Antenna Sector Width @ Distance

Interest Groups Amateur Radio
#1

I’ve probably mis-titled the post, but here goes…

I’m thinking about putting an AIS receiver (RPi / DAISYhat) at my home port, which is 10-15 miles inland. I can stick any VHF omni antenna on the Rx, but being that the purpose is to receive ship signals at sea, I’m wondering if anyone has any ideas as to what might be the best antenna design to go with. I’m not worried about obstacle obstruction (I can probably get it on top of the tallest building: a big grain elevator).

I for an online calculator to figure how wide a 3 degree yagi’s beamwidth (terminology?) would be at a given distance (say the mouth of the ship channel. 10 miles). Anybody know where to find one? I’m about to break out the good old trig’ education but wouldnt mind having someone double checking me

Thanks
Jeff.

#2

The single most important criteria for long range vhf signal reception is height. VHF signals are line of sight, and altitude increases range.

I would start with a simple dipole cut for the 161MHz frequency. Since the transmitters are going to be using decent power, that is likely all you will need. I would also mount the Pi close to the antenna, to reduce power loss in the coax.

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#3

I never was any good at math… http://www.mathwarehouse.com/triangle-calculator/online.php

1.0467191248988 miles wide is the number that I come up with for your example (3 degrees at 10 miles)

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#4

Thanks guys. Knew I’d what I needed here.

Walter, would that have to be hung out in the open, or could you say string it along the edge of a parapet wall?

#5

It would depend what the wall is made of. If it contains conductive material, that could cause interference. This would be the case for nearly any antenna. If you do have metal in the wall, then an antenna design like a jpole may be better.

The dipole will offer better performance though.

#7

I would look into a folded dipole, which can actually provide some gain, and is compact enough to work on board as well. They are simple to make, but the harness must be measured pretty carefully.