For those of you not particularly interested in ham (or other) radio antenna geekery, you may as well just skip the rest. Thanks for stopping by and have a great day!
Back on the 5th, I mentioned that when I got the tower back up I looked at one of the antennas for the first time since before Ian. Briefly, because it looked good and nothing was obviously wrong, I assumed it was good. In reality, it had a failure of something that apparently was going bad and was going to fail. The swept VSWR went from 1.1 or 1.2:1 up to over 5:1. For reference, this is for the six meter ham band and is the antenna I put up last January.
The story begins with the comment thread to that Nov. 5 post started by drjim. I went down the road of the connector/cable interface and thought I had found the issue. Until I touched it again and the VSWR went from that 1.1:1 back up to 5:1 again. That was on the 6th and I wasted most of the afternoon in an endless loop of doing something, finding the VSWR was fine, doing the next step to reassemble only to find it was awful again. Then I'd undo the change and it stayed awful.
On the 7th, I realized that going outside, walking essentially the length of my yard between the antenna and the ham shack to sweep the antenna wasn't the way to do this. Any technician knows that when you're trying to fix an intermittent problem, you need to come up with ways to make it fail again. That's when I switched to the NanoVNA H4 (I think it's this model) that I bought at the Orlando Hamcation last February so that I could watch the test equipment while touching things and found the intermittent problem in far less time than one walk back to the shack.
The problem was that Philips-head screw in the upper right corner of the picture with an orange wire going to it as well as the end of a piece of coaxial cable. I would bump into that accidentally or touch that coaxial cable and the VSWR would jump radically, but I didn't realize what was causing that until I touched that right cable while watching the NanoVNA.
That screw doesn't tighten down normally. Not visible in this view is that it screws into a Delrin insulating cylinder. The screw at the top left also does that, but it tightens properly; the one on the right never stops turning.The appearance of this area and the corrosion evident after just under a year of being outside is a bit of a surprise to me. The manual cautions about coating this area with anything other than clear Krylon spray paint. I didn't have any last January so I omitted that. Bad idea.
DO NOT, UNDER ANY CIRCUMSTANCES, APPLY ANY TYPE OF SEALANT OR COATING TO THE DRIVEN ELEMENT, T-ARMS OR CONNECTOR ASSEMBLY, OTHER THAN KRYLON® CLEAR COAT. ANY OTHER COATING WILL ADVERSELY AFFECT THE SWR AND VOID YOUR WARRANTY
I will coat that area once this is fixed.
As for exactly what the fix is, I don't know at the moment. The insulator holding that screw is black Delrin, which I assume is black because it has carbon black in the mix. That's often done for plastics left outdoors to protect against sunlight. I have some white Delrin that I could make a replacement insulator from. Since it seems the #6 screw must have boogered up the Delrin it might be that simply going to a #8 screw, next bigger size, would help it grab well enough. That's not an unusual fix in the hobby machinist world.
A PrintScreen dump from the Windows PC app I use with my NanoVNA, called (appropriately enough) NanoVNA-App by Some Dood calling himself OneOfEleven. This was taken the first time it looked like I was done, back on the 7th. The VSWR plot is in the lower right hand corner, and you can see the absolute minimum value is 1.003:1 a virtually perfect match at 50.6125 MHz which is a frequency I have never operated on (and probably won't). The top right is the same information displayed on a different scale, called Return Loss; top left is the impedance displayed on a Smith Chart while bottom left is the signal seen at port 2 from port 1 and is nothing because port 2 isn't connected to anything. When that screw connection I've been talking about opens, that lower right trace never goes below 5:1
I apologize for probably wasting your time, but I look at the photo and I think of:
ReplyDeletehttps://www.engineeringtoolbox.com/electrode-potential-d_482.html
It's something that I've tried to design the big stuff around. The problem is that something like this is a kit and you get what they ship unless you buy all new parts.
DeleteLast June I replaced the galvanized boat winch that cranked it over with a stainless winch. I think galvanized winches last about five years.
Maybe I ought to sink a sacrificial anode in the yard.
perhaps it's my eyeglass Rx, but I think I'm seeing dissimilar metals screwed tightly togetther Don't even need a saline environment if you're within 100 miles of the coast. I can remember chewing on a piece of aluminum foil with a silver amalgam filling
DeleteI'm sure there are. The Philips head screws, go through stainless hardware, crimp on terminals which I think are ordinarily tinned (tin-lead solder), stainless lock washers, and through aluminum. The major elements of the antenna are aluminum. The bracket is aluminum, the N-connector (other end of the orange wire) is steel and looks to be silver coated which is common in radio connectors.
DeleteI've heard many times that the beach side here has the saltiest air in the US. I can literally see the difference in things from the east side of the house to the west side.
Wouldn't hurt to look up how to fit a sacrificial zinc. There are lots of boat stores in your neighborhood and there's always the net.
Deletenot to belabor the point but...
DeleteI don't think a sacrificial lamb is ever a bad idea (to appease the gods) but...
I'm talking about about very highly localized electrochemical corrosion that has been increased due to a highly saline environment.
The corrosion can be reduced/delayed (but not avoided) with the application of a Krylon-type spray, but, if it can be accomplished, the use of the same metals/(preferably the same alloys) in a junction such as this.may reduce the corrosion/weakening (due to electromotive differential) to near zero.
(and, of course, all elements/compnents of Chinesium should be avoided like the plague)
Just my one penny.
And I don't mean to minimize or belittle that point. I'm painfully aware of that corrosion. I've been working on it for decades.
DeleteAt some point, and I don't know where that is, if I cut back wires in that area to terminate them in a compatible metal (if the required crimp-on connectors even exist) I'm going to de-tune the antenna. Electrical length is everything. The lengths we're talking about are pretty small, but the impedances in this area are all over the place and I don't have a feel for the changes I'll see.
What brand antenna is that? It's sure not an M2! I'd run a tap in it first before I just ran the larger screw in. I've used Delrin many times for insulators, and it machines nicely, as I'm sure you know. It probably "fretted" the threads from vibration, causing the screw to loosen. I'd put a dab of Loctite Threadlocker Blue in the stick formulation on it, too. The elements vibrate in the wind, and make things get loose.
ReplyDeletehttps://www.amazon.com/Loctite-Blue-Threadlocker-Stick-Single/dp/B01LVW5294
That's a Directive Systems & Engineering antenna. The model is their DSEJX5-50. I had never heard of them until I started reading the FrontRange6Meter groups.io. A lot of people were speaking highly of them.
Deletedrjim, not really sure if cyanoacrylate (in the threadlocker) sticks to Delrin very well, I know for a fact that Hytrel does a good bond with cyanoacrylates. Of course, Hytrel is a LOT softer but still has good UV resistance. Ah, plastics....
DeleteI didn't know DS made 6M beams. I'm more familiar with their VHF/UHF stuff, although 6 does qualify as "Low VHF". I'd put anti-sieze on all those bolted connections, and put a little "tupperware" type box over it to keep some of the moisture off. It really helped in Long Beach, where we were about 5 miles from the beach.
Delete@Igor - It really doesn't have to stick to the Delrin. It fills up all the little spaces and gaps in the threads, and then turns solid in the absence of air. I wouldn't use it on super heavy duty stuff, as it's just intended to vibration-proof things. I've used it before to hold steel fasteners in plastic on my radio controlled cars, and it works "As Advertised".
That stick format is a new one on me. I have two colors of LocTite liquids, green and red, and another company's version of blue liquid. I could go out in the shop to find the bottle and mention who made it but that would take years of searching and undoubtedly cost the life of at least one Sherpa. Unless I don't need to find it, in which case it will be on top of something with a spotlight on it.
DeleteOK, I'll ask the dumb question. Why is there a ground strap going to the driven element in the first place? I'd be very surprised if the capacitance between the two ring terminals has anything to do with performance. So, the screw there attaches the element to the frame, and you do need an insulator for the screw - frame, but it doesn't need to be threaded. A machine screw going through a shoulder bushing with an insulating washer on the other side should do it. Of course, this is from looking at just one picture - I could be missing something important.
ReplyDeleteAs far as using ONLYKRYLON - what a bunch of crap. I doubt it'd adhere to Delrin anyway, and so you'll still end up with little micro-gaps where salty moisture can infiltrate. Maybe the Krylon will fill gaps? Even so, I'd still use something where I could be sure of a good seal all around. I've been using Gardner-Bender Duct Seal, which comes in a brick at your local big-box home improvement store. It has its down sides. I know a guy who recommends wet suit repair glue - I haven't tried it. Even liquid electrical tape could be good.
Why is there a ground strap going to the driven element in the first place?
DeleteYou're not seeing the driven element. This is part of matching the coaxial cable from the radio to the antenna. The cable from the radio isn't exactly visible in the picture but the end of it attaches to the big round connector in the middle with the orange wire being the center conductor of the coax. The shield of the cable is attached to ground, which is everything except those two metal rods at the top of the picture that are insulated from ground by the black plastic.
The stuff about Krylon I interpret as saying "we tested this and it worked: if you use anything else, we don't know if it will work, so it voids your warranty."
OK. Got curious enough to RTFM. I keep seeing hams call their coil of coax a "balun". Aaaaarggghh! I don't know why anyone does that. But I didn't pick up on those coax ends as being what they were. With just a quick read, I don't know what it's doing, or whether it's part of the T-match. But whatever. Not my monkey. :) Mongo just push button on LDG tuner.
DeleteAt 5:1, I could push the button on and LDG tuner, too, but two things stop me. The first is reconfiguring my station to use the LDG instead of the builtin tuner with the 3:1 limit. The other is that I have measured data showing it worked before, and works even better if I jiggle that wire just right, so why settle for a broken antenna? Does it even have the right antenna pattern if the driven element isn't working right?
DeleteYet another concern is never forget if your tuner is inside near the radio, the transmission line still has the bad mismatch, and that can drastically increase its loss.
I was just joking about my "Mongo" level of being non-knowledgable about that match on that antenna. Well, in general, I don't know as much I could about the various match methods.
DeleteIn your shoes, I would be quite anal about getting all that very clean, protected, and all buttoned up. Irrespective of SWR and trans-match units, I've been bit a few times by loose connections.
Sacrificial anode ground rod?
ReplyDeleteI have three ground rods, all copper, why not another one?
DeleteIf it's still under warranty, I'd hit them up for a couple of those insulators. Tell them what happened, and they might send you a couple new. I haven't used Liquid Electrical Tape, but I have friends who swear by it for sealing connections like on your antenna.
ReplyDelete