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Old 16 September 2011, 05:14   #1
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Is anyone using a Moonraker 450 Vhf Aerial

Hi

I currently have a vtronix raider antenna like the one in the picture below on my rib.




The in the specifications its 3db gain , it is only 600mm long , when the rib is in the water the tip is only about 2.5m above sea level, mounted on top of A-frame.
when operating around Fleetwood , We rarely receive anything , and IF we do its Liverpool Coastguard Very Faintly.
I have cheked all connections and the coax for continuity etc , thats all ok , I assume the aerial is Rubbish !!.

I am concidering one of these



its a Moonraker 450 4.5db gain approx 1.5m in length.

Is anyone using one ?

Any Comments or Suggestions

Alan
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Old 16 September 2011, 07:10   #2
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I have a similar one, I don't think it is exactly the same, but works perfectly and I am in the same area as you. But to be honest there is not usually a lot a radio traffic in the morecambe bay area.
I have attached picture of where it is mounted. A friend who as vtronic raider has commented that I seem to get more radio traffic the he does, so maybe the extra length does help.

Keith
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Old 16 September 2011, 07:19   #3
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The higher gain has to be a bonus. You'll have to be careful to remember to drop the ant. when on the road - it maybe prudent to fit it on the starboard side as they're a bit of a branch catcher.

Don't ask me how I know this...
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Old 16 September 2011, 07:42   #4
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Be careful when looking at quoted gain. There appears to be a 'marine' gain that is quite often quoted and is about twice the real gain. Classic marketing ploy to catch the unwary.
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Old 16 September 2011, 08:12   #5
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Thanks for the reply,s

I think I will try the new aerial a go , Its ordered and on its way

Our intention is to use the rib extensively off the N.W coast of Scotland next year in the area between Skye and Cape Wrath , So it would be prudent to have the best radio signal possible.

A.
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Old 16 September 2011, 08:21   #6
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Quote:
Originally Posted by Erin View Post
Be careful when looking at quoted gain. There appears to be a 'marine' gain that is quite often quoted and is about twice the real gain. Classic marketing ploy to catch the unwary.
Duley noted ,Thankyou , I think the main "Gain" I will have is height as Vhf is Line of sight .
And the current aerial (that came with the boat) is so rediculously small , Possibly ok for the solent and the like , But Not for my intended use.

Thanks A.
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Old 16 September 2011, 09:40   #7
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Your new antenna will outperform the old one because it has more gain not because it’s longer. Longer doesn’t equal higher – well not much, anyway.

Gain is achieved by focusing the signal. (All antennas are designed to do is manipulate the signal, they are passive, they don’t add power to the signal, they just focus it differently – there are no ‘high power’ antennas).

Here we go with the old football analogy: If you could see the signal radiating from a basic antenna it would look like a football stuck over the antenna. The signal would be radiating out equally in all directions, up down and all around.
As antenna gain increases the football becomes flattened, it’s more like a donut now, so that more of the signal radiates out towards the horizon and less points up and down. As gain continues to increase, the radiation pattern looks like a dinner plate with most of it pointing out to the horizon and very little pointing up or down.

This is good as long as the horizon and the antenna stay still. If the antenna waves around, the high gain dinner plate will only be looking at the horizon for part of the time, it’ll spend a lot of time looking at sea and sky. This is why sailboats have low gain antennas mounted on their masts.

Power boats tend to have higher gain antennas because they are considered a more stable platform than sailboats, but you must make sure the antenna stays vertical as much as possible. Swept back antennas might look sexy but they don’t perform very well!

Mounting the antenna higher gives you more range because it can ‘see’ further. All things considered height trumps other aspects, but is tough to achieve on a RIB. So you have to use gain even though any boat is an unstable platform and the flatter radiation pattern hurts.

So, fitting a longer antenna because it would appear to make the antenna ‘higher’ is a little off beam. Generally speaking the longer the antenna the higher the gain but the more it will waft around.

For this reason I think a 3 dBi whip mounted on a pole would be a good system for RIBs.

By the way, gain is usually measured two ways – dBd and dBi. The first is gain over a ½ dipole antenna and the second is gain over a theoretical isotropic radiator. As long as you know which is which you can compare, but vendors often don’t tell you which measure they’re using.

I think your Raider is 3dBi and your Moonraker 4.5 dBd, this means you’ve gone from 3 dBi to 5.6 dBi. (Add 2.14 to a dBi figure to get dBd). You’ve got a lot more gain, but do keep that antenna vertical.

Wow, sorry to ramble on!
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Old 21 September 2011, 13:31   #8
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Just a Quick update.

Received my new Moonraiker 450 aerial yesterday .

I plugged it into my vhf radio last night , (aerial just propped up on back of boat).

and What do I get " Liverpool Coastguard" clear as anything booming in.!!

I then plugged the vtronix aerial in , and virtualy nothing, but a very very feint noise in the background.

Plugged the moonraiker back in , and signal was there as strong as anything .

Just to satisfy my own curiosity , I did a continuity test again ,inner of pl259 plug to tip of aerial ,and a "inner to outer of coax test to check the insulation wasnt shorting out, everything checked out ok on the vtronix.

The verdict is the vtronix is just a load of CRAP !!! , but it can stay fitted , just in case.

and The moonraiker is Fantastic , Just got to get the A frame drilled and fit it now.

MARINE-450 156MHz MARINE ANTENNA | eBay
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Old 21 September 2011, 13:34   #9
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Good job! Sounds like the Vtronix was faulty.
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