to a standard test dipole having a gain of 1 (unity) known as dBd, or to the "isotropic" antenna, which is a theoretical only perfect antenna known as dBi.
Without referring to the boring tables, roughly speaking a gain of 3dB refers to a doubling of power and 6dB a quadrupling of power.
In reality there is no gain on an antenna as it is a passive device it is not capable of amplifying.
Gain is simply a term used to define the direction of maximum radiation...ie the power on a beam antenna (like aTV aerial) is gathered up and radiated in one direction only the gain being the difference in the`one direction and what would have been radiated all around if there were no directional elements there.

In colinear and vertical antennae fitted to boats and vehicles, it is the angle of radiation that is important, and gain comes from the fact that the RF is collected up from the top and bottom of the antenna and radiated out sideways.
The 5/8 wave antenna is probably the most efficient at doing this.

Hope this explanation makes some sense to somebody, but it is pointless bandying about dB's without some meaningful explanation.

Steve

 

[Oldhand]

Re: Explanation of Antenna Gain..for those interested

Not sure as to what you mean by "RF is collected up from the top and bottom of the antenna and radiated out sideways". Vertical radiation pattern of a colinear or vertical antenna is determined by the RF current distribution along the element(s) unless there has been a change in physics since my early professional life.

 

[steverow]

Re: Explanation of Antenna Gain..for those interested

In laymans terms, I thought that was the best way to describe it, rather than current distribution along the length of the antenna...and then we have the added complication of why most radiation occurs in the first few inches of the antenna...and a host of other issues including Q of the loading coil which affects the radiation pattern...so I did chose my words carefully.
The 5/8 wave "flat tops" by virtue of the untuned 1/8 section at the top.
The angle of radiation is mainly affected by the counterpoise.

Steve.