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Someone correct me if I am wrong, but, I believe you would not want to use the 6' antenna. Those are intended for powerboats that will be on the level. The 3' antennae have wider lobes of signal radiation than the 6'. When you are heeled over, the 6' antenna will have maximal power directed toward the water and the sky.
It's 3 dB for sailbaots vs 6 dB for powerboats, not 3 ft vs 6 ft.

Marine VHF frequencies are 156-162 Mhz, which is a wavelength right around 6 ft. The 6 ft antennas are full-wave, the 3 ft antennas are half-wave. For esoteric engineering reasons, you should avoid the full-wave antennas unless you know what you're doing. Stick with half-wave. (Most "6 ft" antennas are just a 3 ft half-wave antenna on top of a 3 ft fiberglass pole, so don't assume you're getting more for your money by buying the taller antenna. Likewise, the 8 ft antennas are a 3 ft antenna on top of a 5 ft fiberglass pole. The height affects how far your antenna can see over the horizon, which is why sailboats try to mount it on top of the mast. The Coast Guard puts tall antenna towers on top of big hills near the coast to maximize range.)

3 dB vs 6 dB (vs 9 dB) determines the beam pattern. Imagine dropping a donut so the antenna goes through the donut hole. The lobes of the donut roughly correlate to signal strength - it's stronger out to the sides, weaker up/down.

A 3 dB antenna has fairly round lobes. It's signal strength is weaker overall (only 3 dB), but more evenly distributed across a wider range of angles. A 6 dB antenna has stronger signal, but it's like a squashed donut - the lobes are stretched out closest to the horizontal. So if you tilt the antenna a bit, you quickly get into the smaller parts of the lobes and your signal strength is actually weaker than for a 3 dB antenna. West Marine actually has one of the best pictures demonstrating this that I've seen.


Anyway, since a sailboat is frequently heeling, your antenna is unlikely to be perfectly vertical. So if you get a 6 dB antenna and are heeled, you're likely trying to transmit and receive at an angle where a 3 dB antenna would actually perform better. That's why they recommend 3 dB antennas for sailboats, 6 dB antennas for powerboats (which tend to stay level more than sailboats).

And yeah, the antenna affects both transmit and receive signal strength. There's a common misconception that your antenna only matters for transmit. The math for receiving and transmitting is symmetrical. So whatever dB gain the antenna gives you for transmit, it also gives you for receive. (Think of it as talking through a cone megaphone so you can be heard further. But you can also put the cone up to your ear to hear further.)
 

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Picture below in 3d. To bolster Solandri's comments. He's 100% right on his monopole antenna theory. My microwaves EE degree is rusty (gave up EE for IT years ago), but I can confirm everything he is saying.

Below is a rudimentary pattern diagram, it doesn't show the minor lobes formed from the monopole or accurately reflect the different distortions in 3 dimensions for the particular frequencies in question, but it helps define the conversation better.

Keep in mind too, quality of cable, and connectors, and solder joints matter as to how much attenuation happens. The orientation of the antenna matters too (again as Solandri has stated, the 3dB pattern with a heeling sailboat).... Keep in mind those patterns do you no good if half or more of the lobes go under water (and they do), severely limiting range. Generally speaking the taller the location of the antenna the better, to the point that the antenna is half the width of the lobe.

We won't get into land obstructions, noise, the moron that leaves his mike on when you are trying to transmit, or the varying of impedance of air.

I will stick by my original statement though. Its a trailerable boat, put the antenna on the stern rail, get a nice 6ft antenna to get it higher off the water (despite it still being 3db, and likely 3 foot of that 6 ft is antenna the rest is height above water - see my height statement from before)... and live with the fact that it's not likely to transmit more than 5 miles at 5watts. That Cat 22 is not likely to make any tans-Atlantic voyages, so if you are within sight of land with it, the 3db rail mounted antenna will serve you well. Also it'll be a lot easier to deal with when you trailer her home (put my 3db shakespeares on top of the mast on both my Capri 25 and S2 7.9s, the extra cables were a PITA to deal with for anything more than 2 times a year to trailer). My C&C 32 is on an inland lake... the antenna is on the stern rail, WHY? cause I'm on an inland lake, and it was infinitely easier to deal with cabling it up. I might eventually move the antenna back to the top of the mast, but it will likely also be during a conversation about hauling the boat out to larger water.
 
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