I've been working with Xnec2c to model a 40 meter vertical antenna I want to build for field QRP. This antenna is short at 25 feet and uses a 1:1 unun at the base with 4 radials.
The model has the load placed 1/4 of the way up (~6 feet). The model load is 35uH and 17pF. I've used toriods.info to pick an FT37-43 to have 10 turns. The web app reports that the inductor will have 12 pF. So I'm adding a trimmer cap in series.
My understanding is that b/c the antenna length is not close to being resonant, the Q must be quite narrow. I recall from my basic LRC theory, that R can widen the Q. So I added 20 Ohm to the model and sure enough the SWR flattens somewhat.
It seems like a really bad idea to add a resistor to an antenna in this way. The gain without the resistor is ~2.9 dBi. That's almost half power.
Is using a resistor like this done ?
First plot is with resistor and the second without.
I created 4 wire radials with excitation cards for each with -0.25 V with the vertical segment having 1 V. Then used the Sommerfeld-Nortonoption in the GN card. The losses were substantial for varying parameters. The antenna went far out of tune.
This really brought home to me the concept of the radials and RF ground.
I finished building the antenna with the same design spec with the exception of a wider range trimmer capacitor. I tuned the trimmer cap in my basement under very non-ideal conditions, to the lowest SWR of 18.
I then hung the antenna down my back stairs and connects three radials. The SWR was ~13 so I swapped the 1:1 unun for a 9:1 and the SWR lowered to ~1.3 without a tuner. This is all done at 7.125 +- 0.300 MHz.
I made a contact to someone doing POTA. Very satisfying.