Electrical query - Should a VHF aerial have continuity?

[StephenW]

Salvaged a rib-type (1.2m'ish ) vhf aerial from a marina waste skip with thought of it providing a useful emergency aerial if the main aerial in the mast is ever lost for any reason. Only apparent damage was a broken plastic mount, short length (15cm) of cable attached.

Have bought a connection and suitable cable but when assembled & tested there is continuity between inner and outer connections.

Unsure whether there should be or not???

Basic arrangement is aerial with short length of its own wire, joint , then URM 76 Low C cable connected to UHF plug for VHF radio.

I've broken the connection joint (OK, I should have tested before finally joining) and there is no continuity between outer sheath and core in the new cable and UHF connector at the radio end.

However, the short aerial cable connected to the aerial has continuity between inner and outer cable - should this be so?

Shooting in the dark a bit here so appreciate feedback from anyone with good electrical knowledge.



r

 

[Heckler]

Salvaged a rib-type (1.2m'ish ) vhf aerial from a marina waste skip with thought of it providing a useful emergency aerial if the main aerial in the mast is ever lost for any reason. Only apparent damage was a broken plastic mount, short length (15cm) of cable attached.

Have bought a connection and suitable cable but when assembled & tested there is continuity between inner and outer connections.

Unsure whether there should be or not???

Basic arrangement is aerial with short length of its own wire, joint , then URM 76 Low C cable connected to UHF plug for VHF radio.

I've broken the connection joint (OK, I should have tested before finally joining) and there is no continuity between outer sheath and core in the new cable and UHF connector at the radio end.

However, the short aerial cable connected to the aerial has continuity between inner and outer cable - should this be so?

Shooting in the dark a bit here so appreciate feedback from anyone with good electrical knowledge.



r

It depends, on my mast head vtronix there should be a continuity reading, I couldnt find it in a quick google but I found this one, note the warning about continuity, http://vtronix-marine.com/pdf/399-1-m.pdf
Stu

 

[ianj99]

There doesn't need to be and usually isn't on a whip aerial I beleive. The RF sees the 50ohm impedance of the aerial so its not open circuit to this, which is what matters.

 

[starfire]

Short answer, it depends.

The type of aerial within the outer case can be one of 3 types.

1. A length of wire a 1/4 or 1/2 wavelength at 156Mhz, which will be open circuit when tested with a DC meter

2. As above, but with a matching network which normally will be a short at DC.

3. An aerial formed from ribbon cable with a section used as a matching network, also a short at DC. This is the most common type in the fiberglass whip style antenna.


What you can measure with a multimeter bears no relation to what happens at RF frequencies.

The only way to test is with a SWR meter, which will tell you if the load (antenna) is good, bad or near enough, at the working RF frequency.

Someone in the marina may have one, the local chandlers/mast rigger/radio ham etc may have one you can borrow.

 

[StephenW]

OK many thanks - helpful.

It's a Banten DB/6 glassfibre outer casing aerial as you've guessed.

Seems to mean my electrical joint wasn't as duff as I though it was.

Hopefully will work if I join again - I was concerned something had melted and the wires were shorting

 

[lw395]

The quick test is 'does it receive?'
Antennas can appear open or short at DC, depending on the matching network.

 

[William_H]

As said yes most likely the DC continuity from centre to outer is correct. Try it on receive then if OK transmit you will soon know if aerial is bad.
It is often quite easy to cut the fibreglass tubing where it goes into metal base to expose innards. The innards are simply lying loose in the tube. You can often get away with shortening the f/g tube by an inch or so to allow it to be glued together. This way you can attach a completely new cable.
good luck olewill

 

[Bilgediver]

Salvaged a rib-type (1.2m'ish ) vhf aerial from a marina waste skip with thought of it providing a useful emergency aerial if the main aerial in the mast is ever lost for any reason. Only apparent damage was a broken plastic mount, short length (15cm) of cable attached.

Have bought a connection and suitable cable but when assembled & tested there is continuity between inner and outer connections.

Unsure whether there should be or not???

Basic arrangement is aerial with short length of its own wire, joint , then URM 76 Low C cable connected to UHF plug for VHF radio.

I've broken the connection joint (OK, I should have tested before finally joining) and there is no continuity between outer sheath and core in the new cable and UHF connector at the radio end.

However, the short aerial cable connected to the aerial has continuity between inner and outer cable - should this be so?

Shooting in the dark a bit here so appreciate feedback from anyone with good electrical knowledge.



r

Most marine VHF antennas will show continuity if you measure resistance between shield and conductor . This is due
to a built in matching circuit. Tell us the make and model and someone may confirm in more details.

 

[VicMallows]

Current Vtronix antennas show a specific DC resistance when tested. If I recall correctly, it's 10k Ohm. (Earlier models would measure nearly a short-circuit).

Quite a clever idea ... the 10k resistance makes no difference at RF frequencies, but enables you to easily check for BOTH open-circuit AND short-circuit in the cable/connectors/etc.