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What an excellent drawing for us of little knowledge.

I TOTALLY AGREE! awesome pickie that shows the lower the polar (looks like a doughnut) the more signal is DIRECTED towards the distant receiver. Some think of GAIN as extra power. A 25w transmitter fed into a 50 ohm resistance will only produce 25w dissipation. SHITZA!! Getting technical again!! Basically, RF radiates from an antenna in all directions so most is wasted as the radio we want to reach is on the horizon... not mars! 5/8 or co-linear antennas reduce the "waste into space" but as that diagram shows, TILT the antenna and you create a "null" or dead spot towards the horizon which defeats the purpose. A 3 phase powered triode valve linear running 289KW would solve the problem.. start fires, bring down aircraft... pre-cook your catch... and forget the need for a vasectomy either!!!.... :lol:

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A 6dB antennnae will still give a good "throw" to Horizon while on a heel.

Yes it will if not for its length to create 6db. About 8 feet at 156mhz!!!!... at the top of the mast???

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We have to replace the fibreglass antenna on our fishing boat probably every second year - they just are not that tough if things get caught on them - such as a line with something big on the end, or the road cover. I cant see the rear of a yacht being that much better!. You can get stainless folding mounts for them, which I have thought about. But then I suspect the antenna will just snap.

 

I will say it again!! Go for a SIMPLE base loaded (has a coil / capacitor to match it to 50 ohms inductance) stainless steel whip! The only draw back is you need a good "counter poise" or earth (steel boats.. no problem!). The shield or outer part of the coax is phase neutral and needs some conducting surface to connect to.

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Ground planes/counterpoise are another topic again. The mast should be connected via lightning protection to the keel - good counterpoise. Therefore the ss 3dbi Gain antenna on the masthead is good. A proper Ground plane from the stern rail is unlikely (but possible). Hence my recommendation earlier in this thread for a F/G whip on the stern - no ground plane needed :) Simple and works! Low wind resistance, and redundancy. Main VHF via masthead, AIS via transom antenna.

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One more comment then I will go cook dinner on my triode!

Modern day transmitters and receivers like to "see" 50 ohms reactance (like resistance but with magnetic flux). No matter what the antenna, the closer it is (with feed line) to 50 ohms then the maximum amount of RF is dissipated by the antenna (sent out). Any CB'ers here? You will know what SWR is then. 1 to 1 is the target which means all the energy produced by the radio is leaving the antenna. In other words, a properly tuned 1/2 wave with high quality low loss feeder will out do a stacked high gain vertical any day that's not tuned right! Sitting in the marina at Sulphur Point using a 5/8 whip I could clearly hear Auckland boats on simplex (no repeater) but the mast head 1/2 wave could too.. just not so clear. Heel the boat over and I guarantee you the 1/2 wave will out do anything.

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Ground planes/counterpoise are another topic again. The mast should be connected via lightning protection to the keel - good counterpoise. Therefore the ss 3dbi Gain antenna on the masthead is good. A proper Ground plane from the stern rail is unlikely (but possible). Hence my recommendation earlier in this thread for a F/G whip on the stern - no ground plane needed :) Simple and works! Low wind resistance, and redundancy. Main VHF via masthead, AIS via transom antenna.

 

DUH!!! at 156megs you don't need a ground plane the size of the Queen Mary! The mast itself UNEARTHED is enough! Your push-pit is ENOUGH!! glass whips break and contain their own ground plane which is why they are so long and get in the way of decent size snappers coming over the stern. A 1/2 wave s/s base loaded whip earthed to the mast or push-pit / davits is for a yacht, a sensible choice.

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Ok WD, how about an explanation? As I said, I'm not a radio engineer, nor a ham, I've just picked up a few things over the years. What I know about Ground Planes/Counterpoise comes from HF, where the consensus is 10sqft of metal in RF contact with the water. I do understand that VHF is different due to the different frequency range. What are the required specs for a counterpoise in the VHF freq range?

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KM & Redline, I'd suggest for low power Plotter and AIS a low power Netbook - Trade me has them under $300.00. Load Linux and OpenCPN, load the NZ Charts, All free. With the right power setup, you can get it down to about 10w - like a single bulb. It can be connected to a speaker, and provide a loud alarm if a vessel approaches with AIS!

Cost does NOT include the AIS Transponder!

10W is 8 single bulbs on my boat :)

 

The only reason I mention plotter is you can get some with Class B AIS installed for close as the same coin as just a AIS transmitter. Knot massively happy with the idea of assembling a few things into a plotter/AIS. I know many have and it works OK but they are generally on larger drier boats used by people who know things or have the time to play with things, none of which are me.

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Hey KM, I've seen the cheap plotters with receivers, but not yet with a transponder... I'm interested if you know of one. Basically the plotter is a PC anyway, often running Linux in the background hidden from you by a menu system. Same issues as with a small PC, but then you can also use it for email, weather forecasts etc... Either via HF, or via satphone.

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Same issues as with a small PC, but then you can also use it for email, weather forecasts etc... Either via HF, or via satphone.
I'm yacht racing fella knot cruising complete with 10min Facebook updates :) :)

 

Yes that ONWA was one I had in mind. I've used the 8" which is identical to the 5.6" bar the size. I found it OK enough I would buy one myself.

 

Satph, no HF for me.

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There is a tactical advantage in getting gribs and weather fax, and being able to display the progression on your screen, which is why the big race boats do it. You can too, without great expense, albeit not quite as fancy... Also being able to send an email home is good for family. But not so much in the Solo Tasman, as you will have a tracker :D

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Ok WD, how about an explanation? As I said, I'm not a radio engineer, nor a ham, I've just picked up a few things over the years. What I know about Ground Planes/Counterpoise comes from HF, where the consensus is 10sqft of metal in RF contact with the water. I do understand that VHF is different due to the different frequency range. What are the required specs for a counterpoise in the VHF freq range?

 

HF.. 1 to 30mhz (;longer wave length) needs a LONGER or larger earth. Shorter the wave length..less and so on. 156mhz needs about the same in wave length, which in this case is less than two metres.

If you were transmitting on 7mhz you would need an earth conductor of about 40 metres OR LONGER.

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I was going to stay out of this thread. I try not to come across as a know it all, but it seems that is the way it is. But there is just too much inaccuracy going on in this thread (but I say that in a nice way, so no one please, don't take offence) so I will attempt to explain and I will try to keep it simple. But as I said to KM on Friday, when it comes to Antennae theory, it's so damn complex.

Firstly, seeing as it is the last discussion, the ground plate on a Hull has nothing to do with being a "ground" in itself. It would have to be huge if that were the case. It is there for the sole purpose of coupling to the Water. The Water is the ground. For HF, where the transmission power can be anywhere from 100W up, the plate is all about getting that power to the Seawater. HF(High Frequency) is AM (amplitude modulation) which means huge swings in Voltage. If you grab a Backstay when someone transmits, it'll deck ya. That same power going out the antennae is also going out the ground plate and the area of a ground plate serves two things. It allows a good conduction area and also stops the water boiling on the plate which causes bubbles and loss of conducting area. However, via capacitance, it is also acceptable to have an earth plate mounted on the inside of a GRP Hull. It will couple to the water on the other side just fine.

VHF(very high frequency) is FM (frequency modulation). It works so differently to AM, you can not compare the two in it's operation and antennae requirements.

Let me go back to the beginning about propagation of the radio wave. It has been bugging me about that 6dB and 9dB thing. Oh and dBi is also inaccurate. dBi is not used because although in theory you could have an Omni directional pattern, in actual practice, it is impossible. Firstly because we are working with a Rod of some form. So there is no transmission at the top or bottom of that rod. What we end up with is a Transmission wave shape looking more like a Donut. This in itself is slightly directional, to a gain factor of 2.1dB over the theoretical omni transmission wave form. Dbd is the term used, which is dB over Diapole. Dipole is the type of antennae we are using.

OK, I know AC would have yawned and left by now. So to those that maybe still reading. The ground is created in the base of the Antennae, which is coupled to the outer braid of the Coax feed cable. The antennae itself is cut to set measurements. The wave zips up the coax and into the transmitting element at the speed of light and it reaches the end of the Element and does something rather weird. It bounces back down the Element again. If the element has been "tuned" correctly (cut to the exact length) the bounce back interacts and creates a "standing wave". This results in a "load" that the transmitter has to push into. That load is what gives the transmitter "Power". Power (Watts) is the Sum of Voltage squared divided by the resistance, which is our impedance. Yes I said Impedance. Reactance has been used in the threads and the two terms are the same. This impedance and the transmitting power pushing into it is what creates a magnetic field around the Element and that field radiates out away from the Antennae as the Radio Wave.

Now AIS is a slightly higher frequency than the standard VHF international frequencies we use. But the same Antennae will actually be close enough to transmit and receive AIS on. For absolute perfect transmission/reception, a dedicated antennae will be better, but it is not essential.

Even an Antennae with a gain of 9dB will push to the Horizon when a vessel is Heeled. The reason for this is easy to misunderstand if you take IT's picture diagram as being the sole transmission "picture". What needs to be drawn around the 9dB shadowing is a different colour at 6dB and 3dB. So even at an angle, the 6dB, or greater angle, 3dB will still push out to the horizon just fine. Something else that is peculiar to VHF, is that at angles of up to 60deg, the Transmission wave actually drags along the seawater, instead of bouncing off it. From 60 to 90 degree of angle, the wave bounces. This also causes the wave to "drag over" the Horizon slightly. Or over and around Hills slightly. So hence why you can sometimes pick someone up that is actually out of sight. As Frequency increases, the angle gets less and less till finally it is clear cut line of sight. FM does NOT bounce like HF AM does.

Anyone still here????

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Yep. All good wheels! Too complex for some I expect.

What about the ground plane for vhf? Should it be connected to the water? WD was talking about lengths for the ground plane, but did not explain what or how for a boat.

Yes I know some vhf antenna require ground some don't. For those that do, what's best?

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Nope no ground plane for VHF FM. It works differently to HF AM. You could think it this way. AM has the wave produced between ground and Antennae. AM goes from Ground to sky. To receive AM, think of it like a mail box with the letter slot on top of the box. Problem is, all the rain and dirt falls in through the slot too. So hence why AM tends to have background noise etc, because anything from space can "fall" into it. FM is like having the slot o the side, so only the Radio wave enters. Hence why FM is so much clearer. FM radiates out of the Antennae element due to that reactance. Not between antennae and ground.

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