High Gain Collinear AIS
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. Comparative
performance of Nasa and SR162Over the last year, I have been
experimenting with different
aerials to try and improve my reception distance at home.
I would be interested in any comments,
Location
My home is in Scarborough, North Yorkshire, UK about ½ 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.
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.
Commercial 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.
Commercial 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.
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
- Cut 4 lengths of RG58 coax 63.9 cms long
- Strip 7 mm down to the center conductor
- Strip a further 10 mm of outer cover
- Gradually twist the screen back to to cover, leaving
3 mm of screen
exposed
- Carefully run solder round the 3mm of exposed braid
to bind
the braid
- Cut back the dielectric to the centre conductor
leaving 1
mm
exposed
- Tin the centre conductor with solder
- Match up the end with a similarly prepared end of the
next section of coax
- 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.
- Solder both centre conductors to both other screens
- Connect all 4 sections of coax in the same manner
- Attach the feed wire in the same manner, you may wish
to
add a connector on the feed wire.
- Solder a 46.3cm length of plain uninsulated copper
wire
to
the top end centre conductor
- Check the continuity from top rod to centre of aerial
plug also checking there is no short across the plug
- Wrap PVC insulating tape round the joints
- 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
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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 ¼ wave top rod with ¼ wave
shorted coax and ¼ 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.
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).
- Construct 8 sections of RG58 coax the same
manner
as in the Mk 1 aerial above up to step 7
- 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
- Continue with steps 8 to 10
- When all 8 sections are soldered together check the
continuity
- Slip the sleeves over the joints
- Again check the continuity
- Hang the coax up and seal the sleeves onto the RG58
coax
cover with araldite at each end of the sleeve
- Leave the araldite overnight to dry
- 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.
- Cut a length of about 1½ metres of RG213
coax
- Cut about 25mm of sleeve off one end the RG213 coax
- Unbraid the braid on the coax back to the sleeve. The
coax
tail will be required if you fit the ground plane
- 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.
- 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.
- Put all the cable gland except the nut
onto the RG58 coax before joining the RG58 & RG213 coaxes
- Obtain a copper or brass washer that will just fit
over the
cable gland bolt
- Make a cup to hold the ground plane
- Cut a 65.3 cm (¼ wave) length of 28 mm
copper
tube for the ground plane
- 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
- Cut a further 1 cm ring of the same tube
- Saw through the ring to split it open
- By filing the saw cut open and squeezing it
together,
create an insert that just fits into to copper tube
- 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.
When completed the cap should be a tight fit in the copper tube
- This is how they assemble
together
- 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
- Push the copper/brass washer tightly down over the
gland
bolt so that the plastic nut will tighten onto the washer and braid
- 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
- Solder the top rod on to the end of the RG85
- Put a 10mm offcut of RG58 sleeve over the top joint
& fill with araldite to strengthen the joint onto the cable
- 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
- 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
- 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
- Put the copper tube over the RG213 and push into the
copper/brass cup
- Assemble the bottom two sections of the pole and mark
on
the outside of the bottom section where the next section ends
- 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
- 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.
- Cut the RG213 so that when a connector is added the
whole
of the connection will be within the bottom section of the pole
- Put on the N type jack
- 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.
- 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 ½ 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.
- 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
- 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.
- 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.
- 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.
- When assembled clamp the pole into a TV aerial
mounting
bracket round the aluminum tube.
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) will
alter the electrical
dimensions, as it would affect the velocity factor of the aerial. Glass
fibre is OK, carbon fibre is not.
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.
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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.
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 |
¼ wave |
1 |
46.3 cm |
| Middle Coax |
½ 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
