VHF areils

[Guest]

Lets talk VHF ariels and the why and hows. I don't know much but in a thread about AIS our beloved Island Time mentioned something about newer ones and longer or shorter ones, something like that. Since then I have had a gentle suss to find there are differing alsorts when I thought a VHF ariel was a VHF ariel.

 

So I want a areil for the VHF to go on the top of the mast, I am assuming as that's where most go it is the best spot. I also want to fit AIS but are told it's best to have a areil for each rather than use a splitter. The preference was for the AIS, or one of them, knot to be up the mast, nasty windage and all that.

 

Who knows this stuff and what say you on the best way for me to go is?

[Island Time 1,230]

Hi KM, me again!

My thoughts are (without the technical crap which you can have if you like) a 3dbi gain wire antenna for the masthead and main VHF. This will give the best performance over a wide angle.

A standard fibreglass whip mounted on the stern rail for the AIS. Gives redundancy (with the correct cable connectors and length) - if the rig comes down simply switch connectors. Simple and practical.

[Jon 360]

I T has it correct

I have a splitter and it works fine but that doesn't give you a backup.

Then again having our AIS arial up top gives us ships showing on the screen at over 40nm

[Jon 360]

But if I was doing your boat I would do both,

Run a splitter and have a fg whip as backup

Pm me and I will tell you where to buy

[wheels 543]

Seeing as I used to do this in years gone by.

I suggest a whip type for the top of the mast if you want to put one there. If you would rather go rail mount, then as high a Gain as you can afford, because you don't have the height. An antenna’s gain is a measurement of its focused radiation and sensitivity pattern. So lets say you have an Antenna that radiates in a perfect sphere. That is called "Omni Directional" or "Isotropic" depending on what school you got dragged up through. Gain is measured in dB (decibels) and the Omni antenna would be said to have a gain of 0dB. or lets say a light bulb with no light shade. By making the Antenna longer, you start to make the radiation pattern more "focused". Like adding the light shade. So a small handheld radio will have a Antenna of about 1dB in gain. Making it longer still, as in a Whip, you will get an even more focused radiation pattern. the energy that would have been radiating up into the sky above, is being pushed out toward the Horizons more. A Whip will have about a 3dB gain.

Oh by the way, every 3dB increase is double the power.

The bigger Fibreglass jobs are even more focused and you can achieve a 6dB gain and the big 5m'ish jobs will have around 9dB of gain. But who in their right mind would want a 5m antenna for their VHF. So then we get into the "collinear" designs. They look about the same as the smaller Fibreglass rod jobs, but they have windings called "elements" stacked inside or sometimes seen wound around the outside of the glass rod. These things have 9dB and even much higher gains levels.

[Island Time 1,230]

Hey wheels, that's why I said I'd leave out the technical crap! Most people don't want to know why, just want an answer!

 

Lets see if I can give a plain English description of gain.

Positives;

Higher gain gives a stronger output = longer range.

 

Negatives;

Higher gain means a narrower angle of horizontal transmission - So they are not so good when heeling, and may not work at all at high angles -esp 9dbi ones.

 

Therefore;

A 3 db Gain antenna at masthead is the best compromise, as it gives a good signal at all normal angles of heel.

 

Ok?

[Steve Pope 243]

I am also in the process of putting a Pacific VHF aerial up the top of the mast and have a splitter for AM/FM radio and maybe for he AIS that still has to come. Like Knot me I am flying a bit blind on this. Should I have 2 aerials up there?? also I have run the VHF co-ax through the same gland as the mast head wiring, could that be a problem?? (interference?)

[Island Time 1,230]

No, you should not have 2 Ariels up there! The cable gland should be OK, I'm sure your masthead is not using much power! If there is a problem, it's more likely to be when you transmit, your LED lights (Right!) at the masthead might glow! Should not though, if the Ariel coax is good and well shielded.

 

Splitters can be problematic, and responsible for quite a bit of loss in the signals. I have a 0 loss splitter available with VHF/AIS and FM radio ports if you want one send me a PM. Same for LED masthead lights if you need one.

 

One base loaded stainless whip of 3DBi gain is the ideal for the masthead....

[Guest]

Good question Steve, I was thinking the same thing.... while corners of my brain was just repeating windage.... windage. It makes one wonder just how much windage would 2 cause? But to know that you'd need more inputs.

 

Lads, nice answers both by the way, you mention big and FG ones which I know are BIG, we're only a baby 30fter. Am I to assume you mean one of those skinny stiff wire ones around 18" long and a 2mt odd outrigger like off the bum somewhere?

 

If so, the one at the back could be smaller if it got a little more height like on a small structure that's above the tiller for other gear to hang off. Say the top of a 18" being on the same level as a 2mt would be = similar results???

[Guest]

Ah, you 2 are talking about wiring touching other ones like masthead light/s or windgear. Is that a big no no?

[Chewing Gum 17]

I don't know much about aerials but we do have one of those skinny short wire ones which is probably ancient and found it performed pretty well as we were able to talk to Maritime Radio down most of the North Island West coast on the VHF. It is quite high - mast is about 16 metres

[Island Time 1,230]

Hi Guys,

 

Like this on the masthead http://shakespeare-marine.com/antennas. ... nna=5241-r

or http://www.pacificaerials.co.nz/MarineP ... tenna.aspx

 

Low weight, low windage, good performance

 

Standard glass whip for the stern, or, if you are really finicky, another one of these mounted as high as you can. A 6db gain one might be better,

[idlerboat 116]

I am going with the stainless mast top "Thingy" for the reasons stated previously.

and..they are far less prone to birds beaks (may not be a problem in NZ but here plastic covered wound helicals get the covers eaten).

I have found that high quality, low loss coax makes a huge differance.

We have no problem in clear tx and rx to a high level. Often hitting repeaters way more than expected.

We have no splitters or joiners, and surgicaly clean and corrosion free connectors.

As an aside...thunderstorms in line with vessel to intended station have knocked out our transmission more than anything else we have expirienced.

We also run HF becuase vhf just dosnt cut it in more remote areas.

Sat phone is great (and I would like one) but dosnt allow broad "open channel" emergency comunications so be careful ...

Dont trust vhf in close coastal passages with high local terain in remote areas.

Vhf should always be considered "line of sight".

[Guest]

Oh good golly I'm being edumicated. But god stuff so that's all good.

 

How about this puppy for the masthead for the VHF? 2 birds one stone = less windage???

FREQUENCY RANGE 156/162 MHz

GAIN AVERAGE 3 dB

IMPEDANCE 50 ohms

POLARIZATION Vertical

SWR <1.4 at 156.8 MHz

MAX INPUT POWER 50 W

DC GROUND Reads 10K ohms

ANTENNA LENGHT 850 mm (33")

ANTENNA WEIGHT 345g (12.2oz)

TERMINATION Cable

 

and this one for the bum for the AIS.

FREQUENCY RANGE 156/162 MHz

GAIN AVERAGE 6 dB

IMPEDANCE 50 ohms

POLARIZATION Vertical

SWR <1.4 at 156.8 MHz

MAX INPUT POWER 50 W

DC GROUND Yes

ANTENNA LENGHT 2.3 m (7.5')

ANTENNA WEIGHT 300g (10.6oz)

TERMINATION Cable

 

Those the sort of numbers I should be looking for??

[Island Time 1,230]

yep, looks fine

[Guest]

I suggest a whip type for the top of the mast if you want to put one there. If you would rather go rail mount, then as high a Gain as you can afford, because you don't have the height.

 

Being a radio amateur I've been caned severely here for getting technical... so, I will say this. Yachts heel over (unless a multi-hull) so high gain antennas are not advisable (due to their "polar diagram"). Stick to the BASE LOADED HALF WAVE unless you can keep the antenna upright. Go for double shielded high quality coax if mast head mounted as AIS is 2watt (Class and will lose some power due to resistance.

Personally I would go for 2 separate antennas if running VHF coms and AIS.

[Guest]

I noticed some mention about emergency use of VHF as opposed to HF. This will cause a stir... if voyaging well away from shore and you really need to get help, apart from the EPIRB slash Sat Phone... get a "Airband" hand held transceiver and seal it in a bag set to 121.5mhz. If you have stepped UP into your life raft and see a vapor trail or flashing lights in the night sky (Aliens don't monitor..?) you will be heard by the passing aircraft. By international convention, all aircraft have monitoring on that frequency. Its illegal to transmit without a license etc etc but not in a genuine emergency... being in a rubber round boat that's only guaranteed to survive 30 days I think qualifies!

[Island Time 1,230]

Totally agree re the Ariels. the only reason for a 6db one on the stern is that, should you lose the rig, the boat should be upright! And the 6db one should give you a bit more range. The 3dbi one for the masthead, stainless not glass, is the primary one. Stern one for AIS and emergency antenna.

[too_tall 15]

Wetdream, a hell of a lot of people think they understand how radio works, but really, don't. No understanding of fresnel zone, no understanding of how gain really works, no understanding of how atmospheric conditions effect radio transmission, etc. Your advice, and Island time also, cover what I consider to be the most useful - without getting overly technical.

 

The masthead only needs to be 3dB. There is quite sufficient height on most masts to get between good and great range from that height. a high gain on the stern as a emergency antenna can be useful, but there is quite a chance that if the rig is no longer, you may well have lost your mounted high gain off the stern also. Put one in a locker somewhere if you can store it and mount it in am emergency. It will never get the range that a 3db up high will anyway, so mounting it for the off chance that you lose the one on the mast is, in my mind, counter intuitive.

 

An Airband hand held is quite useful in the right situation. At night, you can see nav lights on aircraft from quite a considerable distance. By daylight the vapour trail is even more distinctive assuming no cloud cover. But, a satellite phone with a hand held GPS, and an EPIRB would make me feel more comfortable should I be bobbling on the waves enjoying my lilo boat.

[Guest]

Stick to the BASE LOADED HALF WAVE

That's knot technical at all Wetty But all good as when I combine all the posts you all are painting a very good picture for this lad.

 

I do carry a old 121.5 EPIRB as well as a full fruit 406. A handheld VHF will be in the grab bag. I have a Satph but no HF. If she goes down all switches will be flicked.

 

Good point about the whip areil TT but as I want to use that for the AIS I'll ponder what damage limitation things I can do.

 

If I carried a spare smaller whip, a 4.5fter I think I saw and only a 3db, does anyone know what sort of range that would get when used with a VHF? Just thinking what one of those would be like for a stashed just in case spare. Screw that onto the damaged big whip base and ...... ?