AIS antenna and Coax as a ground plane

Dellquay13

Dellquay13

Well-Known Member
Joined
19 Feb 2021
Messages
1,306
Location
Boat at Milford Haven, Home in Chesterfield
Visit site
I’ve just bought a Shakespeare stubby heliflex AIS antenna to go on the push pit, rx only for my nasa AIS.
I don’t need AIS tx as I don’t have an AIS transponder, but I would like the option of the stubby as an emergency VHF antenna.
The website details said the 20m coax acts as the ground plane in TX mode, and should not be shortened.
I am going to have to loosely coil 17m of excess cable (in a fig8?) and secure it somewhere. Does it need to be near the antenna to act as a ground plane, or can it be 2m away behind the receiver? Does the coil need to be at 90deg to the antenna ie horizontal or can it be perpendicular, face on or side on?
One thought I had was to run the excess cable around the cockpit inside the coamings several times, like a hearing aid induction loop, but it would be a right faff to do and secure.
TIA
Chris
 
Last edited:
If you coil it up under the deck at your stern, it will act as an inductor in the outer screen and actually help to choke off unwanted currents flowing back to your RT.
 
neil_s said:
If you coil it up under the deck at your stern, it will act as an inductor in the outer screen and actually help to choke off unwanted currents flowing back to your RT.
Click to expand...
If he had a transponder it might?

I suspect the gains from shortening the cable to the correct length outweigh other (rather dubious in RF terms) suggested advantages. Their request not to shorten the cable ‘because it acts as a ground plane’ makes no sense RF wise. Transmission line (including Co-ax) can certainly act to transform impedances but in the application specified it’s not supposed to!
 
john_morris_uk said:
If he had a transponder it might?

I suspect the gains from shortening the cable to the correct length outweigh other (rather dubious in RF terms) suggested advantages. Their request not to shorten the cable ‘because it acts as a ground plane’ makes no sense RF wise. Transmission line (including Co-ax) can certainly act to transform impedances but in the application specified it’s not supposed to!
Click to expand...
I guess I don’t need a ground plane for the original intended use as an AIS rx antenna, but it may be more important if I need to repurpose it as an emergency antenna for my fixed VHF
 
Dellquay13 said:
My maths isn’t good enough to work out the diameter or number or turns needed to tune an inductor to ch16 for emergency vhf use
Click to expand...
You don't need to! All you'd be doing is adding a bit of inductive reactance to help reduce current flowing down the outside of the coax. You could get clever by adding a lambda/4 sleeve underneath the stubby - but that would just confound the whole idea of a short antenna.
 
neil_s said:
You don't need to! All you'd be doing is adding a bit of inductive reactance to help reduce current flowing down the outside of the coax. You could get clever by adding a lambda/4 sleeve underneath the stubby - but that would just confound the whole idea of a short antenna.
Click to expand...
Indeed. The only thing that really matters for it to be an effective emergency antenna is that it presents 50 ohms impedance and how high it is. (Assuming it actually radiates!)
 
Dellquay13 said:
I’ve just bought a Shakespeare stubby heliflex AIS antenna to go on the push pit, rx only for my nasa AIS.
I don’t need AIS tx as I don’t have an AIS transponder, but I would like the option of the stubby as an emergency VHF antenna.
The website details said the 20m coax acts as the ground plane in TX mode, and should not be shortened.
I am going to have to loosely coil 17m of excess cable (in a fig8?) and secure it somewhere. Does it need to be near the antenna to act as a ground plane, or can it be 2m away behind the receiver? Does the coil need to be at 90deg to the antenna ie horizontal or can it be perpendicular, face on or side on?
One thought I had was to run the excess cable around the cockpit inside the coamings several times, like a hearing aid induction loop, but it would be a right faff to do and secure.
TIA
Chris
Click to expand...
I have a NASA AIS receiver connected to a stubby aerial from an old handheld VHF and mounted about 1.5m above the water. I receive ships at 8+ miles which is good enough for me. My coax is about 4m long, no excess. I have also tried the stubby connected to my 25W VHF and it worked on both transmit and receive although I have not tested the VHF transmit range.

www.solocoastalsailing.co.uk
 
The question really relates to the effectiveness of the stubby antenna on transmit as an emergency VHF com antenna. Much depends on the design of the stubby antenna.
Now a basic antenna is a 1/4 wave length dipole conductor (rod) 17 inches long connected to the centre of the coax cable. This however requires a ground plane connected at the end of the coax shield. A ground plane is a conductive plate or structure in excess of 17 inches radius. if you use a stern rail or a ali mast this works OK as a round plane.
Now some antenna use some jiggery pokey using a coil and capacitor in a manner such that a ground plane is not necessary. Typical long antenna in a grp tube for mounting on cabin top of GRP mobo. I don't claim to understand how they work but assume coax cable sheth has some part in ground plane simulation.
Does your stubby antenna use this technology or is it a simple 1/4 wave dipole compressed by winding the wire into an internal coil. The latter I suspect. (any experts here?)
As said a long coax cable especially of smaller diameter RG 58 will have significant losses over distance. (A 17m cable might lose half of transmit power in losses. )
If it were me and not knowing any better I would cut the cable to the length needed to get to the AIS and VHF radio. I might be tempted to connect the outer of the coax at the antenna base top the stern rail to provide ground plane.
Bare in mind that actual power loss in watts or percentage only relate in a logarythmic manner to range and perceived strength. So lose half your power probably no one will ever notice. The height of the antenna does make a difference.
Having said all that I fitted a 17 inch rod in an insulator to my stern rail on a .5 metre pole with coax screen connected to pole. It works fine for me and my needs. So yes having lost several masts I would advocate having an mergency stern rail antenna available. ol'will
 
You must log in or register to reply here.

Share this page

Top