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Erratic SWR readings lately..


WSAK691

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For about the last week I've been dealing with a sort of frustrating issue with my SWR numbers.

I've got a yagi antenna installed outside about 16 feet high that went up about a month ago. When it was newly acquired I spent the time with it on the ground connected to my nanoVNA nudging the tuning block back and forth to get a nicely centered low SWR in the GMRS ranges I desired. I tightened the set screws nice and firm to lock everything in, then spent the better part of an afternoon getting it erected on my mast. Ran everything with a nice LMR-400 type coax.

Everything stayed just about as desired until randomly one day I fired the radio up and noticed that my SWR reading (From my Surecom SW-102 that just sort of lives in-line) was reading about a 2.3 SWR.

Now a little bit more context might be necessary about how I operate this setup, and I apologize for this post getting a bit long winded. I have a sort of makeshift "tower" setup consisting of a 4x4 fence post concreted about a foot and a half into the ground, and then a few sections of masting pole (from the TV antenna aisle of the home improvement store) mounted onto it. At the top of that is actually a VHF antenna for MURS. My GMRS yagi is mounted further down on a PVC arrangement that the runs down parallel to the steel mast. This arrangement is set up so that I can grab the PVC that runs down and manually twist it to point the yagi in different directions. I'll include some pictures below.

So as stated before, I noticed that my SWR suddenly spiked one day, and my assumption was that my tuning block must have moved around. Now, I've got LMR-400 run up to this antenna, which is a bit of a thick and stiffer type coax. It's got a bit of a slacked loop at the top by the so-239 connection point on the yagi to accommodate the twisting movement of the yagi, but there's definitely a bit of torque being applied on that connection point due to the stiffness of the LMR-400.  It's zip tied off a bit below to support the coax at that spot.  So with that being said, my concern was that just the slightest bit of movement at the tuning location must've caused the antenna to go out of tune..

I then spent the better part of an afternoon bringing the pole down, fussing with the tuning block again with the whole pole arrangement laid down and propped up slightly. No matter what I did, my SWR's wouldnt budge from about 2. Suddenly a breeze came through and blew the yagi from it's propped up position, spinning it down and making contact with the ground. No big deal,.. I rotated it back up and again to be upright and not touching anything, and bingo, my SWRs were suddenly back down to where I was wanting them. At that point it occured to me that the way I have this yagi set up is completely electrically isolated up in the air (with it being on PVC). Wind had been blowing and putting a charge on the antenna, and when I brought it down and had the little mishap where the antenna touched the ground, it allowed the accumulated electrons to run off.. So that was the lightbulb moment. Everything is fine, I've just got to get some sort of grounded path in my line to this antenna..

My simplest way of doing that was more on the radio side. I took a length of wire to the middle screw on the power outlet at the desk and connected that to the grounding lug that's on the Surecom SW-102, which sort of remains permanently in-line of the radio setup, whether it's powered on or not, it maintains continuity as just a "connection point" like any adapter would.. I fire it up periodically just to see what things are doing. But this accomplished my goal of getting the circuit contacting my service ground.

So here's my only remaining issue. On the VNA, it shows an SWR of 1.68 across 467. but on the Surecom it reads 1.38 when keyed up. And it does so whether my grounded wire is connected to the Surecom meter or not... It also seems to move around a slight bit day by day. I've seen it get as low as 1.01, and then randomly another day it's 1.4..

Why am I getting these shifts in SWR, and why are the readings between my meters inconsistent? Anyone have any suggestions on what to look into?

 

 

 

 

 

 

 

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Wow.  I have no answers and I’m really not trying to be a jerk but I’m so glad I simply put up my antenna and it works.  This type of stuff would drive me nuts and right out of radios.  I hate chasing junk like this.   Good luck. I hope it works out.  

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1 hour ago, WRXP381 said:

Wow.  I have no answers and I’m really not trying to be a jerk but I’m so glad I simply put up my antenna and it works.  This type of stuff would drive me nuts and right out of radios.  I hate chasing junk like this.   Good luck. I hope it works out.  

Everything does still work fine. I'm just being a bit hyper-vigilant over my SWR numbers. I thought I had a pretty good grasp on "antenna-ry" tuning, and such, but this is just puzzling me.. I just kind of want to understand what is causing this. 

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Thanks for the tips. I think I'm realizing that I need to do a better job of weather proofing my connection points as well. It's a soggy say here and my SWRs are .3 higher than they were yesterday,.. despite no other change.

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I would suspect the 239/259 connector at the antenna is either mechanically loose, or electricaly loose. When it hit the geound it made connection again. I generally put some silicone grease on mine to help keep water out, and I prefer the solder connectors over the crimps for those. N connectors do seem to behave a little better in those conditions but I haven't swapped them all yet. Many yagis are built out of non conductive booms, or electricaly isolated elements.

 

Fwiw, I have 3 yagis on a 40' pvc mast and another 40' pvc mast with a yagi and murs jpole. No static issues in 4 years, although one mast does have a grounding wire from top to bottom.

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