DIY Short Vertical Wire Loaded Antenna: Using 20 Ohm Resister to Widen Q

Question

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.

UPDATE MODEL

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.

BUILD

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.

Answer 1

Great work so far!

It's reasonable to add a resistor to increase the bandwidth - if you're not using a tuner, so you need it to work in various situations and across the band, then it's your only real option. It will be less efficient but that's the trade-off. Many HF antennas are worse.

uild and measure it in situ before adding resistance. I think you'll find the input R is a lot more than you expect, because of ground losses, and these will also reduce the Q and increase the bandwidth. The same loss of course, just the power is being dissipated in the ground instead of in the resistor.

Most kinds of NEC can simulate a Somerfeld-Norton ground. Try simulating with a lossy ground and see what happens to the impedance. Keep the wires several radii above the ground for most accurate results. Use this table of earth parameters to select something reasonable for your situation:

Description	Conductivity (S/m)	Relative permittivity
Fresh water	0.001	80
Salt water	5	81
Pastoral, low hills, rich soil.	0.0303-0.01	14-20
Flat country, marshy, densely wooded	0.0075	12
Pastoral, medium hills and forestation	0.006	13
Pastoral, medium hills and forestation, heavy clay soils	0.005	13
Rock soil, steep hills, typically mountainous	0.002	12-14
Sandy, dry, flat, coastal cities, industrial areas	0.001	5
Cities, heavy industrial areas, high buildings	0.001	3

These are from the SuperNEC GUI URM, sadly very hard to find online. The manual is part of the supernec package which you can download.

Finally - In the 40 m band, 24 feet is almost a quarter wave tall, so you should get enough bandwidth for your part of the band. Couldn't you add some top loading (a T or even just a single wire hanging to one side) to make it resonate, instead of using an inductor?

Answer 2

Is using a resistor like this done ?

Occasionally (the best known example is probably the terminated folded dipole, which places the resistor at a point that has zero current at frequencies where the antenna is resonant), but there are a few things to consider:

1. Efficiency. I'm sure you realize this already, but lowering the Q like that will definitely knock a few dB off of your transmitted signal — about 2.8dB judging by the graphs you posted.

2. Power handling. I know you said QRP, but how QRP is QRP? You'll be putting almost half of your output power into the resistor, which then has to get rid of it as heat. So make sure you use a resistor with a high enough power rating (or several higher-value resistors in parallel).

3. Inductance. Wirewound resistors and many kinds of film resistors have more inductance than a bit of wire of the same length, because they're basically coils. When you add an R to your model, it will assume it's a pure resistance. Either make sure to get a low-inductance type, or be prepared to tweak your loading coil specs to compensate.

All that said, I tend to agree with tomnexus that, in the case of a vertical, you probably already have a 10 or 20 ohm resistance in the form of ground loss, so if your model isn't already taking that into account, it's lying to you, and you need to either add ground loss to your model before you go further, or just build and test (without the resistor).