Here's a long one to explain what I did.

The prompt I gave was

====
How would you recommend a vertical antenna with minimum fixed length be created for use on amateur radio bands of 40-6 meters, including the use of an unun, and no more than 16 radials?

====

The output was
====

1. Antenna Design: Vertical Length

Minimum fixed length: 6.6 meters (≈22 feet)
- This is a compromise length, roughly 1/4λ on 20m and usable with matching for lower bands.
Tapered or loaded vertical:
- A straight 6.6m radiator is simplest.
- A top-hat (capacity hat) or loading coil at the base can improve 40m efficiency.

2. Ground System: 16 Radials

16 radials are sufficient for a practical setup.
Length: At least 3m-5m each (longer is better for lower bands).
Elevated vs. Buried:
- Elevated radials (10-15 cm high) improve efficiency.
- Buried radials work but need more (≈32+ for efficiency).

3. Impedance Matching: 9:1 or 6:1 UNUN

A 9:1 UNUN is often better for multiband operation.
A 6:1 UNUN may work well if the antenna presents a somewhat higher impedance across bands.
Use a tuner at the rig for fine-matching.

4. Feedpoint & Matching Considerations

Direct feed with UNUN:
- Connect the UNUN at the base.
- Coax shield to the radials, center to the vertical.
Common-mode choke recommended at the feedpoint to reduce RF on the coax.

5. Band-Specific Performance

Band	Operation Strategy
40m	Electrically short, may need a loading coil or tuner.
30m	High impedance, matched by the UNUN/tuner.
20m	Near 1/4λ, good match with 6:1 or 9:1 UNUN.
17m-6m	Shorter fractions of λ, mostly efficient with tuning.

6. Optional Enhancements

Capacitive Top Hat (wires extending from the top) improves 40m efficiency.
Loading Coil at the Base (~100µH for 40m) helps on lower bands.

Final Thoughts

This design provides a simple, durable, and effective vertical covering 40-6m.
Trade-offs: Efficiency on 40m will be low unless a loading coil is used.
Best tuning method: Use an external tuner near the feedpoint for best results.

Would you like a detailed EZNEC model for this design? 🚀

====
I then said "yes" to the model and got this:
====

I'll generate an EZNEC model for a 6.6m vertical antenna covering 40m to 6m, using a 9:1 UNUN, 16 radials, and an optional loading coil for 40m. The model will include:

✅ 6.6m vertical radiator
✅ 16 radials (elevated slightly for better efficiency)
✅ 9:1 UNUN for impedance matching
✅ Optional loading coil (~100µH) at the base for 40m operation
✅ Real ground (Sommerfeld-Norton) for accurate modeling
...

Here’s your EZNEC input file for a 6.6m vertical covering 40m-6m with 16 radials, a 9:1 UNUN, and an optional 40m loading coil.

How to Use This File:

Copy & paste the content into EZNEC’s .EZ file format.
Run the simulation to check impedance & radiation patterns.
Adjust radial lengths or coil value for best results.

...
CM 6.6m Multiband Vertical Antenna (40m-6m)
CM Uses 9:1 UNUN, 16 radials, optional 40m loading coil
CE
GW 1 22 0 0 0.05 0 0 6.6 0.002
' Vertical element (Wire 1), 6.6m tall, 22 segments
GW 2 10 0 0 0.05 3.5 0.1 0.05 0.002
GW 3 10 0 0 0.05 -3.5 -0.1 0.05 0.002
GW 4 10 0 0 0.05 0.1 3.5 0.05 0.002
GW 5 10 0 0 0.05 -0.1 -3.5 0.05 0.002
GW 6 10 0 0 0.05 2.5 2.6 0.05 0.002
GW 7 10 0 0 0.05 -2.5 -2.6 0.05 0.002
GW 8 10 0 0 0.05 2.6 -2.5 0.05 0.002
GW 9 10 0 0 0.05 -2.6 2.5 0.05 0.002
GW 10 10 0 0 0.05 5 0.1 0.05 0.002
GW 11 10 0 0 0.05 -5 -0.1 0.05 0.002
GW 12 10 0 0 0.05 0.1 5 0.05 0.002
GW 13 10 0 0 0.05 -0.1 -5 0.05 0.002
GW 14 10 0 0 0.05 3.5 -2.1 0.05 0.002
GW 15 10 0 0 0.05 -3.5 2.1 0.05 0.002
GW 16 10 0 0 0.05 -2.1 -3.5 0.05 0.002
GW 17 10 0 0 0.05 2.1 3.5 0.05 0.002
' 16 radials, each 3.5-5m long, slightly elevated

LD 0 1 2 2 100.0
' Optional loading coil (~100uH for 40m) at segment 2 of vertical

GE 0
GN 2 0 0 0 13 0.005
' Real Ground (Sommerfeld-Norton), Dielectric 13, Conductivity 0.005
EK
EX 5 1 1 0 1 0
' Feedpoint at base of vertical

TL 1 0 50 450 9 0
' Transmission line simulating 9:1 UNUN (primary 50Ω, secondary 450Ω)

FR 0 1 0 7 7 14 21 28 50
' Frequency sweep for 40m-6m

RP 0 37 361 1000 90 0 1.0 1.0
' Radiation pattern analysis

EN

====

There were issues in EZNEC with some wires over top of each other, but I was able to change them.

Give it a shot!

73!

Cam, W4XXV

On Sun, Feb 16, 2025 at 1:27 PM Rick Armstrong via nrv-hams <[email protected]> wrote:

I agree with you both about the house cleaning thing!

Cam, let us know how it works! Here's the reference I used when first putting up my random wire with 9:1 unun here in NC.
https://hamuniverse.com/randomwireantennalengths.html It includes suggested lengths, and ones to avoid. If you have trouble 'tuning' it maybe you could try one of these 'good' lengths. I’m really curious how ChatGBT's decided what it did.
Cheers everyone and 73, KS4XO

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Kevin
Sent: Sunday, February 16, 2025 10:50 AM
To: NRV-Hams: A mailing list for members of the New River Valley (VA) Amateur Radio Club. <[email protected]>
Subject: Re: [nrv-hams] Antenna Modeling the EZ way

Building an antenna is always than cleaning the house!
Sounds like a fun experiment.

On Sat, Feb 15, 2025, 10:03 PM Cam Coble <[email protected]> wrote:

> Since it was raining like crazy, and I don't want to clean the house,
> I thought about some of the antenna experiments I'm wanting to do. I'd
> really like to build a multiband vertical that is simple to deploy for
> possible inclusion with the club loaner "Shack Pack". I got to playing
> around in EZNEC, watched a few videos, and thought... let's ask ChatGPT!
>
> Sure enough, given the right prompts, it gave me a solution within my
> parameters, and even spit out usable text to save for importing into EZNEC!
> Yes, there are a few errors, but ChatGPT helped solve them.
>
> I ended up with a 6.6 meter vertical with radials, and requiring a 9:1
> unun.
> Once the weather calms down, I'll build this thing and give it a
> whirl. I don't expect a crazy performer, just something workable.
>
> 73!
> --
> Cam, W4XXV
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