I had a great, but minimalist Field Day this year, where I finally got to transmit on my Icom IC-705, operated on 6 meters for the first time, and experienced QRP operation for the first time.
As part of this effort, of course I put up an HF antenna for use on Field Day; an 80M/40M fan dipole.
My friend Mike, KG7HQ, hooked up his portable antenna analyzer to it and found it looked okay for 80M, but was way off-frequency for 40M. It was showing a VSWR dip closer to 6.6 MHz rather than the 7.2 MHz expected for 40M operation.
I finally got a few minutes to get my PocketVNA VNA running and calibrated, and I took a closer look:
As can be seen from the above, the 40M portion of the fan dipole definitely appears to be a bit too long. Doing some rudumentary calculations using the classic dipole length formula (468 divided by frequency), I determined that the 40M dipole was about 2 feet, 9 inches too long.
So today, I went out into the rain, lowered the 40M dipole legs and shortened them by about 2 feet, 6 inches. Running another scan on the VNA, I can see that the 40M dipole now resonates within the 40M band, albeit still a little low:
It’s always comforting and reassuring when there is some evidence that theory and practice sometimes match, especially when it comes to antennas and RF.