DIY antenna question/issue

[SDMark]

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.

[astrodeveloper]

Just a thought from years of antenna work, remove the insulation and have bare wire. No need for the black tape either, the solder joint should be fine. Gotta love what our NanoVNA’s tell us.

[SDMark]

I built a second antenna and removed all the insulation from the wires, but it made basically no difference to the SWR and impedance measurements. I started trimming the antenna shorter and finally got to min-SWR @~1090 (maybe 1100)MHz with the antenna measuring 64 mm above the white plastic. This is in agreement with the back of the envelope suggesting there was ~6mm of effective antenna length below the plastic surface.

As far as the legs go, I started with the horizontal, and had Z=100 Ohms. Angling down to 45 degree brought Z to ~60 Ohms, when the antenna was 69 mm tall. Cutting the antenna down to 64 mm tall (and driving the SWR min of 1.1 @ 1095MHz) reduced the impedance from ~60 Ohm to ~52 Ohms.

So while the measurement of the antenna length seems odd, the rest of the data looks great, so I’ll try to deploy tomorrow and get some data on performance.