High vs Low Gain VHF ariel for a cat.

Noddy

Noddy

Well-Known Member
Joined
22 Jun 2005
Messages
621
Location
Thames Estuary
Visit site
Hello,

I'm replacing my VHF ariel this winter. I'm not endowed with any real understanding of how a radio works but:

I read a short article on VHF ariels. What I took from it was that the usual type of ariel (looks like a welding rod) is qiute low gain. This is because when a yacht heels the signal is spread of over a wider vertical angle so the ariel will be effective at greater angles of heel.

I sail a cat, so heeling is not an issue. (At 45 degrees I am not talking on the radio and may well need an underwater ariel if it gets any more exciting).

So, assuming the boat is the right way up, would a High gain ariel (looks like a 6ft white fishing rod) give me an advantage by focusing the signal into a narrower vertical beam.

Would this advantage be in both transmitting and receiving?
What sort of gain (forgive pun) would it give, and how does this compare to cost?

Thanks in advance
Paul
 
In theory yes, but where are you going to put the antenna? If you are going to put the white stick 'high gain' thing low down, at or near coachroof level, then I think you would be better off with a standard yacht type 'not as high gain' antenna at mast height. Height is far more critical than the few db's difference you will get from a power-boat type antenna.
 
If you are going to put the aerial at the masthead use a 3' whip because even on a cat the movement at the masthead is such that a very narrow pattern will spend a lot of time 'looking' away from the horizon. If you want to mount it at deck level then one of those 54" fishing rod types can be used. The masthead whip will outperform the deck level fishing rod because height is the big advantage with vhf aerials because they are 'line of sight'.
When comparing gain make sure you are comparing apples with apples. A 1/2 wavelength 34" whip has a gain of 0, but is often quoted as being 3dB gain. In fact the claim should be 3dBi, the i indicating gain above a theoretical aerial called an isotropic aerial in which the radiating pattern is a sphere. This aerial is entirely theoretical. It would theoretically have a starting point for measuring gain 2.14db below the 1/2 wavelength aerial and with a bit of rounding up allows the claim that the 1/2 wavelength aerial has a gain of 3db over the isotropic. You can have a situation where a 34" low cost aerial claims to have a 6db gain, but it's all to do with fudging the figures!
 
Thanks a lot for the info. Couple of questions:

[ QUOTE ]
If you are going to put the aerial at the masthead use a 3' whip because even on a cat the movement at the masthead is such that a very narrow pattern will spend a lot of time 'looking' away from the horizon. If you want to mount it at deck level then one of those 54" fishing rod types can be used. The masthead whip will outperform the deck level fishing rod because height is the big advantage with vhf aerials because they are 'line of sight'.

[/ QUOTE ]
I don't understand this; surely if I have 10 degrees of heel at the masthead I also have 10 degrees of heel at deck level. So mounting the thing at the top of the mast will add the advantage of height - everything else being equal
Is this right??

A disadvantage might be extra windage aloft but cats have chunky rigs.

[ QUOTE ]
When comparing gain make sure you are comparing apples with apples. A 1/2 wavelength 34" whip has a gain of 0, but is often quoted as being 3dB gain. In fact the claim should be 3dBi, the i indicating gain above a theoretical aerial called an isotropic aerial in which the radiating pattern is a sphere. This aerial is entirely theoretical. It would theoretically have a starting point for measuring gain 2.14db below the 1/2 wavelength aerial and with a bit of rounding up allows the claim that the 1/2 wavelength aerial has a gain of 3db over the isotropic. You can have a situation where a 34" low cost aerial claims to have a 6db gain, but it's all to do with fudging the figures!

[/ QUOTE ]
/forums/images/graemlins/confused.gif I'd need that in earth language please.
I can see the need for a common base measure, but is a 6dbi ariel twice as powerful as a 3dbi one?

Sorry if I'm being thick
Thanks again
Paul
 
If you do not heel significantly, a 9dB gain would be fine. I would recommend a Shakespeare aerial (they make fishing rods as well, so know all about whipping).

You should get an excellent range.

On Play d'eau, I use both 9dB gain and 6 dB gain aerials. The 9dB far exceed the performance of the 6dB.

Proves it for me.
 
It's to do with motion rather than heel angle, I suppose. At the masthead the movement is exaggerated so a less focused, more rounded radiation pattern will have more of the signal pointed at the horizon at any one time.

The gain thing has nothing to do with power because the aerial doesn't increase the strength of the signal, it just focuses it. Because the signal you are trying to receive, or hit with your transmission, is usually at the horizon, more or less, the part of the signal that points above or below the horizon is 'wasted'. By focusing the signal you can achieve a gain, but only if the signal remains focused. This is difficult to achieve at the top of a moving mast. (Think shot gun versus rifle if it helps).

As a rule, a 0db gain antenna is about 1.5' long, a 3db antenna is about 6' long and a 6db antenna is about 10' long. The latter types are typically encases in fibreglass for a fishing rod-like appearance wheras the 1.5" whip is usually stainless steel.
(Each of these antennas can claim to be about 3db 'better' if you add a little i after the db so that a 1.5' whip is often referred to as a 3dbi antenna. However, it's the relative performance that is important).

A 6' antenna wafting around at the masthead may be similar in performance to the shorter s/s whip by the time all the pro's and con's cancel each other out, but the fibreglass rod will be vulnerable to uv degradation, bird strikes and bridge strikes! That's why you rarely see them deployed thus.

If you are still awake and haven't yet lost the will to live, I could go on: The most important factor in the performance of you VHF radio is to have an antenna system with properly sized cable, superb connections and a good quality antenna appropriate to it's location and environment. Top of mast - inaccessable, lots of motion, birds, bridges, relentless sunshine (except in UK) - short, low gain, stainless is best. At deck level where it can be lovingly cared for and movement is less, a fibreglass rod type is OK.

Apologies to any radio engineers and purists for the huge generalisations in the above!

(And, before someone else mentions it, I am aware that the i after 3dB is omitted on the antenna spec on my website. It will be corrected by the end of today. How embarrassing is that!)
 
Lot of science from peeps who know much more than me. However I did look into this as I have a snowgoose with a rear gantry and the existing antenna is on that rather than the masthead. It is only a standard whip and has worked OK up to now but as I am going to put a new one at the masthead (DSC and AIS have to be accommodated) I asked if a high gain one would be better. Everyone at the show who should know said don't waste your money, you won't notice the difference. I think the general opinion was that unless you have a problem with reception usually associated with masts at a very low level, high gain antennae are more trouble than they are worth and at the masthead the motion will be greater even on a cat. Don't do it. Even with a standard whip at 13 above WL I had excellent reception.
 
[ QUOTE ]
It's to do with motion rather than heel angle, I suppose. At the masthead the movement is exaggerated so a less focused, more rounded radiation pattern will have more of the signal pointed at the horizon at any one time.

[/ QUOTE ]
It still seems to be about angles rather than motion to me!
Firearms analogy very helpful: but it seems to be a relatively narrow gauge shotgun rather than a rifle.

[ QUOTE ]
A 6' antenna wafting around at the masthead may be similar in performance to the shorter s/s whip by the time all the pro's and con's cancel each other out

[/ QUOTE ]
Standard ariel is cheaper and easier. I guess this is the answer to my question. If its lots of effort and expense and I'm not going to gain anything then I might as well just replace what I've got.

I just don't want to do that without questioning the pros and cons.

[ QUOTE ]
Apologies to any radio engineers and purists for the huge generalisations in the above!

[/ QUOTE ]
Thankyou, Thankyou, and Thankyou Again!

Paul
 
I agree. I think it would be difficult to notice 3dB or 6dB difference in gain. And what is the advantage? The extra gain will only be needed when you are on the limit of transmission or reception, and then all it does is turn a weak and difficult to receive signal, into one that is only slight less so difficult to receive and still weak. 3dB or 6dB could easily be lost by purchasing an inferior radio set, or having poor quality connections.

I think the best thing you can do is try to have a good quality, reliable, marine grade aerial as high as possible.
 
You can think of gain as in a torch. Take the bulb out of the torch, and you have a low gain light. Put it in the reflector and you have a beam of light that brighter in the beam, but non-existent elsewhere. The buld isn't brighter, doesn't use more power or emit more light, just that you use that light better and more effectively.

Now to antennas.
The simplest (theoretical) antenna is like a bulb, and emits its radiation in a sphere - in all directions. Very useful if you don't know which way to point your beam, or can't control its orientation.

The most complicated puts out a signal in one direction like a beam. Useless in this marine application.

Next is one that puts out a signal like a disc. All round the compass, but almost none strianght up and straight down.
But thats limiting if the disc gets tilted (like on a heeling mast).

A compromise is the stick type antennas (they're not always aerial, sometimes they're terrestrial). Generally the longer they are the more like a disc radiation, and the shorter more like a sphere of radiation.

Choose from there.
 
Yes but bear in mind IMHO a cat is more susceptible to mast failure* so do have a deck level emergency antenna. Also do use large sized low loss antenna cable as using the thin RG58 over 15 metres or so can negate any gain you might get from a higher gain antenna.
Obviously the higher gain antenna are designed for Mobo so should be fine for a cat.

* A wild statement that is sure to ruffle feathers. Based on the stiffness of a cat unable to heel to a gust versus the resilience of a keel boat by being able to heel to a gust. Obviously we hope the cat's rig is really strong to handle the extra load. So we hope my statement is wrong. good luck olewill
 
[ QUOTE ]
Yes but bear in mind IMHO a cat is more susceptible to mast failure* so do have a deck level emergency antenna.
* A wild statement that is sure to ruffle feathers. Based on the stiffness of a cat unable to heel to a gust versus the resilience of a keel boat by being able to heel to a gust.

[/ QUOTE ]

Not actually a wild statement, as a cat's rig has to take the stresses of the gusts, whereas a monohull will heel over in order to relieve the stress. However, British cats in aprticular are built with thicker mast wall sections and stronger rigging than their equivalent mono designs in order to cope with these extra loads.

I have a masthead antenna which is the vhf, and a 1.1m whip on the davit/solar panel gantry for AIS. I am able to swap over aerials for VHF/AIS so have an emergency aerial inbuilt which is up and tested.

I achieve nearly double the range using the masthead antenna
 
You must log in or register to reply here.

Share this page

Top