Constructing an aerial for AIS (& VHF)

N

Nudge

Well-Known Member
Joined
15 Apr 2009
Messages
191
Location
West Coast Scotland
Visit site
I know that this has been covered in other threads but here's my recent experience...

Wanted to construct a seperate aerial for the AIS so got some data from the radio ham chaps that advised:-

1. Length of exposed centre of coax cable (to be the actual aerial) should be 7500/Frequency which, for AIS is approx 7500/162MHz = 46.3cm
2. Braid "foldback" length i.e. braid from the exposed part of the centre folded back over the cable is 7500/freqency x velocity factor
VF for the type of cable that us nautical types use is 0.66 = 30.6cm
3. Mounting gap from the bottom of the braid foldback before securing is >3750/frequency = >23cm

(I am given to understand that CH16 is ~171MHz so a VHF aerial should be made with that frequency figure for the half wavelength)

The Ham boys usually stick this up a 1/2" conduit for mounting, but my eye fell on the wife's spare riding crop which is a whippy length of fibreglass. Stripped off the handle and tip, wrapped the top in double sided tape and coiled the 463 exposed coax centre round the end. Taped on the foldback braid and covered in heat shrink.

Of course they could have been taking the p!&& with all their data but I mounted it up and works a treat.

The only snag was explaining to my missus why I was seen lashing her crop enthusiastically around the garage shortly before it disappeared.....
 
Made our AIS aerial as a J pole antenna from scrap materials. All I used was a length of 2.5mm2 copper single strand cable bent as a J pole, a length of conduit and some RG 58 cable and connected it to our Easy AIS unit. Encased in a old length of conduit and mounted on back rail. Has worked perfectly for 3 years now and seams to have a good range.
 
Coincidentally, I just yesterday built up an antenna of the same design as described by the OP and using the same formulas. The design is pictured here:

http://www.hamuniverse.com/w4bwsverticalbazooka.html

My plan was to use on board (in cabin) for AIS receive untill I get round to fitting a second permananent antenna or building an RF switched splitter for the masthead one. Will also make a good emergency VHF, raised on a halyard, should the need arise.

I used cheap RG58U co-ax (velocity factor 0.66) and cut for resonance at 160MHZ or therabouts. Swept on a VNA, VSWR at resonance was approx 1.3:1, though it was quite narrow - from memory 2:1 Bandwidth approx 5MHz. In practice it will be distorted by nearby objects, particularly when used in-cabin, but should be more than adequate for RX use.

Regards,

Rob
 
Mine is a suitable length of 15mm copper water pipe with centre core of coax soldered inside bottom and and plugged with filler paste.The other end is capped with plumbing end cap.Its well painted to stop salt water getting at copper.You can secure it low down/to rails etc using domestic pipe clips or through bolt it to a mount by adding a brass skin fitting with compression connector.
My £5 AIS receiver is a suitably modded old Realistic scanner fed into my laptop.It can only monitor one chanel so updates are slower.
 
Last edited:
You must log in or register to reply here.

Share this page

Top