How do VHF whip antennas fail?

[colemj]

I have had two VHF antennas fail in the past. Not struck by lightning or anything like that - just suddenly giving a high SWR. I was just helping a friend troubleshoot his VHF, and it turned out his antenna had failed.

These were all typical whip antennas with a loaded base coil. Cutting one open reveals a simple short coil of thick solid copper wire/rod, which presumably is used to match the impedance.

Replacing the female-female connector that screws into the antenna base didn't improve things, so it wasn't that component. It can't be the steel whip, as that is just a piece of wire. The internal matching coil was solidly connected at both the whip and coax ends on the one I cut open, but I don't know about the other two. A dummy load in place of the antenna shows the coax is fine, and a new antenna on the old coax works perfectly.

So what actually fails in these to give a high SWR?

Mark

 

[Don S/V ILLusion]

One common problem with antenna ‘traps’ (coils) is moisture and dirt buildup over time. Some manufacturers design drip holes to alleviate the moisture issue but that allows easy access for dirt and bugs. Others are supposedly sealed to prevent any intrusion which is almost inevitable given the environment.


Or it could be faulty design or. construction of the loading coil. Or it could be (have been) corrosion where the PL259 connects to the antenna. Or corrosion where the antenna mount connects to the mast which interferes with the efficacy of the counterpoise which the mast serves as.

if you have/ can borrow an antenna analyzer, it should solve the mystery.

 

[colemj]

I've now had my third VHF antenna failure. It was a Vesper VHF-AIS antenna, and I also had one of these fail in the past. The other failure was a Metz.

When the Metz failed, I took it apart. Doing so was difficult because it consisted of a thick copper coil connecting the center conductor to the whip encapsulated inside a casing filled with epoxy. All the epoxy had to be dug/chipped out to get to it. I found nothing visibly wrong with it and everything looked like new. I don't know why it failed, or the mechanism of that failure.

The first Vespar failure I threw away without investigating.

Here is the second Vespar failure. The failure mode is obvious - the center conductor is connected to the coil and whip through a capacitor and the lead on the capacitor corroded off. The Metz was different electrically, as it did not have a capacitor connecting the center conductor. Just the coil. I also think the Metz used the coax shield/mast as a counterpoise, where this one does not.

The Vesper is tuned to 159MHz with a 6db bandwidth to be centered for both VHF and AIS. I suspect this capacitor is part of that tuning.

However, the capacitor and the antenna design is not very robust. The housing is hollow pipe, and while there was no apparent moisture inside (and I removed it in the rain after 4 days of non-stop rain), no doubt normal humidity itself was enough to corrode the fragile capacitor lead.

This was a great antenna while it worked. The center tuning and flat wide bandwidth gave 1:1 VSWR at both VHF16 and AIS. However, I will not be buying another one. This is made easier by the fact that they no longer sell it after the Garmin takeover.

Mark

 

[JoeWhite]

Nice job investigating it. It appears that moisture is leaking in from the top. On your next new one, I would strengthen the top with epoxy. This will make it water proof also. It will not interfere with its performance. It is worth a try.
It is possible that the person who tuned the antenna may have broken the seal at the bottom side by not holding it with a wrench when loosening or tightening the top nut.
I am not sure how old my mast top antenna is, but it works just fine after 24 years. It is the original antenna.

 

[jssailem]

I too have a Vesper antenna. It is very convenient to have the mid-band width capturing both the AIS and the VHF bands. It has been flawless for the past 10 years. Let me know what you do, so I can prepare for what appears to be the inevitable. I was saddened when I learned Garmin had bought out Vesper. I am sure it was good for the Vesper owner. The Marine industry lost an innovative radio company.

 

[colemj]

Nice job investigating it. It appears that moisture is leaking in from the top. On your next new one, I would strengthen the top with epoxy. This will make it water proof also. It will not interfere with its performance. It is worth a try.
It is possible that the person who tuned the antenna may have broken the seal at the bottom side by not holding it with a wrench when loosening or tightening the top nut.
I am not sure how old my mast top antenna is, but it works just fine after 24 years. It is the original antenna.

There is no tuning this antenna, and nothing to turn. Well, I suppose cutting the wire whip shorter would be a form of tuning it, but it comes shipped all put together, and that is how it was installed.

I also assume moisture got in from the top given where the visual corrosion is, but it was already epoxied into the housing, and that was still good. The weird thing is the iron oxide rust. This is really thick on the capacitor lead, and the solder joint connecting it to the antenna. But there is no iron metal that I can find in it - everything is copper and stainless.

I searched for when I bought this antenna, but can't find anything. It's definitely not 24yrs old. It seems to be a less robust design than all other loaded whips I've owned.

I too have a Vesper antenna. It is very convenient to have the mid-band width capturing both the AIS and the VHF bands. It has been flawless for the past 10 years. Let me know what you do, so I can prepare for what appears to be the inevitable. I was saddened when I learned Garmin had bought out Vesper. I am sure it was good for the Vesper owner. The Marine industry lost an innovative radio company.

I have a spare antenna that is supposed to be similarly tuned for operating in both bands, so will probably throw that up there. It is an inexpensive Tram: https://www.petra.com/tram-1604-ais-vhf-39-marine-antenna.html

Costs $42. We have one mounted as a second antenna on our arch and it seems to work as well as the Vesper. Don't know why it is so cheap compared to others. On the other hand, having taken others apart, I don't know why they are so expensive. I mean, that Vesper antenna was $125, and you can see what it consists of.

Mark

 

[kloudie1]

For Reference: Lightning strike failure of a Shakespear VHF base .. I suspect there may have been a little water inside that vaporized quickly..

 

[colemj]

OK, now that you mentioned lightning strikes, we have had two additional antenna failures that I wasn't counting above. One vaporized completely leaving only some burned and melted stub of coax behind. The other one had its antenna melted down like a candle stick - ending up ~6" long with rivulets of metal running down it and a pool of metal at the bottom. They were mounted together on the mast head one on each side on arms. One obviously took the strike, but a lot of energy went into the other to melt stainless like that. I don't actually know that one failed, as I just tossed it on principle, and the radio it was connected to was a slag pile after anyway.

Mark