VHF Antenna repair.

jamie N

jamie N

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The fitting at the end of my antenna is broken, in fact the 'aerial' part and the mounting body are now separated. The body is in good nick, as is the aerial. My query is, on the coax, obviously the core/centre is connected to the aerial, but where does the shield connect to, or is it left open circuit?
As an ROV guy, I should know, thus (in embarassment), I'll purchase a virtual glass of wine for the correct answer!
 
I'm looking at bypassing the antenna, and only using the body of it, as it's a convenient mount at the mast head. I envisage connecting the core to the aerial, and there being nothing else around to connect the shield to, leaving it open circuit. I'm seeking to confirm that this 'should' be correct. The aerial would be 'moulded' into the body of the antenna fitting.
 
jamie N said:
I'm looking at bypassing the antenna, and only using the body of it, as it's a convenient mount at the mast head. I envisage connecting the core to the aerial, and there being nothing else around to connect the shield to, leaving it open circuit. I'm seeking to confirm that this 'should' be correct. The aerial would be 'moulded' into the body of the antenna fitting.
Click to expand...

What aerial?
 
There is now way as yet to determine if there are any essential components in the body/mount. Manufacturer and model number would be required, or perhaps we could recognise it from a picture.
 
jamie N said:
A picture isn't possible. What essential components would there be?
Click to expand...

Very often there will be a low value inductor.
When building a home made half wave dipole you would wrap the incoming coax around the pole. In a more commercial product it could be a spiral of circuit track on a PCB. Some designs also use a resistor to match impedance.
 
If the rod antenna is around 17 inches then it is a quarter wave antenna. The centre of the coax goes to the bottom of the rod. The outer of the coax goes to the mast in a lead as short as possible. Thus the mast makes a ground plane to the coax outer. If it is longer than around 17 inches then antenna has much jiggery poky to match 5/8 wavelength or something else to the 50 ohm coax often not needing a ground plane. In my experience the latter more often in a GRP tube.In which case have a new antenna if you can't see where the connection is made. good luck olewill
 
Cheers Will; the mast's wooden though! The aerial is in the order of 50cms. I'm going to take a number of wraps from the coax centre around the lower end of the aerial, soldering the centre to the aerial, then potting it. The shield will 'just' be terminated at the bottom of the pot.
 
How are you going to test it?
Do you regard the VHF as safety kit?

A lot of off-the-shelf antennas have more subtle stuff going on than merely grounding the outer of the coax and connecting the inner to a wire of a certain length.
I would suggest buying a new one.
 
lw395 said:
How are you going to test it?
Do you regard the VHF as safety kit?

A lot of off-the-shelf antennas have more subtle stuff going on than merely grounding the outer of the coax and connecting the inner to a wire of a certain length.
I would suggest buying a new one.
Click to expand...

Thanks for your reply, but what more subtle stuff?
Also, the antenna is only for the traditional VHF in the cabin; I use a handheld for the majority of the comms onboard, so this item isn't the primary safety system, but point taken.
 
jamie N said:
Cheers Will; the mast's wooden though! The aerial is in the order of 50cms. I'm going to take a number of wraps from the coax centre around the lower end of the aerial, soldering the centre to the aerial, then potting it. The shield will 'just' be terminated at the bottom of the pot.
Click to expand...
The antenna really needs a ground plane. This can most easily provided by attaching a piece of wire from the end of the shield of the coax and run it down the mast. The wire should be same length as the antenna. ie about 17 inches. If you want to do it really well run 4 wires of that length down each of 4 sides of the wooden mast.
Go on I dare you to tell to me it is a round mast with no "sides" olewill
 
It IS round!!! :)
The shield is connected though, by virtue of the fact that the 'lower' end is grounded at the VHF, or have I got it wrong, that the shield return should run down the mast for 'only' 17"? Apologies if I'm appearing thick today!
 
Last edited:
jamie N said:
It IS round!!! :)
The shield is connected though, by virtue of the fact that the 'lower' end is grounded at the VHF, or have I got it wrong, that the shield return should run down the mast for 'only' 17"? Apologies if I'm appearing thick today!
Click to expand...

The signal in the shield coming up the wire is going the wrong way, it needs to be going opposite to the signal in the antenna. So, you need wires from the end of the screen going back down.
Ok, so "signal in shield" sounds barking, but its there, it has to be to oppose the signal in the core.
 
Bear with me on this, but if the wire from the end of the shield is 'going down', then where does it terminate? The shield is already terminated at the VHF plug, thus there's continuity from the top of the shield to this point, so if there's a separate wire from the top of the shield, back down the mast, where does this go?? I appreciate that I might be missing the blindingly obvious here, and thanks are due for this consideration; virtual glass of wine again!
 
jamie N said:
Bear with me on this, but if the wire from the end of the shield is 'going down', then where does it terminate? The shield is already terminated at the VHF plug, thus there's continuity from the top of the shield to this point, so if there's a separate wire from the top of the shield, back down the mast, where does this go?? I appreciate that I might be missing the blindingly obvious here, and thanks are due for this consideration; virtual glass of wine again!
Click to expand...
The wire does not terminate, it ends just the same as the top end of the wire (antenna) attached to the signal core, it needs to be the same length as the antenna as well..
You must remember that this is radio frequencies, not dc (not by a long chalk). OK if you say that to a microwave engineer they'll laugh you out of the room, but they are the weird ones!
 
jamie N said:
Thanks for your reply, but what more subtle stuff?
Also, the antenna is only for the traditional VHF in the cabin; I use a handheld for the majority of the comms onboard, so this item isn't the primary safety system, but point taken.
Click to expand...

There may be an inductor or two. The feed to the radiating element may be connected somewhere other than the end. The body of the mounting may form a capacitor at VHF, which is tuned with the inductive part of the antenna.

Letting somebody with no understanding of RF mend an antenna is like asking your Mum to knit you a spinnaker.
A new one is not expensive in the scheme of things.
At the very least, anything 'mended' ought to be tested with an SWR meter.

The other thing is, simply soldering it will probably break as soon as you need it.
 
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