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Old 14 January 2010, 04:41   #21
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Originally Posted by Polwart View Post
I think your misunderstanding me. I'll post a piccy later.
Ah, that would be good

Ah too, I just read Malty's post.

Think I see where you are coming from!

Anyway, outside for a bit.

Sorting out a two battery set up on a rib.

Probably back on here for advice when the whole lot melts!
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Old 14 January 2010, 05:08   #22
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The best advice is to have good wiring, both in terms of power connection to the set and unbroken coax to the twig.

The actual range difference for adding a few feet is only a couple of miles, but being just out of range is a matter of just a couple of miles.
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Old 14 January 2010, 05:35   #23
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I think Malthouse has addressed the point I was getting at.

Here's my sketch to illustrate the point I was asking/making - i've added Malthouse's 2/3rds rule as dotted red lines.

Option A - an antanae of length x mounted on the transom, effectively at sea level.

Option B - a handheld radio opperated at head height

Option C - an antanae of length x mounted on an aframe or pole, effectively 'y' above sea level.

Option D - a shorter ariel mounted on the same pole or aframe.

Can the handheld actually give better range than a long aerial mounted on the transom?

Would a little very short ariel (like a handheld) give nearly as good performance as a big whippy one (which have storage/snapping issues) if mounted on a suitable pole/frame?

oops - forgot to attach the pic
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Old 14 January 2010, 06:07   #24
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Can the handheld actually give better range than a long aerial mounted on the transom?

Would a little very short ariel (like a handheld) give nearly as good performance as a big whippy one (which have storage/snapping issues) if mounted on a suitable pole/frame?
Great sketch

Handhelds are more forgiving of being used at an angle but this combined with the lack of 25w is always going to limit their range much more than even a stubby for a fixed set at the same height.

A very short rubber ducky mounted above head height would be a good trade off. The only down side is that I find they are less good at receiving.

Our RIB had until recently a Pacific 1m stainless whip on the A frame, which was a good all rounder and pretty robust. We are about to change that to a Seamaster Pro (still Pacific) 0.3m version, but because of the Pro mount it can be changed to something bigger if the occasion suits. The previous setup meant I could talk to Jersey CG (35 miles away) as soon as I got to either side of Alderney, with clear comms both ways; so if the smaller one is less effective I will know.
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Old 14 January 2010, 08:51   #25
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Would a little very short ariel (like a handheld) give nearly as good performance as a big whippy one (which have storage/snapping issues) if mounted on a suitable pole/frame?
Now there lies a bit of radiographic geometry....... I forget the physics behind this, but the theory is the longer the antenna, the more "disc" shaped the transmitted signal will be.

So, two extremes - a 3" stubby and a 4m whip which somehow manages to defy gravity & stay vertical. The stubby will produce a more "ball" like transmission patern,whereas the 4m will produce a near disc, which in theory gives better power density as it;s only going in "one direction" as opposed to going everywhere.

This is where practicality comes in - on a rib you are not perfectly aligned to the horizon at all times, and even less so with a handheld, so the fact that the stubby on the handheld is nice & compact also helps because you dont need a spirit level down the side of your handset!

Conversely, a lo-o-o-ong whip on a rib will maybe fire it's signal straight over the top of the recieving set's antenna..... should the rib be at an angle when transmitting becaue of rolling in big waves, the transmitting "disc" will roll with it.

Somewhere in the middle is a bhappy medium. I have both a 1.25m whip with a patch panel under the consol so I can easily swap to to a Vtronix raider. Haven't noticed a difference in either in either TX or RX.

(And before you ask, it's a phased introduction of technology - eventually one will be the AIS stick & double as the VHF backup antenna. At the moment, I'm swapping them around to find out which works best with the VHF.)
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Old 14 January 2010, 15:09   #26
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Does anyone know the wavelength at 156mhz?
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Old 14 January 2010, 15:26   #27
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Does anyone know the wavelength at 156mhz?
1.9230769230769231 m
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Old 14 January 2010, 16:25   #28
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1.9230769230769231 m
I bet you knew that off the top of your head Brian

The most Idea antenna length for this wavelength is the wavelength it's self, so 1.9230769230769231 meters
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Old 14 January 2010, 16:29   #29
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I bet you knew that off the top of your head Brian

The most Idea antenna length for this wavelength is the wavelength it's self, so 1.9230769230769231 meters
Andy,
Ermmm............................................. ..NO had to work it out

Have to be careful with anntena tho' some are base, loaded to make them Electomagnetically "look" longer.......
Antenna physics are wild......one reason I haven't waded into this thread!!!
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Old 14 January 2010, 17:19   #30
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Plus on 84 I can hear the CG and sometimes the vessel responding.
84 is duplex-you will only get the CG side of the conversation
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