On Fri, Mar 14, 2025 at 04:00 AM, Nico wrote:
Look at the picture and zoom it (not zoomed it does not show up properly),
you'll see a blurred larger line around the feedline. This is the return path
or, the other half of the transmission line. This is a ground line attached to
the ground plane on layers 2 and 3. I was unsure on how to terminate it close
to the antenna attachment point, so I've just ended it close to it
Nico,
In your case the purchased helical is one half of the antenna. The other half is the "buried" side of the transmission line and the ground plane of your board. When making measurements of the antenna the outer surface of the shield of your connecting RG316 cable is now also part of the antenna. Grab the coax with your hand or attach a wire to the SMA connector nut on the VNA and you will see the SWR change. The situation is similar to what happens on a handheld radio transmitter where the operator is capacitively coupled to the radios antenna system.
You can try reducing the RF current on the coax cable with ferrites but you will not get rid of it entirely. And at this high of a frequency you will still have a considerable length of coax shield radiating until the ferrites.
Roger Need