Help me resolve this VHF antenna mystery

[Lomax]

Friend of mine had got hold of a new & unused Glomex VHF antenna, still in original packagaing, that he wanted me to help install on his boat. It's of the fibreglass tube variety, 1.5 m long, I think the model is RA112. Upon removing it from the packaging, we discovered that it had at some point been bent, approximately 1/5 of the way from the top, which had caused a small crack in the fibreglass. Considering the cost of these things, I opted to repair the crack with epoxy and once covered with a little paint it looks perfectly fine. The damage was really quite minor, a couple of parallel cracks about 4 cm in length on one side only. No moisture could have entered since it's never been outdoors, or even out of the (plastic) packaging. But once installed my friend says it does not work. He tried transmitting to the local marina (I was unfortunately not present for this) and told me all they heard was static bursts as he keyed the transmitter (he spoke to them via his handheld). Reception was unfortunately not tested. Now it's my understanding that an antenna of this size (half wave?) will have a tuned coil at the base, connected to a simple metal rod, and that the fibreglass tube is merely there to support the metal rod. If this is correct, then I cannot understand how it could possibly have been broken by minor damage to the fibreglass tube. It measures a short at DC, which I understand is normal for an antenna of this type (with the coil looking like a short at DC). I have it with me here now, and was thinking I'd try it on my own radio, but got a little paranoid; what if there really is a short, also at 160-ish MHz, might this damage my radio - even if I don't transmit? I am loath to simply throw it away without figuring out what (if anything) is wrong with it. Has anyone taken one of these things apart?

 

[kwb78]

Now it's my understanding that an antenna of this size (half wave?) will have a tuned coil at the base, connected to a simple metal rod, and that the fibreglass tube is merely there to support the metal rod.

That would be a typical construction for that type of antenna. Sometimes fibreglass antennas have an element made from a PCB, however they tend to be for shorter wavelengths than VHF.

I have it with me here now, and was thinking I'd try it on my own radio, but got a little paranoid; what if there really is a short, also at 160-ish MHz, might this damage my radio - even if I don't transmit?

You won't damage anything by just connecting it - you can check for reception without any problems. Reception may still be reasonable even if transmission is not though. Many antennas will read a short when tested with a multimeter, but that doesn't mean they don't have the proper impedance at the operating frequency. I think the risk of damage to your radio by transmitting with it connected is minimal. Most modern radios will not be damaged by being connected to an antenna with poor SWR, especially transmitting for only a short period. You can also set the radio to low power (1W mode) to further reduce risk.

Poor quality or damaged coax and connectors can be very detrimental to the effectiveness of transmissions so it's worth double checking that. If the coax is old, make sure it's not corroded internally.

Other than basic checks, it's difficult to do a lot without a few tools - if you have access to an SWR meter you can check whether the antenna is properly tuned. A damaged one will have a high SWR and not be effective for transmission.

 

[William_H]

I agree with above. If he could not contact a nearby marina something is seriously wrong beyond just a bad antenna. Cable not connected to radio or radio not working correctly. Yes try the antenna yourself. I reckon it will be fine. ol'will

 

[chriscallender]

You won't damage anything receiving and even if you transmitted briefly into a shorted antenna (which depending on the length of coax connecting the radio to the antenna could look like a short, open circuit or anything else), you'd be very unlikely to do damage - this will be exactly the scenario the designers have thought about when designing the protection (present in any modern radio). Turning transmit power down to low power (only 1W) would make it even safer and is better for antenna testing anyway.

I'd try receiving and if that worked, make a few brief test transmissions on low power to a handheld to see what sort of range you get.

My experience is that its quite common for marinas not to answer anyway, usually the person with the radio is just busy with something else. So perhaps everything is already fine.

 

[MontyMariner]

Now it's my understanding that an antenna of this size (half wave?) will have a tuned coil at the base, connected to a simple metal rod, and that the fibreglass tube is merely there to support the metal rod.

Not sure about the metal rod, I thought that antenna wire was wound right up to the top around the tube. If the tube has cracked, it probably broke the wire. The short you are measuring is probably across the tuning circuit in the base.

 

[Lomax]

Thanks guys, you gave me the confidence to connect the antenna to my boat's VHF, and I've been listening on the shipping channel for the last hour or so. I seem to receive much less traffic than I do with my own antenna (which admittedly is in a much better position at the top of the mast) and there's a constant helicopter like choppy noise in the background while receiving, which is not present when using my own antenna. This regardless of whether it's ship or shore transmitting. Seems odd that this kind of interference would be caused by a bad antenna, but what do I know? Have not tried transmitting yet, but when/if I do I'll make sure to only use 1 W (should be the default on the general channels anyway).

Not sure about the metal rod, I thought that antenna wire was wound right up to the top around the tube. If the tube has cracked, it probably broke the wire. The short you are measuring is probably across the tuning circuit in the base.

Oh, I see. That might explain the issues then. Can anyone confirm?

 

[kwb78]

I would be surprised if it was a continuous wound wire to the top of the tube for a VHF antenna. There is no benefit to doing that since marine VHF full wave is only about 1.9m. I think it's probably a 5/8 wave whip with a loading coil at the base. The Glomex website doesn't give any details of its construction though and I can't find anything that shows it elsewhere. The website does say that it is DC grounded which means the short you see on the connector is to be expected.

I still think the cable is a likely source of the problem and worth eliminating - unfortunately that antenna has captive RG58 which is not great coax anyway and you'd have to dismantle the antenna to replace it.

 

[Lomax]

I would be surprised if it was a continuous wound wire to the top of the tube for a VHF antenna. There is no benefit to doing that since marine VHF full wave is only about 1.9m. I think it's probably a 5/8 wave whip with a loading coil at the base. The Glomex website doesn't give any details of its construction though and I can't find anything that shows it elsewhere.

This seems to me the most likely as well. Firstly because I don't see the benefit of a coil construction at VHF frequencies, unless you want a very short antenna, secondly because although not super cheap, at ~£60 they're not that expensive either. Surely an all coil construction of this size would cost several times more. But then I'm back to square one; what has happened to this antenna to make it behave like this?

I still think the cable is a likely source of the problem and worth eliminating - unfortunately that antenna has captive RG58 which is not great coax anyway and you'd have to dismantle the antenna to replace it.

That's a good point; it is indeed a very poor quality coax, stiff & horrible with a sparse wire screen over foil construction. But at least it uses tinned copper conductors, and it's in perfect shape, with no punctures, cuts or signs of being crushed - so I still don't see how the cheap cable alone could produce the observed results?

 

[Lomax]

Maybe I should give up and cut the thing open, you know, for the science?

 

[Martin_J]

Whereabouts are you located?

There just might be someone on this forum nearby with a power/SWR meter for the VHF band that can easily check out the antenna and it's feed cable for you. If you're in the Solent area, I'd be happy to help with a meter.

As to construction, I would expect that fibreglass tube to just be holding up a straight wire antenna. The base load might not be a coil that you might find in the cylindrical shaped part at the bottom of some other VHF antennas though. It could be just some tracks printed onto a PCB that I would have said has now got a little damp ( or waterlogged) but you say that this is a new antenna.

Only hearing static upon transmit could be a faulty radio trying to transmit... although perhaps check if your friend was using shorepower at the time, or had some other noisy switched mode power supply running nearby and check that the power supply leads to the VHF look good. Perhaps check 12v at the back of the VHF when transmitting.

 

[Daverw]

I did see one sone time ago that had a narrow long circuit board inside that was the antenna, this was a 2m radio ham one so they do exist. The board had a number of inductive coils along it.

 

[Lomax]

Ok, I'm going to cut through the fibreglass tube a little below where the damage was, and see if I can remove it. If the antenna uses a PCB rather than a simple wire this may have been damaged where it was bent, and that might be possible to repair.

 

[Lomax]

Nah. It's exactly how I imagined it, apart from the zig-zag bends in the wire. I guess these are to prevent it rattling inside the tube. No damage whatsoever. Problem must lie elsewhere - I'll have to cut the tube at the bottom as well...

 

[Lomax]

Ha. There was never anything wrong with this antenna - it's just a piece of junk. I was able to remove the base quite easily after heating it up with a hot-air gun, to soften the glue. What I found inside did not impress me; there's no tuned coil, or any other interesting gubbins, just a bent piece of brass wire soldered to the coax. The side of the bend that's soldered to the screen measures 43 cm, and the other side (which is soldered to centre conductor) measures 116 cm. Cheap and nasty. I can't believe they have the ***** to charge £60 for this laughable excuse for an antenna. I wish I could get the hours I wasted on it back, but at least I now know what not to buy in the future: anything "made" by Glomex. Case closed; I'll keep the brass wire for something, the rest goes in the skip (where it always belonged).

 

[Lomax]

And to think I praised up Glomex to my mate - how embarrassing! I guess this is the perfect example of what happens when engineer-led companies with a long tradition and good reputation get taken over by greedy bean-counters who have no concept of value. Glomex join an endless list of companies I now mourn as the walking corpses they have become, with few employees, no engineers, no manufacturing, no quality control - barely even an office - just an empty husk of a brand acting as a vessel for cheap Chinese tat, and a vehicle for boundless corporate greed. I prefer to remember them as they were before they became zombies, and only buy their products second-hand.

 

[MontyMariner]

Thanks, useful knowledge
Maybe not dissimilar to the radar reflectors that, when cut open, were found to contain screwed up tin foil

 

[Refueler]

Most radios today have diagnostic menu that runs a check over all parts incl antenna ..... no need to PTT

Most radios will also warn you not to transmit without an operating antenna connected - 25W can blow the RF unit in the radio. My mate has such a blown radio if anyone wants it. 1W for short tests OK

OK Question for OP : Did guy have a splitter for VHF + AIS when he tested it ? Has the co-ax plugs been stressed ? Given the cracked FG tube - it takes quite a bit to do that .. imagine its built to stay on a mast in all extremes ... the stress that caused the cracks could easily have damaged low down at base - causing fault of antenna ..

 

[Lomax]

OK Question for OP

You should read the thread before posting.

 

[Refueler]

You should read the thread before posting.

OK - sorry I bothered you.

Bye

 

[st599]

Ha. There was never anything wrong with this antenna - it's just a piece of junk. I was able to remove the base quite easily after heating it up with a hot-air gun, to soften the glue. What I found inside did not impress me; there's no tuned coil, or any other interesting gubbins, just a bent piece of brass wire soldered to the coax. The side of the bend that's soldered to the screen measures 43 cm, and the other side (which is soldered to centre conductor) measures 116 cm. Cheap and nasty. I can't believe they have the ***** to charge £60 for this laughable excuse for an antenna. I wish I could get the hours I wasted on it back, but at least I now know what not to buy in the future: anything "made" by Glomex. Case closed; I'll keep the brass wire for something, the rest goes in the skip (where it always belonged).

Looks like a J-pole antenna, the shorter section is the tuning circuit, so it doesn't need a tuning coil. Interesting design.

Monopole antennas all have the problem that they either need a ground plane to work against or a tuning circuit, J Poles or Slim Jims are good ways of doing that.