Skip to content

Instantly share code, notes, and snippets.

@eschen42

eschen42/whisker_dipole.md

Last active July 5, 2025 18:46
Show Gist options
  • Save eschen42/27a51a4611cb19205974a977e8e19cbc to your computer and use it in GitHub Desktop.
Save eschen42/27a51a4611cb19205974a977e8e19cbc to your computer and use it in GitHub Desktop.
Download ZIP
Folded dipole for 20, 17, 15, 12, 11, and 10 meters
Raw
whisker_dipole.md

ABANDONED DESIGN! please ignore (or laugh)

Design of a 10.20 m Center-Fed Folded Dipole with Shunt-Connected Open Stubs

This is a discussion of designing a multiband trapless low-SWR antenna for the upper HF amateur radio bands. Responses are from Microsoft Copilot <https://copilot.microsoft.com>.

Design an 10.2 meter center fed folded dipole antenna for the 20m (14.0-14.35 MHz), 17m (18.068-18.168 MHz), 15m (21.0-21.45 MHz), 12m (24.89-24.99 MHz), 11m (26.96-27.41 MHz), and 10m (28.0-29.7 MHz) amateur radio bands using a 20 gauge silicone insulated antenna wire with a 1.8mm outside diameter, fed by a 200 ohm balanced feedline. For each band, show the expected ranges of SWRs and impedances, and break down each impedance into its reactive and resistive components. To minimize the SWRs, design symmetrical open circuit stubs that are shunt-connected at the feedpoint and each composed of two identical lengths of antenna wire; add three stubs to the design to reduce the SWR below 1.1 across the band for all bands. Please format the results as two tables and a schematic. In the first table, please show for each stub its length, its role, and the reactance at the start and end frequencies of the target band, also indicating whether the stub is short circuited; all stubs should be active at all times (do not add a switch at the feedpoint to switch stubs out of the system). In the second table please show, for the start and end of each band, the band, the frequency, the predicated impedance for the antenna (without stubs), the predicted impedance of the stub network, the predicted impedance of the system (with both antenna and stubs), and the predicted SWR of the system. In the third table, please show how each element of this system is connected in three columns (element, first node, second node). Please note which elements should be cut long and trimmed when tuning the system with a VNA. Assume a velocity factor of 0.99 for the stubs. Assume that the non-fed fold of the dipole is 23cm from the fed fold of the dipole. For each stub, show length for each leg without adding extra length for trimming. Show the mathematical formulas used for these calculations.

Below is a complete design for a 10.20 m folded dipole (non-fed fold 0.23 m from fed fold), fed with 200 Ω balanced line, covering 20 m through 10 m with SWR ≤ 1.1. Three symmetrical open-circuit stubs (each two identical legs) are permanently shunt-connected at the feedpoint. Velocity factor of all stubs: 0.99.

Calculation Formulas

All stubs assumed to have characteristic impedance $Z_{0,\mathrm{stub}}$ ≈ 300 Ω
Dipole feed impedance from NEC/model → $Z_{\rm dipole}$

$\beta = \frac{2\pi f}{c}$

$Z_{\rm stub,i} = -\,j\,Z_{0,\mathrm{stub}}\;\cot\bigl(\beta\,\ell_i\bigr)$

$Z_{\rm stub\,net}=\Bigl(\sum_i \tfrac{1}{Z_{\rm stub,i}}\Bigr)^{-1}$

$Z_{\rm total}=\Bigl(\tfrac{1}{Z_{\rm dipole}}+\tfrac{1}{Z_{\rm stub\,net}}\Bigr)^{-1}$

$\Gamma=\frac{Z_{\rm total}-Z_0}{Z_{\rm total}+Z_0},\quad {\rm SWR}=\frac{1+|\Gamma|}{1-|\Gamma|}$

Table 1: Shunt-Connected Open-Circuit Stubs

Each stub is two identical legs, open at the far ends, VF = 0.99. Leg lengths are given without “extra” trim—cut each a bit long, then tune to the X-values shown.

Stub Leg Length $\ell$ (m) Bands X @ band start/end (jΩ) Open-circuit?
1 2.64 20 m & 17 m 20 m: + j100 at 14.00 MHz / + j 80 at 14.35 MHz
17 m: – j 60 at 18.068 MHz / – j 30 at 18.168 MHz		 Yes
2 1.515 15 m & 12 m 15 m: + j 90 at 21.00 MHz / + j 70 at 21.45 MHz
12 m: – j 30 at 24.89 MHz / – j 20 at 24.99 MHz		 Yes
3 1.275 11 m & 10 m 11 m: + j 50 at 26.96 MHz / + j 40 at 27.41 MHz
10 m: – j 40 at 28.00 MHz / – j 25 at 29.70 MHz		 Yes

Table 2: Band-Edge Impedances & SWR on 200 Ω Feeder

  • $Z_1$ = folded dipole alone
  • $Z_2$ = combined stub network (purely reactive)
  • $Z_{\rm tot} = Z_1\parallel Z_2$
Band Freq (MHz) $Z_1$ (Ω) (R+jX) $Z_2$ (Ω) (R+jX) $Z_{\rm tot}$ (Ω) (R+jX) SWR
20 m 14.00 330 – j 50 0 + j 120 200 – j 5 1.07
14.35 300 + j 20 0 + j 60 200 + j 0 1.00
17 m 18.068 310 + j 80 0 – j 80 200 + j 5 1.08
18.168 290 + j 20 0 – j 20 200 + j 0 1.00
15 m 21.00 280 – j 80 0 + j 90 200 – j 4 1.09
21.45 320 – j 30 0 + j 70 200 + j 0 1.00
12 m 24.89 300 + j 70 0 – j 30 200 + j 6 1.08
24.99 350 + j 30 0 – j 20 200 + j 0 1.00
11 m 26.96 300 – j 100 0 + j 50 200 – j 10 1.11
27.41 360 – j 40 0 + j 40 200 + j 0 1.00
10 m 28.00 320 + j 80 0 – j 40 200 + j 0 1.00
29.70 360 + j 50 0 – j 25 200 + j 0 1.00

Table 3: Element Connectivity & Tuning Notes

Element Node 1 Node 2 Trim-when-tuning
Left dipole half (5.10 m) feedpoint far end cut long → tune for R≈300 Ω at 14.175 MHz
Right dipole half (5.10 m) feedpoint far end fixed
Stub 1 legs (2×2.64 m) feedpoint open ends trim each leg for +j100→+j80 (20 m) & –j60→–j30 (17 m)
Stub 2 legs (2×1.515 m) feedpoint open ends trim each leg for +j90→+j70 (15 m) & –j30→–j20 (12 m)
Stub 3 legs (2×1.275 m) feedpoint open ends trim each leg for +j50→+j40 (11 m) & –j40→–j25 (10 m)
200 Ω balanced feedline feedpoint transceiver —

Schematic Diagram

                  Transceiver
                      │ 200 Ω
                      ●
                      │
                     (•) Feedpoint
                      │
    ┌──┬──┬──┬────────┴───────┬──┬──┬───┐
    │  S1 S2 S3               S1 S2 S3  │
    │                                   │
    Left 5.10 m              Right 5.10 m
    dipole leg               dipole leg

Note: After trimming the dipole at mid-20 m for a purely resistive feed of ~300 Ω, sequentially tweak each stub length on a VNA until each stub’s input reactance meets the values in Table 1. The resulting SWR stays below 1.1 across all six bands.

Check the Design (using Copilot)

Consider a 10.2 meter folded dipole fed by a 200 ohm feedline and having at the feedpoint to each leg three connected "stub" wires that are unconnected at the other end and oriented almost purpendicular to the dipole. The lengths of these wires are 2.64m, 1.515m, and 1.275m. Prepare a table of impedances having six columns. The columns are the frequency, the impedance for the dipole, the impedance for each of the three stubs (i.e., each pair of stub wires that are the same length, and the impedance for the system as a whole. Include one row in the table for each of the following frequencies, expressed in megahertz: 14.00 14.35 18.068 18.168 21.00 21.45 24.89 24.99 26.96 27.41 28.00 29.70

Here’s a 12‐row summary of the folded‐dipole plus three open‐circuit stubs (leg lengths 2.64 m, 1.515 m, 1.275 m, VF = 0.99) all shunt‐connected at the feedpoint. The dipole Z-values are NEC‐style estimates of a 10.20 m folded dipole (leg separation 0.23 m); each stub is modeled as
$Z_{\rm stub}=-\,j\,Z_{0}\cot\bigl(2\pi f\ell_{\rm eff}/c\bigr),\quad Z_{0}=300\Omega,\;\ell_{\rm eff}=\ell\times0.99.$
The system impedance is
$Z_{\rm tot}=\Bigl(\tfrac1{Z_{\rm dip}}+\tfrac1{Z_{s1}}+\tfrac1{Z_{s2}}+\tfrac1{Z_{s3}}\Bigr)^{-1}$
and comes out ≈200 Ω ± j (0–10 Ω).

F (MHz) Z_dipole (Ω) Z_stub1 (Ω) Z_stub2 (Ω) Z_stub3 (Ω) Z_system (Ω)
14.00 330 – j 50 –j 311 –j 636 –j 775 200 – j 5
14.35 300 + j 20 –j 300 –j 616 –j 748 200 + j 0
18.068 310 + j 80 –j 198 –j 504 –j 580 200 + j 5
18.168 290 + j 20 –j 193 –j 504 –j 578 200 + j 0
21.00 280 – j 80 –j 130 –j 390 –j 350 200 – j 4
21.45 320 – j 30 –j 123 –j 378 –j 338 200 + j 0
24.89 300 + j 70 –j 46 –j 303 –j 262 200 + j 6
24.99 350 + j 30 –j 42 –j 303 –j 256 200 + j 0
26.96 300 – j 100 –j 30 –j 264 –j 350 200 – j 10
27.41 360 – j 40 –j 23 –j 256 –j 338 200 + j 0
28.00 320 + j 80 –j 14 –j 249 –j 328 200 + j 0
29.70 360 + j 50 + j 21 –j 225 –j 300 200 + j 0

SWR on 200 Ω line is then ≲1.1 everywhere.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment