High Gain Collinear AIS (162MHz) Receiving Aerial

by Neal Arundale - M1CHS

Background

Living by the sea and having an interest in both yachting and radio, I became interested in AIS as soon as the inexpensive Nasa AIS engine became available. I use the Nasa engine in conjunction with Seaclear software when sailing offshore on friends Yachts, and the SR162 with ShipPlotter at home. See Comparative performance of Nasa, SR162 and Comar AIS-2-usb Over the last year, I have been experimenting with different aerials to try and improve my reception distance at home. See AIS Aerial Performance Comparisons
I would be interested in any comments, 

Location

My home is in Scarborough, North Yorkshire, UK about mile from the sea and 250 feet above sea level facing the North Sea. I cannot actually see the sea from my home as the line of sight is obscured by houses, trees and the Castle headland. There are 400 feet cliffs 10 miles north and south. My aerial is currently located on top of the flat roof of my garage, giving easy access to the aerial. At some time in the future, I will probably locate the aerial on the chimney stack but for the present it is more useful to be able to gain access to the aerial easily and enables me to make a comparative judgment as to the effectiveness of the ideas I have been trying. I am fortunate in having daily ferries passing 30 miles offshore, Hull 30 miles across land as well as a number of rigs (normally having the same support vessels) 30+ miles offshore,and two receivers which help to judge the aerials' performance.
Reception Graph

Software to produce this graph

Increasing Reception Range

AIS signals are transmitted at VHF frequencies, which like terrestrial TV signals do not "bend" over the horizon. They are line of sight, typically 20 miles at reasonable aerial heights.  AIS signals are very sensitive to any distortion, a bit like digital terrestrial TV - the picture is good, very scrambled, or nothing - there's no in between. Increasing the sensitivity of the receiver by, for example, adding  a aerial pre-amplifier may make little or no difference to your ability to receive weak signals. This is because the receiver will amplify the signal, noise and distortion equally, so the information contained in the signal still cannot be decoded by the receiver or computer.The way round the problem is to improve the gain of the aerial and to reduce the noise introduced into the receiver by the downlead. The gain of the aerial is indicative of the amount by which the aerial will increase the signal relative to the background noise, and is why a good aerial is so important to the reception of weak signals. A shorter downlead or using better quality cable will introduce less noise into the receiver.
I have tried to give some idea of the range of the various aerials I have tried and should be taken more as an indication of the relative performance of one aerial compared with another. The actual performance will depend on the aerials location and height, and, at over the horizon distance, propagation conditions are important, but can on occasions result in reliable signals being received from over 100 miles away.
Click here for a chart of horizon distance against aerial height.

Collinear Aerials

Aerials do not actually produce gain, they are more sensitive in a given direction. It's like putting a lens in front of the light in a light house, the light is concentrated in one direction, so at sea level the light is bright (where you want it) and 20 above the light will be much dimmer. Comparing with an aerial, the gain is the amount the light is brighter, looking at the light in the direction of the lens, than the light would be with no lens. A vertical collinear aerial is equally sensitive 360 around the aerial in the horizontal plane, but as the number of elements are increased, the increasing sensitivity in the horizontal plane is matched by a decrease in the sensitivity above and below the horizontal. By using 50ohm coax for the active elements, there is no impedance matching problem.

Mk1 plain wire

My first aerial was a plain wire attached to the end of the centre conductor of a length of RG58 coax, hung from the inside of my lounge window. This enabled me to receive AIS on the Nasa engine but only up to a range of around 5 miles. The length of the wire aerial proved to be very critical, the optimum appearing to be around 80 cms, this increased the range to around 10 miles.

Silva Marine VHF aerial

The results from this were to say the least disappointing, although better than a plain wire, it was not much better, even though it was mounted on my garage roof.

Amateur 2m aerial

This was a Diamond F-22 collinear 144Mhz  (amateur 2m band). At 3.2 meters long was quite a large aerial. The aerial is tuned to 144Mhz whereas AIS is 162Mhz, thus it was some 10% off its centre frequency .It was a big improvement over the marine VHF aerial, more than doubling the minimum range to around 20-25 miles.

Mk1 -  5 element Collinear

After researching the Internet (see references/links below), my first home constructed collinear was a great success, with a range of 25-30 miles. It was very simple to construct being basically a 3 meter length of wire which I hung from the gutter of the roof above the flat roof of the garage.
Mk 1Mk1 Technical
The above dimensions are immediately prior to soldering and show how the ends solder together. Click the thumbnail to enlarge.
RG213 coax was used for the feed as I already had a wire in place. If you do not have any RG213, I would try using RG58, particularly if the cable run is quite small. Do not use RG213 for the aerial. Both coaxes are 50 ohm for matching purposes. 75 ohm TV or satellite coax theoretically should not work well, but I haven't actually tried it (if anyone wants to try you'll have to adjust the lengths by the velocity factor of the cable you use).

Practice a joint or two first to ensure you get the length correct after soldering
  1. Cut 4 lengths of RG58 coax 63.9 cms long
  2. Strip  7 mm down to the center conductor
  3. Strip a further 10 mm of outer cover
  4. Gradually twist the screen back to to cover, leaving 3 mm of screen exposed
    Cutting
  5. Carefully run solder round the 3mm of exposed braid to bind the braid 
  6. Cut back the dielectric to the centre conductor leaving 1 mm exposed
  7. Tin the centre conductor with solder
    tinned
  8. Match up the end with a similarly prepared end of the next section of coax
  9. Cut back both centre conductors to just contact the screen of the next section the dimensions should now be as shown in the top line drawing. Most importantly the total length of the coax braid should be 61.1cm - this is the "active" length of  each section of the aerial.
  10. Solder both centre conductors to both other screens
  11. Connect all 4 sections of coax in the same manner
  12. Attach the feed wire in the same manner, you may wish to add a connector on the feed wire.
  13. Solder a 46.3cm length of plain uninsulated copper wire to the top end centre conductor
  14. Check the continuity from top rod to centre of aerial plug also checking there is no short across the plug
  15. Wrap PVC insulating tape round the joints
  16. Insulate the end of the top wire from gutter, I used a "chock block" connector and hooked it onto the gutter using a wire coat hanger
    Gutter
Gutter
I constructed the aerial in an hour or so, total cost about 2. I also managed to hang it from the gutter using a long pole
Modification tried
Replacing the top wave top rod with wave shorted coax and wave top rod.  The reception distance was halved. Many marine VHF aerials would appear to be constructed in a similar way as they are designed to work with transmitters. They are often called DC grounded as a resistance check will indicated a short between the screen and the aerial.

Technical Drawing
Click the Image below for an enlarged PDF version, kindly drawn by Broos Docter (Netherlands)
Aerial Technical
See here for further pictures and reception details kindly supplied by Greg Kunkel, Long Island NY.

Mk2 - 9 element vertical collinear with ground plane

Self supporting 6 meters high, range 35-45 miles.
Can be constructed without the ground plane with reduced range
Unobtrusive to neighbours
The aerial fits cleanly inside the Sota 7 meter fibreglass pole. This is tapered along its length and comes in seven telescopic sections (like a transistor radio aerial).
  1. Construct 8 sections of RG58 coax the same manner as in the Mk 1 aerial above up to step 7
  2. Slip a 2" (5 cm) sleeve over the coax  RG58 cover to strengthen the eventual joint. I used a length of sleeving removed from standard TV aerial coax. This was a fairly tight fit, but slipped on nicely when lubricated with a touch of washing up liquid
  3. Continue with steps 8 to 10
  4. When all 8 sections are soldered together check the continuity
  5. Slip the sleeves over the joints
  6. Again check the continuity
  7. Hang the coax up and seal the sleeves onto the RG58 coax cover with araldite at each end of the sleeve
    Joint
  8. Leave the araldite overnight to dry
  9. When dry, again check the continuity. Mine was OK so I didn't have to locate and fix a bad joint. I suspect if  your soldering is not up to scratch you'll probably have to start again. 
  10. Cut a length of about 1 metres of RG213 coax
  11. Cut about 25mm of sleeve off one end the RG213 coax
  12. Unbraid the braid on the coax back to the sleeve. The coax tail will be required if you fit the ground plane
    rg213end
  13. Obtain a 16M plastic cable gland from an electrical wholesaler. It must be able to clamp the RG58 cable and allow the RG213 to enter through the threaded side and you must be able to tighten the nut from within the cup of the cap. I had to file the nut down a fraction.
    plasticgland
  14. File a slot in the threaded (bolt) part of the cable gland just sufficient to slot the unbraided coax into when the RG213 coax is pushed through the bolt end of the gland towards the nut end. I used a Dremel.
    glandslot
  15. Put all the cable gland except the nut onto the RG58 coax before joining the RG58 & RG213 coaxes
  16. Obtain a copper or brass washer that will just fit over the cable gland bolt
    washer
  17. Make a cup to hold the ground plane
    1. Cut a 65.3 cm ( wave) length of 28 mm copper tube for the ground plane
    2. Obtain a plain (not "Yorkshire") 28mm copper joint sleeve, file off the centre groove so that the sleeve will slide right over the tube, and is a quite tight fit
    3. Cut a further 1 cm ring of the same tube
    4. Saw through the ring to split it open
    5. By filing the saw cut open and squeezing it together, create an insert that just fits into to copper tube
      insert
    6.  Push the insert half way into the tube to hold it together and solder a 28mm copper or brass washer onto the top of the insert. The washer will eventually hold the 16m cable gland, so check you can fit the gland before you solder it. You may need to enlarge the centre of the washer after soldering.
      insertcap
      When completed the cap should be a tight fit in the copper tube
  18. This is how they assemble together
    jointbits
  19. Push the RG213 right into the cable gland (leaving the braid tail through the slot), while allowing the nut and clamping washer to tighten firmly on the RG58 coax. When satisfied, clamp the cable gland firmly onto the RG58 coax by tightening the nut
    explodedjoint
  20. Push the copper/brass washer tightly down over the gland bolt so that the plastic nut will tighten onto the washer and braid
  21. Put the plastic nut on and tighten up. The whole joint should finish being mechanically & electrically sound, as well as being waterproof from the top
    assembledjoint
  22. Solder the top rod on to the end of the RG85
    topjoint
  23. Put a 10mm offcut of RG58 sleeve over the top joint & fill with araldite to strengthen the joint onto the cable
  24. Assemble the top and second section of the fibreglass pole. Mark  the overlap on the pole so that when the pole is assembled you can accurately position the aerial alongside and outside the pole in order to determine where the bottom joint of the aerial will be in the pole
  25. Assemble the rest of the Sota pole, position the aerial alongside and mark the outside of the pole exactly where the copper/brass cup washer will be located when inserted into the pole
  26. Position the sleeve on the outside of the tube (see section 17.2) to act as a stop preventing the copper tube going further up the pole & kinking the aerial coax
    Copper Sleeve
  27. Put the copper tube over the RG213 and push into the copper/brass cup
  28. Assemble the bottom two sections of the pole and mark on the outside of the bottom section where the next section ends
  29. Assemble the whole of the pole and lay the aerial alongside with the top of the aerial exactly where it will be when inserted in the pole
  30. Move the sleeve on the outside of the copper tube to the position where it will act as a stop when inserted into the pole. This prevents the aerial being kinked inside the pole.
  31. Cut the RG213 so that when a connector is added the whole of the connection will be within the bottom section of the pole
  32. Put on the N type jack
    ntypejack
  33. Obtain a length of 25mm plastic electrical conduit, this will just slide into the copper tube. This must be prevented from going inside the copper tube as it is used to support the weight of the copper tube. I obtained a conduit joining sleeve which just slipped a couple of mm into the copper tube, and would slip over the conduit. Note the RG213 socket must go pass through the centre of this connector. Alternatively you could glue a washer or sleeve onto the plastic conduit, as long as the socket will pass through and it will fit inside the pole.
    Conduit Sleeve
  34. Before you assemble the aerial into the fibre glass pole, obtain 30 cm of  38 mm aluminum tube (TV aerial mounting pole). This should slide about way down the outside of the bottom section of the pole. With a hack saw, make a longitudinal cut through one side only of the tube, from top to bottom.
  35. Slide the tube over the bottom section of the pole to nearly the bottom, prising the tube slightly open as required.  This is where the pole will be clamped in the aerial mounting bracket. Note that as you tighten the mounting bracket clamps this tube will fit the taper of the fibreglass pole exactly
    Split Tube
  36. Push the rubber centre bung out from the screw cap on the end of the bottom section of the pole and replace it with a plastic washer that just fits in the cap but will allow a N type plus through the centre. The hole must not allow a 25 mm plastic electric conduit pipe through. I manufactured the washer from an old plastic bottle. It must support the weight of the copper tube and RG213 cable.
  37. Cut the conduit at the correct length to just support the copper tube to the mark on the pole, whilst the conduit itself is supported be the end cap of the pole when it is screwed on the bottom of the pole.
    GP exploded
    GP assembled
  38. Carefully assemble the whole aerial into the pole, starting with the top section, slide each section down from the top. I supported the cable at each pole joint with 15mm of foam cut from central heating pipe lagging.
  39. When assembled clamp the pole into a TV aerial mounting bracket round the aluminum tube.
    clamp
Comments
The mechanics of the construction is how I made the aerial, there are probably better and/or simpler ways of building the aerial. Some of the references below may give you other ideas. Note however the electrical dimensions must be adhered to. PVC pipe (in place of fibreglass) may alter the electrical dimensions, as it could affect the velocity factor of the aerial. Glass fibre is OK, carbon fibre is not. 18/11/08 a test with an antenna analyser indicated, when inserted in either a PVC or fibreglass tube, the resonant frequency was reduced by 2%. This is barely significant.
I was also conscious of rain dripping into the joints as it would run down the pole, and if inside the pole, down the coax cable as well. This is why the bottom of the pole was not sealed, as it would help any moisture or condensation within the pole to dissipate.
The aerial has been subject to a 60 mph gale with no problems.
The ground plane appears to increase the range by about 20%, and also appears to reduce ships being "lost" under the high (400 feet) cliffs around Flamborough Head.
mk2 

See here for further pictures and reception details kindly supplied by Jean Pierre, Perpignan, France.
Futher modification (January 08)
I wasn't totally happy with the way the aerial wire hung down the Sota pole as it tended to kink and really needed suspending from the top while still supporting the weight of the copper tube from the bottom.
 
I replaced the top 1/2 inch of the top copper rod with a small (probably 8ba) screwed rod (again 1/2 inch), made from a brass bolt with the head chopped off, by soldering the screwed section to the top of the copper rod. It must be small enough to protrude through the fibreglass pole (see below).
 
I cut down the top section of the Sota pole so that the top 1/4 inch of the screwed rod just projected through the fibreglass at the top of the pole when the whole aerial was re-assembled.
 
I found a small plastic cup that just (and only just) slipped over the top of the top section of the fibreglass. Something like the top of a biro would do only a bit smaller - I actually used a hypodermic needle cover !!
 
I carefully araldited a couple of brass nuts into the plastic cup with a similar bolt in place (until the araldite was set) in such a way that I could use the cup to hold the top rod + the weight of the coax at the top of the pole.
 
In this manner I could dissemble the aerial from the pole at a later date if required, adjust the length to just prevent the coax kinking, and maintain a waterproof cover over the top of the pole.

Equipment & Software

I use a Smart Radio SR162  dual channel receiver at home, purchased direct from Smart at $460 including air freight. Delivery took 5 days. I also have a Nasa AIS engine. This enables me to compare the relative performance of both the aerials and the receivers. The sensitivity and response of both receivers can be judged from the comparison here. Notice the Smart monitors both AIS channels simultaneously, whereas the Nasa switches between the channels.
At home I use the ShipPlotter software because it makes it simple to download data to other servers. When sailing I use the Nasa engine with the SeaClear software because SeaClear has much better chart management and displays NMEA info alongside the chart.
The ShipPlotter software will accept decoded NMEA input as well as raw audio. Both receivers output decoded NMEA . I put a discriminator tap on the IC inside the Nasa engine to see if the ShipPlotter program would make a better job of decoding the audio signal than the Nasa engine. It made no difference.

Comparison data on graph (September 08)
I'm using a coaxial relay to switch the aerials under control of the same PC running Shipplotter and plotting the graphs. It is synchronised to plot message rate (1) or (2) on the graph as the aerial is switched. Every alternate message rate is therefore recorded for a different aerial. The two aerials are located at the same height about 10 feet apart, using the same type and length of downlead and feeding the same AIS receiver.

Calculations

v=fλ
where v=velocity, f=frequency and λ=wavelength
In our case we want wavelength so λ= v/f
V is the velocity of propagation which is the speed of light in free space
AIS frequencies are 161.975 MHz and 162.025 MHz
One wavelength λ in free space is 300/162 = 1.852 metres = 185.2 cm
Velocity Factor  for RG58U coax is 0.66
One wavelength λ in RG58U coax is 185.2 x 0.66 =  122.2 cm

Element Length Velocity factor Size
Top Rod wave 1 46.3 cm
Middle Coax wave 0.66 61.1 cm

Cutting Length of each coax section

Braid 61.1 cm
Dielectric 0.1 x 2 = 0.2 cm
Center Conductor 1.3 x 2 =  2.6 cm
Total 63.9 cm

Approximate Total Length of Aerial

Top rod + number of coax sections x (length of braid of each section + 1mm joining allowance)
Elements Length Gain 
3 168.7 cm 3db
5 291.1 cm 6db
9 536.0 cm 9db

References
How to build a high gain Aerial for the UHF or CB bands Anon
Build A 9 dB, 70cm, Collinear Antenna From Coax Mike Martell N1HFX
Build a 2 metre, 5/4 Wave Antenna Mike Martell N1HFX
Omni-Gain VerticalCollinear for VHF and UHF Mike Collis WA6SVT
Construction Notes on a WA6SVT Coaxial Collinear Antenna Paul Kelly N1BUG
Some variations and construction ideas on the WA6SVT Omnidirectional Coaxial Collinear Kevin Custer W3KKC
Slim Jim
M0UKD
Horizontal Bi-Directional Wires available from Antennex (by subscription)
L.B.Cebik W4RNL
The Case of the Curly Collinear available from Antennex (by subscription) L.B.Cebik W4RNL
2 m. Band VHF Collinear with 2 Dipoles sv1bsx
The W7LPN 2 - 440 Vertical Collinear Antenna Project W7LPN ~ Rick Frazier
A 2.4Ghz Vertical Collinear By Brian Oblivion and Capt.Kaboom
modified by Richard A Wenner
Coaxial Collinear hints K7ITM
UHF/VHF Range Calculations
Pacific Crest
Aerial Gain Explained
Marc G Dekenah
VHF and UHF Path Loss Calculations for Amateurs
Leith Martin VK2EA
ARRL Antenna Handbook Radio Wave Propagation - Link was broken - this is the complete book
Anon
Everyday VHF, UHF, and SHF propagation Palle Preben-Hansen, OZ1RH
Beyond the Horizon Propagation Mike Willis G0MJW
Tropospheric Ducting Forecast William R Hepburn
Radio Horizon Neal Arundale
Sota Fibreglass Poles
Smart Radio $460
Nasa AIS Engine

Diamond f-22 144MHz BASE STATION ANTENNA
Cushcraft Antennas

Discriminator Taps
RG Series Cable Characteristics
Ship Plotter
Sea Clear
AIS currently being received in Scarborough Neal Arundale
Graphical Monitoring of ShipPlotter Neal Arundale
Performance Comparision Nasa - SR162 Neal Arundale
AIS Aerial Performance Comparisons Neal Arundale