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• Tuning time!

  greenscum • 04/26/2020 at 10:17 • 0 comments

  I got the proper connectors to connect the antenna to my NanoVNA netwrok analyzer so I could start tuning the antenna. My plan it to tune it to 52MHz and hope that the copper pipe gives me enough bandwidth to keep under 1:1.5 SWR across the entire 6M band: 50-54MHz. I left the pipes long so I could trim them down until I get to the right length.

  Before tuning, this is what my NanyVNA is reporting:
  

  
  When you zoom in, you can see that the antenna starts out with best matching for 41MHz. This matches with my predictions. The most exciting thing is that the bandwidth for the <1:1.5 SWR or ~-15dB of signal loss is about 4MHz, perfect to cover the 4Mhz of the 6M band!
  
  Bring on the hacksaw!
  
  I used a measuring tape and sharpie to mark off 1/2" sections so I could measure how much each set of cuts would change the resonant frequency. After cutting off the first 1/2" section from each pipe, I measured this:
  

  As you can see, that 0.50in changed the resonant frequency to about 42.5MHz or 1.5MHz per 0.50in. My -15dB signal loss bandwidth has stayed within desired parameters as well!

  Next I cut off an entire inch from each pipe, expecting the new resonant frequency to be about 46Mhz and lo:

  My prediction was not correct! I continued to make cuts in this fashion until I hit the sweet spot at 52MHz. In making these cuts, I found that having the slightest variance in length between the pipes made a big different in the reflected power. 1/8" longer on one pipe changed my VSWR from 1.09 to 1.22!

  In the end I decided to settle after the following readings:

  I had to cut the antenna a bit shorter than I had initially expects, and I think this is due to the length of the wires from the inside of the box, from the jack to the pipes.

  I also noticed that the readings on my NanoVNA varied depending on the the  position I was holding the antenna. I will do more analysis once I have it mounted, but I believe this is close enough to put on the roof tomorrow!

  73 for now, will post more in the evening.
  

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