[nrv-hams] Antenna Modeling the EZ way

Cam Coble cam at doubleclickdata.com
Sun Feb 16 13:51:58 EST 2025 

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:*

   1. Copy & paste the content into *EZNEC’s .EZ file* format.
   2. Run the simulation to check impedance & radiation patterns.
   3. 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 <
nrv-hams at mailman.qth.net> 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: nrv-hams-bounces at mailman.qth.net <nrv-hams-bounces at mailman.qth.net>
> 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. <nrv-hams at mailman.qth.net>
> 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 <cam at doubleclickdata.com> 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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