Friday, 9 August 2013

AIS Antennas

Testing five homemade antennas and a rubber ducky for AIS reception.


What you need to know: monopole with 2 radials is best value / performance.
Read on to find out why.

Introduction

The antenna is the most important part of your receiving system.
To get better performance, you can:
1) increase the height of the antenna, or
2) use a better antenna and / or better equipment.

Equipment being the same and height being the same, you need the best antenna, hence this review of available easy-to-do choices.


1) Increasing antenna height increases range

On top of a mountain you see further away - it is the same with an antenna (called line of sight). That's why your existing VHF aerial is at the top of your mast - it's up high. The way radio waves travel might increase range an extra 10-30 %, but do not depend on it.
Nm is short for 1 nautical mile, equals 1.852 km or 1.15 land mile.
An inch is 2.54 cm. Divide cm by 2.54 to get inches.

Height of antenna above sea level - distance to horizon:
3 metres (fixed to cockpit railing) - 3.3 nautical miles
10 metres (small yacht mast) - 6.1 nautical mile
20 metres - (large yacht mast) - 10 nm
50 metres (large merchant ship) - 13.6 nm
From experience, AIS or VHF signal range is greater due to the fact that the antenna is also on top of the other vessel.

 2) Better antenna and/or equipment

At home or on a yacht, you can only change the antenna location until you reach available maximum, then you must get a better, more efficient antenna and / or more sensitive equipment.
Commercial equipment costs a lot as they are made by people doing radio all their lives, equipment works out of the box and you have less worries.
The choice is yours: pay 300 dollars for an AIS receiver or pay 30 dollars for a solution on your laptop.

AIS vs VHF antenna

If you have a Marine VHF antenna / aerial it will be fine for AIS.
Radio signals come to your antenna in waves, like sea waves rippling in a quiet anchorage. The distance between the top of the waves is called wavelength, and can be calculated by dividing 300 with the frequency in question. Marine band is between 156-162, AIS is around 162, so AIS wavelength is 1.85 metres, Marine VHF is 1.88 metres.
Wavelength is important to us, as most antennas are either Quarter-Wave (0.45m - 0.48m) or Half-Wave (0.9m, 3-foot whip as you know it) or variations, e.g. 5/8th wave, 1.5 wave etc.
For the reasons above, your antenna will work on AIS frequencies.
Antennas work not only at the "designed frequency": the quarter-wave 46 cm monopole, essentially a piece of wire and the worst performing in the test, received Commercial Radio at 96 MHz, Airport information at 121.85 Mhz and Taxi driver chat around 164 MHz.
Don't worry, give it a go, it will work.


Antenna testing setup and materials / cost

Location: top of a mountain, no building/trees in the way of the signal from the harbor, no clear view of the Celtic Sea.
Mid-range laptop, 15 meters of cheapest coax to simulate cable run to cockpit mounting, RTL stick connected via 60 cm (2 foot) USB extension lead. Antennas 3 meters above the ground.
Materials were 3 mm diameter copper wire (ask house grounding wire in Europe or No 10 wire in the USA), radials are 1 mm center conductors from a coax cable.
Material costs: grounding cable at 2-3 euros per meter, coax cable between 1-2 euros per meter.
A working setup, consisting of an RTL stick, 15 meters of cable and a homemade antenna works out to around 30 euros (I assume you have a laptop). A standalone AIS receiver costs around 120-130 euros plus shipping from Ebay.
Get in a "make-do" attitude and use what's available.


Monopole (fancy name for a piece of wire)
AIS, antenna, RTL-SDR, Software Defined Radio, 820T, testing, review, DIY guide

A 0.46m long wire (1/4th wavelength) connected to the center of the signal cable. 3 versions tested, 1 mm diameter wire from coax cable center, 3 mm diameter wire from house grounding cable, and wire from an extension power lead.
Electrical noise pickup was the lowest with the 3 mm diameter wire, so used that for more testing.
Furthest AIS target is 2.87 nm away.
AIS, antenna, RTL-SDR, Software Defined Radio, 820T, testing, review, DIY guide



Dipole (two wires in opposite direction)
AIS, antenna, RTL-SDR, Software Defined Radio, 820T, testing, review, DIY guide

0.46m length of wire connected to the braid of the coax, so they now form a 92 cm (3 foot) antenna.
Range doubled to 7 nm.

Dipole, AIS, antenna, RTL-SDR, Software Defined Radio, 820T, testing, review, DIY guide


Monopole + 2 Radials
AIS, antenna, RTL-SDR, Software Defined Radio, 820T, testing, review, DIY guide

The center wire of the coax cable is connected to a 46cm wire, then two more wires are added, each 45 degrees down from horizontal / 45 degrees up from vertical.
Radials are wires connected to the braid. Braid are wires circling the hard center wire, when you cut up a coax cable this is the first you meet below the plastic shell.
The antenna looks like a Mercedes sign, or a peace symbol without the circle. All wires are quarter-wave, 46 cm long. The up part is the center wire, the wires connected to the braid are the two wires going down left and right.
Dramatic improvement over a dipole, signal strength up, noise down, maximum range 21 nautical miles, 100% valid signal ratio. Picked up 3 vessels in port 8.27 nautical miles away (line of sight over the city centre, LOTS of electrical interferece)  and a ship 21 nautical miles away.
AIS, antenna, RTL-SDR, Software Defined Radio, 820T, testing, review, DIY guide


Monopole + 4 Radials

Two more pieces of wire added, so four wires stick out 45 degrees from horizontal, each spaced 90 degrees when viewed from the top. Wires point like North, East, South and West when looked from above, with the receiving element at the center of the compass rose.
Maximum range 19 nm, less targets received than monopole + 2 radials.

AIS, antenna, RTL-SDR, Software Defined Radio, 820T, testing, review, DIY guide


Coax Collinear (two pieces of coax connected the wrong way)
AIS, antenna, RTL-SDR, Software Defined Radio, 820T, testing, review, DIY guide

See detailed construction instructions in the hardware guide.
Gave a maximum range of 16 nautical miles with a container ship passing out at sea, but picked up less targets inside the harbor.
This might be a good backup / secondary antenna as extremely easy to make, can be coiled up for transport.


AIS, antenna, RTL-SDR, Software Defined Radio, 820T, testing, review, DIY guide


Rubber Ducky Antenna
AIS, antenna, RTL-SDR, Software Defined Radio, 820T, testing, review, DIY guide

Your handheld VHF probably has a similar antenna, this was from a radio scanner. Picked up, then lost Navaids and passing vessels, with 24 percent of signal error not the best here.
Usable if you have to.

AIS, antenna, RTL-SDR, Software Defined Radio, 820T, testing, review, DIY guide

Conclusion

Three 46 cm wires arranged as a peace-sign / Mercedes emblem offers the best value for money. Existing antennas, whether a rubber ducky, VHF whip or a scanner antenna will work with strong signals, for example a commercial ship within 2-3 miles.
Larger / more expensive antennas and equipment will increase your reception range - but the question remains: 10-20 times the cost for a slight increase in range?

No comments:

Post a Comment