Crazy mystery with VSWR

[Trance]

Maybe someone can share their knowledge and experience with this mystery. I have never encountered this before but here goes. I have a Number 5, 5/8 ground plane antenna. It's mounted to a crank up tower. I'm running 213 as my main run of coax. Tuning on MFJ shows SWR 1.1 R-51 X-1 or 2 depending if it's windy. Tuned to 27.025. Now here's the weird part that I can't explain, I apply about 500w-600w to the antenna system talk for a couple of minutes. I start to notice my SWR going up so I remove my coax and take a MFJ reading. SWR 1.6/1.7 R-36 X-19. Then I wait a couple of minutes without transmitting, take another reading the numbers start dropping. Within about 5-6 minutes with no transmission the numbers all go back to it's original state SWR 1.1 R-51 X-1 flashing 2.
Has anyone encountered this problem or heard of this happening? If you could lend some help with this I would really appreciate it. Thank you in advance.

 

[mackmobile43]

Sounds like a capacitance issue at the feed point due to heating with power applied, possibly a poor or corroded connection there.

 

[Trance]

Thank you Mack, I've been up and down the tower today a few times. I looked at the wire and it "looked" to be fine. I'll inspect the wire at the feed point a little better tomorrow. Thanks!

 

[Captain Kilowatt]

Perhaps heat in the matching coil is causing the turns to expand and shift and alter the inductance.

 

[W5LZ]

Going only by what you've said here, it's a matter of the antenna being fed too much power. Doesn't much matter what you may have been told about it's power capabilities, it's saying that 500 - 600 watts is too much.
- 'Doc

 

[Trance]

Hey Doc, thanks for the reply! Do you think changing out the skimpy little wire connecting the feed point to the coil/capacitance ring will allow the antenna to take the power? That wire always looked like a joke to me from when I first got this antenna. Correction on the antenna maker, it a Zero Five antenna 5/8 WAVE VERTICALS | ZEROFIVE-ANTENNAS but the one that I bought seems to be a low power model. Looks like the Maco design just that it has two more ground planes and a better base mount. But the rest of the antenna looks like a maco v58.

 

[BOOTY MONSTER]

what is a "Number 5, 5/8 ground plane antenna" ??

 

[Trance]

I corrected myself in the previous post. It's a Zero Five antenna 5/8 WAVE VERTICALS | ZEROFIVE-ANTENNAS

 

[Captain Kilowatt]

Hey Doc, thanks for the reply! Do you think changing out the skimpy little wire connecting the feed point to the coil/capacitance ring will allow the antenna to take the power? That wire always looked like a joke to me from when I first got this antenna. Correction on the antenna maker, it a Zero Five antenna 5/8 WAVE VERTICALS | ZEROFIVE-ANTENNAS but the one that I bought seems to be a low power model. Looks like the Maco design just that it has two more ground planes and a better base mount. But the rest of the antenna looks like a maco v58.

It's not the wire that you see connecting the feedpoint to the tuning ring but rather the wire that you don't see that is wound up inside the coil enclosure that would be the issue.

 

[Trance]

Captain, thanks for your post. I wasn't aware that there was a wire within the ring! I thought it was just hollow aluminium tubing with the ends flattened to create it's mounting points. Thanks for your insight on the ring, I'll check it out.

 

[Captain Kilowatt]

Captain, thanks for your post. I wasn't aware that there was a wire within the ring! I thought it was just hollow aluminium tubing with the ends flattened to create it's mounting points. Thanks for your insight on the ring, I'll check it out.

Now you have me thinking. There is no coil in the ring but there should be a coil in either the base of the antenna itself or mounted alongside the base of the antenna. I admit I am not real familiar with your specific antenna but I am familiar with other 5/8 wave antennas and they all have a matching coil somewhere in the base.

 

[mackmobile43]

Now you have me thinking. There is no coil in the ring but there should be a coil in either the base of the antenna itself or mounted alongside the base of the antenna. I admit I am not real familiar with your specific antenna but I am familiar with other 5/8 wave antennas and they all have a matching coil somewhere in the base.

I see they have a couple models and very well could have the problems mentioned.

 

[W5LZ]

All I have to go on are the pictures at the manufacturer's site. So, assuming that I'm seeing what I think I'm seeing, then the insulators are the determining factor for the antenna's power handling ability.
There are two methods for matching the impedance involved with a 5/8 wave antenna, one of them is by inductively matching using a tapped coil, the other is by the use of a coil and a capacitor. That vertical thingy connecting the mount and vertical element appears to be a capacitor. The combination of a coil and capacitor makes a tuned circuit, or a circuit that can change impedance. That's an assumption on my part, so don't take it to the bank, they probably won't cash it.
That vertical thingy connecting the mount to the vertical element could also be another coil (as CK thinks?). (Why would someone use two coils instead of a tapped coil though? It would work, just seems redundant to me. I don't know.)
Either way, I think that vertical thingy is probably the problem, or, the way it's mounted. With the insulator pointed up, I can see where moisture could get into the thing changing either the capacitance or the inductance which could certainly change the SWR. Again, that's just a guess since I'm not familiar with that antenna.
You might ask the manufacturer why you are seeing what you are seeing. They should have some idea.
- 'Doc

 

[W5LZ]

After seeing 'mackmobile43's post, if your antenna uses a torroid for the coil, then that would be my guess as to the problem. That torroid becomes 'saturated' (electrical fields, not moisture) and get's hot, meaning that it's 'over-loaded', can't handle the power fed through it.
And just for grins, it your antenna has a ground radial system as extensive as the one shown in one of 'mackmobile43's pictures, you ought'a have one hel'uva antenna! Wish I had the patience to do that...
- 'Doc

 

[Trance]

Hey guys, actually the antenna I have from Zerofive Antennas is not featured on his site anymore.......maybe this is why lol! Anyway it seem to be the same design as the MACO V5000 Maco Assembly Instructions The only difference is the mounting base and the number of radials. The MACO has 4 and my antenna has 6. The tuning ring in question looks almost the same as the MACO but it's all one piece unlike the MACO's ring is two pieces connected together. Other then that it's like a copy of the MACO V5000. Hope this helps a little more. Thanks again!