What can cause SWR to be one setting one day, and be something different the next day?
For reference, this is a K30 antenna positioned in the center of the roof of my crew cab Frontier truck. I set the SWR on my antenna last night and I was absolutely sure it was at 1.75 on channels 1 and 40. I remember because I had hoped it would be less than that when I was done. It had started out with SWR about 1.1 on channel 1, and close to 3 on channel 40. I kept working and clipping length until it was down equal on both channels. I couldn’t get it below 1.75.
Today I checked it again but at a different location than last night and on channel 1 it was right at 1.5 and channel 40 was closer to 2. I could also hold the PTT down and actually watch the SWR slowly rise above 2 on channel 40. It did not increase on channel 1 when I held it down, it just remained a steady 1.5.
Since I knew the SWR had changed and I wanted to go check it again in an open area, I figured I had nothing to lose by experimenting with antenna placement before going. I moved the antenna further back on the truck cab to see if it visually looked any better (which I think it did, I’m not totally crazy about the center mount for looks). I did this in the driveway at home and checked SWR while still there. This seemed to have the effect of lowering SWR on channel 1 and 40, and it looked better. Channel 1 was under 1.5 and channel 40 was right at 1.5. I thought that was gonna be the fix so I left it like that while I drove to the field again. But on the way I was picking up engine noise and turn signals and brake light noise the whole time I drove. So I moved the antenna back where it had been before when I got to the field. Then I checked SWR again and got the same results as before, almost a 2 on channel 40 when it had been 1.75 last night.
So I adjusted the antenna down about 1/8” (I just loosened the set screw and let it drop down until it bottomed out), and the SWR reading was about 1.7 on channel 1, and about 1.8 on channel 40. The whip is now bottomed out in the base and technically it is still calling for me to cut it slightly to equalize the numbers like they were last night, but they were both equal yesterday even after I had had to slightly raise the whip in the base to get them there, so I don’t understand what has changed from then until now. I don’t want to cut the whip anymore. I already had to take over an inch off of it at 1/8” to 1/4” at a time to get the SWR down under 2 on channel 40, and that felt like a lot. I feel like if I take more off then the SWR is gonna go the other way on channel 1 and there’s no putting it back once it is cut off.
So, am I just nitpicking? Or maybe I’m seeing the cheapness of this little SWR meter being inaccurate? It is a Surecom SW-114. Or am I not understanding all the characteristics of RF signals and atmospheric conditions possibly causing slight changes at different times? I understand it might be different in different areas based on surroundings, but being back in the same area again as last night and seeing different results on the meter prior to making any length changes to the antenna is confusing me.
For now I put it back in the center where it was and as far as I know I’m at about 1.7-1.8 on SWR. Channel 19 is about 1.3. I also still picked up the engine noise at high throttle on the way home, but the turn signal and brake light noise seemed to be gone. I had heard turn signal noise this morning also, but just once. So all that is leading me to believe it could just be more sensitive at certain times based on parameters I don’t understand. Even changing SWR slightly despite no physical changes occurring to the antenna or coax or radio.
For reference, this is a K30 antenna positioned in the center of the roof of my crew cab Frontier truck. I set the SWR on my antenna last night and I was absolutely sure it was at 1.75 on channels 1 and 40. I remember because I had hoped it would be less than that when I was done. It had started out with SWR about 1.1 on channel 1, and close to 3 on channel 40. I kept working and clipping length until it was down equal on both channels. I couldn’t get it below 1.75.
Today I checked it again but at a different location than last night and on channel 1 it was right at 1.5 and channel 40 was closer to 2. I could also hold the PTT down and actually watch the SWR slowly rise above 2 on channel 40. It did not increase on channel 1 when I held it down, it just remained a steady 1.5.
Since I knew the SWR had changed and I wanted to go check it again in an open area, I figured I had nothing to lose by experimenting with antenna placement before going. I moved the antenna further back on the truck cab to see if it visually looked any better (which I think it did, I’m not totally crazy about the center mount for looks). I did this in the driveway at home and checked SWR while still there. This seemed to have the effect of lowering SWR on channel 1 and 40, and it looked better. Channel 1 was under 1.5 and channel 40 was right at 1.5. I thought that was gonna be the fix so I left it like that while I drove to the field again. But on the way I was picking up engine noise and turn signals and brake light noise the whole time I drove. So I moved the antenna back where it had been before when I got to the field. Then I checked SWR again and got the same results as before, almost a 2 on channel 40 when it had been 1.75 last night.
So I adjusted the antenna down about 1/8” (I just loosened the set screw and let it drop down until it bottomed out), and the SWR reading was about 1.7 on channel 1, and about 1.8 on channel 40. The whip is now bottomed out in the base and technically it is still calling for me to cut it slightly to equalize the numbers like they were last night, but they were both equal yesterday even after I had had to slightly raise the whip in the base to get them there, so I don’t understand what has changed from then until now. I don’t want to cut the whip anymore. I already had to take over an inch off of it at 1/8” to 1/4” at a time to get the SWR down under 2 on channel 40, and that felt like a lot. I feel like if I take more off then the SWR is gonna go the other way on channel 1 and there’s no putting it back once it is cut off.
So, am I just nitpicking? Or maybe I’m seeing the cheapness of this little SWR meter being inaccurate? It is a Surecom SW-114. Or am I not understanding all the characteristics of RF signals and atmospheric conditions possibly causing slight changes at different times? I understand it might be different in different areas based on surroundings, but being back in the same area again as last night and seeing different results on the meter prior to making any length changes to the antenna is confusing me.
For now I put it back in the center where it was and as far as I know I’m at about 1.7-1.8 on SWR. Channel 19 is about 1.3. I also still picked up the engine noise at high throttle on the way home, but the turn signal and brake light noise seemed to be gone. I had heard turn signal noise this morning also, but just once. So all that is leading me to believe it could just be more sensitive at certain times based on parameters I don’t understand. Even changing SWR slightly despite no physical changes occurring to the antenna or coax or radio.
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