If you used this BNC to Banana adapter, you are adding about 2 pF or so of shunt capacitance across the feed point of your test dipole. At 500 MHz for example, this is about Xc = 159 ohms. This is significant and will impact where the min SWR will be.
Not sure if this was mentioned above or not. Sorry.
I determined that many of these adapters disrupt the characteristic Z. I came across this years ago as I have old GR bridges. GR, I think, set the spacing at 0.75 inches many decades ago and it was even significant when tinkering with EFHW antennas (the feed point is on the order of 3000 ohms so even a few pF at HF can be significant).
There are different versions of these BNC/Banana adapters and different mfrs so your adapters might be better or worse. I use them mainly for low frequency applications.
