Why different vswr on same antenna?

[Charosenz]

I built two 1/4 vertical antenna using exact same components. Tested and trimmed with Nano VNA. One had a SWR if. 1.1:1. The other was 1.5:1. I cannot for the life of me figure out why one was 1.5. Thoughts? These at 978mhz for ADS-B transmit for my airplane. KE7RQV.

 

[nd5y]

ground plane or counterpoise
proximity to other objects like coax, VNA, your hand
length of coax
position of coax
was the same coax and connectors used between the VNA and both antennas
Anything like that could affect your measurements.

 

[bharvey2]

I wouldn't discount the quality of the solder connection between the antenna element and the SO239 connector.

 

[Charosenz]

ground plane or counterpoise
proximity to other objects like coax, VNA, your hand
length of coax
position of coax
was the same coax and connectors used between the VNA and both antennas
Anything like that could affect your measurements.

Thank you for the comments. Both were tested on the same ground plane with nothing in the proximity using the same coax. I will do more testing but for now its a head scratcher.

I wouldn't discount the quality of the solder connection between the antenna element and the SO239 connector.

You know its funny you mentioned that. I didn't like the solder connection at the SO239 on one of them. But oddly that was the one with better than the 1.1:1 SWR. I felt very good about the solder on the other one and it was the one with the not so good 1.5 SWR. Both have a 0.1 ohm continuity on the vertical to SO239.

 

[KevinC]

Just me, but I wouldn't fret over 1.1:1 vs 1.5:1, you'll never notice the difference.

 

[W4AXW]

Could there be a difference in radiator length between the two? @ 978MHz, a small difference in length could cause the change...

7 3

 

[bharvey2]

From an operating standpoint, KevinC is right. You won't notice a difference but hey, some things just nag at ya....I get it.

 

[Charosenz]

Yes, I am just more than curious about the difference. I did not have acces to the same equipment today to try to figure this out. Hope fully I will tomorrow. Plus I am not getting the performance I should be getting so I am trying several solutions to get better performance. I got Some LMR240 cable to replace the RG316 I was using which will also help reduce some loss.

I did find there was almost 2mm difference. I had trimmed both to hit the bottom of the VNA graph while set for 978mhz. So that was interesting to me. This testing yesterday was done on the bench with a temporary ground plane that was 20% wider than the radiating element. I hope to do testing on the permanent location tomorrow (which is on my airplane.

No fretting here. Just curiosity and a desire to learn some solutions. .

 

[Charosenz]

Update: I thought I might add an update in case this may help future readers. I ended up realizing that my bench testing, as hard as I tried to make this as same as the final installation on the belly of my airplane, it was not close enough.

I originally had a DIY antenna that was using a 14ga copper radiating element. On the bench it tested 1.15:1. On the plane it measured about 1.4:1. The other aspect worth mentioning was that the antenna was close to the edge of the belly of the airplane. Like about 2 inches. Far from ideal.

I took a multi step approach to try to get better performance from my ADSB system. I originally had about 6ft of RG316 between the transmitter and the antenna. It measured about 1.2 DB loss. I replaced this with LMR240. That dropped it down to 0.25 DB. I tested this by seeing how well my positions were being logged by a receiver station about 45 miles away. It was a definite improvement. I was being "seen" at 2300ft.

Next step was to install a new antenna that was more towards the center of the airplane. Plus I used a SO239 with a 2mm copper wire for my radiating element and ensured it was about 1.2:1 SWR. This turned out to be another step in the right direction. I was now being captured by that same receiver but now as low as 1800ft.

My system now performs every bit as good as other systems costing thousands of dollars more.....

Having the Nano VNA allowed to to measure the system so much more accurately that the old needle analog tool. Plus I was able finally to accurately, easily measure DB loss.

While I never satisfied my curiosity for the difference of SWR readings that I originally posted, I did end up with a noticable improvement of my system which really was the most important goal.

Thanks for the suggestions.

Charlie