Glomex antenna.... how does it work?

[aslabend]

I've got a shiny new Glomex stainless masthead antenna to fit and have been playing with the connector (yes, the instructions are appalling but not impossible). It's not the connection I'm unsure about it's the aerial itself. When I was testing that the outer sheathing of the coax was not touching the inner core in the connections, no-matter what I did it was always connected so I took it apart and tested the antenna and found that there is a electrical connection between where the sheath contacts with the actual stainless aerial itself.

Is this correct? If so what's the point of the coax if at the aerial it all connects as one?
Could it be the way I'm testing it ( multimeter testing resistance at 200 ohms)
Any answers/ideas much appreciated?

cheers

 

[William_H]

Antenna

I don't know the glomex however it is not unusual to have an antenna which shows DC connection outer to inner. Some antennae are fitted into a fibre glass tube the antenna itself being fairly thin wire. A coil is connected and tuned with a capacitor centre to outer and the antenna itself is connected to the coil.
This arrangement is often used with a 5/8 wavelength long antenna to give some gain over a 1/4 wavelength whip. This means that more of the signal goes out in a horizontal plane and less upwards which is useful.
Having a DC connection centre to outer can be useful in bypassing any static build up to ground. ie in thundery weather. The very high frequency AC will see the coil as a high resistance because of its inductance.
Inductance... When a current passes through a wire it induces a magnetic field. If the wire is in a coil shape of multiple turns the magnetic field is multiplied by the number of turns. When a magnetic field changes and in rising or falling passes through a wire it induces a voltage into the wire. When current is applied to a wire (especially in a coil shape) the rise in current produces a rising magnetic field which in rising induces a voltage into the wire which is carrying the rising current. This voltage opposes the rise in current. So if the number of turns is enough in the coil you can apply a battery and measure the current and see that it takes time to get the current up to full level. This is caused by the inductance.
Now you can perhaps see in a simplified manner that if the voltage applied is rising and falling to zero then rising in the opposite direction at 160 million times per second (you VHF transmitter) that a coil with perhaps 10 turns 6mm in diameter will have enough inductance that the AC can not flow through it.
Now Ac theory and resonance is far more complex than that but that is part of the story.

Sorry probably not much use to you but don't be dismayed when you multimeter on ohms measures short circuit. good luck olewill

 

[aslabend]

Thanks WilliamH,
it makes a kind of sense, I was wondering if there was something like a balun built in that might be the cause. I'll stick the aerial on the mast and be one of the annoying people who asks the coast guard for a radio check on a saturday morning, if they reply I'll know it works.

Thanks again.

 

[William_H]

Hi Pete

What are you doing up at this time of the morning on the forum? You should be sleeping soundly.
Me? It is 7.15 AM here oolewill

 

[Twister_Ken]

Thanks WilliamH,
it makes a kind of sense, I was wondering if there was something like a balun built in that might be the cause. I'll stick the aerial on the mast and be one of the annoying people who asks the coast guard for a radio check on a saturday morning, if they reply I'll know it works.

Thanks again.

If you wanna radio check with the CG (last resort) use their working channel, not 16. But why not check with a marina, or a VTS station? Apart from anything else, the CG have just about the best aerials around, so are most likely to get you loud and clear. Some marina office at the top of the river is a far better test.

 

[Bilgediver]

What are you doing up at this time of the morning on the forum? You should be sleeping soundly.
Me? It is 7.15 AM here oolewill

He is worried about his Glomex....Yes they have a Balun built in so give a low DC resistance. Just make sure the end of the coax wire is prepared exactly as instructed.

 

[aslabend]

What are you doing up at this time of the morning on the forum? You should be sleeping soundly.
Me? It is 7.15 AM here oolewill

I'm a night owl not a lark. Is there a 7.15 in the morning? Must have seen one once but can't remember it.

 

[aslabend]

If you wanna radio check with the CG (last resort) use their working channel, not 16. But why not check with a marina, or a VTS station? Apart from anything else, the CG have just about the best aerials around, so are most likely to get you loud and clear. Some marina office at the top of the river is a far better test.

Only kidding about the CG Twister_Ken, they're far to useful to waste on checking my radio signal. Not sure I'll try Southampton VTS either, might make a supertanker ram the IOW. Might try the Ark Royal though, she's parked up a couple of miles away......do you think anyone would be in?

 

[alan_d]

Might try the Ark Royal though, she's parked up a couple of miles away......do you think anyone would be in?

Er, isn't she on the Clyde at the moment, offloading ammunition prior to decommissioning?

 

[aslabend]

She was in portsmouth a little while ago (the queen dropped by for something or other).

There will be other boats from the fleet busy doing nothing by the time I'm back in the water, I'll call one of them (or the MOD plod who drift by occasionally).