I built a 1/4 wave spider on a type-N panel jack - using 14 gauge wire; soldered in to the jack for the antenna element, and crimped to rings and screwed on for the ground plane legs.
I followed the plan details and cut the antenna length to 69 mm from the “white plastic” of the jack to the tip of the wire.
I have purchased a NanoVNA and decided to hook it up and test to see how it looks. I first calibrated the VNA using the SMA fittings (open, short, load) and then attached the antenna - with a “Type N to SMA adapter”. The resulting plot shows a SWR minimum around 990MHz
[range is 800MHz to 1300MHz]
I changed up the adapters (started with N-Male to SMA-female, swapped for N-Male to SMA-male & SMA-female to SMA-female) to better reflect how the VNA was calibrated with the SMA-F/F in place before the open/short/load.
The results were different, but still shows this isn’t tuned for 1090 MHz. The long flat bottom ranges from ~900MHz to ~1100MHz ; centered around 1000MHz, and the true minimum is slightly below that.
In each case, the peak impedance is around 1020MHz.
So how should I interpret this? In the case of the first VNA output the min SWR at 980MHz suggests the effective antenna element length is ~76mm not 69mm. My measurement isn’t off by 7mm - could there be 7mm of “additional length” in the fitting itself? That seems unlikely to me.
I accept this isn’t a perfect measurement, since there’s an adapter in between the antenna and the calibrated VNA, but I wouldn’t have thought that could have this big an effect.
I’m hopeful that some of the more experienced antenna builders can give some advice.