Calculator Inputs
Formula Used
The calculator first finds free space wavelength.
Wavelength = 299,792,458 / frequency in hertz
Then it applies the velocity factor and end correction.
Corrected wavelength = wavelength × velocity factor × (1 - end correction %)
The main Slim Jim lengths use these relationships.
- Long side is about 3/4 of corrected wavelength.
- Short matching stub is about 1/4 of corrected wavelength.
- Upper radiator section is about 1/2 of corrected wavelength.
- Feedpoint is a chosen percentage of the matching stub.
How to Use This Calculator
- Enter your desired center frequency in MHz.
- Choose a velocity factor for your conductor or line type.
- Add end correction when insulation or thick conductors are used.
- Set the feedpoint percentage for the matching stub.
- Enter gap, spacing, and trimming allowance values.
- Press calculate and read the result above the form.
- Export the result as CSV or PDF for workshop notes.
Example Data Table
| Frequency MHz | Band Note | Velocity Factor | Long Side | Short Stub | Feedpoint |
|---|---|---|---|---|---|
| 50.150 | 6 meter band | 0.95 | 417.408 cm (4,174.08 mm / 164.334 in) | 139.136 cm (1,391.36 mm / 54.778 in) | 45.915 cm (459.15 mm / 18.077 in) |
| 145.000 | 2 meter band | 0.95 | 144.366 cm (1,443.66 mm / 56.837 in) | 48.122 cm (481.22 mm / 18.946 in) | 15.880 cm (158.80 mm / 6.252 in) |
| 222.100 | 1.25 meter band | 0.95 | 94.250 cm (942.50 mm / 37.106 in) | 31.417 cm (314.17 mm / 12.369 in) | 10.368 cm (103.68 mm / 4.082 in) |
| 433.500 | 70 cm band | 0.95 | 48.288 cm (482.88 mm / 19.011 in) | 16.096 cm (160.96 mm / 6.337 in) | 5.312 cm (53.12 mm / 2.091 in) |
Article: Slim Jim Antenna Design Guide
Basic Idea
A Slim Jim antenna is a compact vertical antenna for VHF and UHF work. It uses a half wave radiator with a quarter wave matching stub. The design is popular because it is simple, light, and efficient. It can be built from ladder line, twin lead, copper tube, or stiff wire.
Why Dimensions Matter
This calculator helps convert radio frequency into physical dimensions. It starts with the wavelength of the chosen signal. Then it applies velocity factor and end correction. These settings matter because current travels more slowly in real conductors than in free space. Insulation, diameter, material, and nearby supports can also change the final tuned length.
Main Sections
The long side is close to three quarters of the corrected wavelength. The short side is close to one quarter of that wavelength. The remaining upper section acts as the half wave radiator. The lower section works as the matching transformer. The feedpoint sits above the shorted end. Moving it slightly can improve match and lower standing wave ratio.
Advanced Building Notes
Use the advanced fields to match real building conditions. A bare copper tube often needs a higher velocity factor. Insulated wire may need a lower value. A small trimming allowance is useful because antennas should usually be cut long. Final tuning is then done by trimming small amounts and checking with an analyzer.
Spacing and Gap
Spacing is also important. Wider spacing can change impedance and bandwidth. It can also make a stronger structure. The gap prevents direct contact at the open end. Keep the gap firm and repeatable. Secure the shorting bar well.
Real World Tuning
Results should be treated as a strong starting point. Real antennas are affected by height, feed line routing, mast material, and nearby metal. Mount the antenna clear of walls and gutters. Use a choke near the feedpoint when common mode current is suspected. Weatherproof outdoor joints.
Best Practice
For best performance, enter the center frequency of your band. Build the antenna a little long. Measure it in open space. Adjust the feedpoint first for match. Trim the long section only after checking resonance. Keep notes and export the table. Good records make the next build easier. Repeat the same measurement method after every change. This prevents confusing tuning choices. Photos also help verify spacing during later repairs.
FAQs
1. What is a Slim Jim antenna?
It is a vertical antenna using a half wave radiator and a quarter wave matching section. It is common for VHF and UHF amateur radio builds.
2. Which frequency should I enter?
Enter the center frequency you want the antenna to cover. For a band, choose the middle of your most used operating range.
3. What is velocity factor?
Velocity factor describes how fast radio energy travels through the chosen conductor system compared with free space. It shortens practical dimensions.
4. Why add trimming allowance?
Cutting slightly long gives room for final tuning. It is safer to trim small amounts than to restart with a short antenna.
5. Where is the feedpoint placed?
The feedpoint is usually above the shorted bottom of the stub. Move it during testing to find the best impedance match.
6. Does spacing affect performance?
Yes. Element spacing can change impedance, bandwidth, and mechanical strength. Keep spacing consistent across the entire matching section.
7. Do I need an antenna analyzer?
An analyzer is strongly recommended. The calculator gives a starting point, but the final match depends on real construction and surroundings.
8. Can this calculator be used for UHF?
Yes. Enter the UHF center frequency. Build carefully because small measurement errors become more important at shorter wavelengths.