Loading Coils
The magical device that makes your antenna longer by using a lot more wire in a much smaller space.
As I assume it is with most folks, people don't really appreciate lab experiments (or understand why we do them) until long after schooling. However, as an old guy who is long past times like that, I use experimentation to learn things all the time.
Today I wanted to make a few loading coils and see how they performed.
First Test
I assembled a simple wire antenna (piece of copper thru an old fiberglass tent rod) and clip the end on my Antenna Test Block.
Then I wrapped some 16ga. magnet wire around a 3/4" SCH 40 PVC pipe (grey conduit, it is an electrical project).
Below is the data I gathered.
Antenna with no coil
| Point Type | Freq. (MHz) | SWR |
| Target Freq. | 144.85 | 4.84 |
| Target Freq. | 440.20 | 1.82 |
| SWR Valley | 145 | |
| SWR Valley | 243 | |
| SWR Valley | 364 | |
| SWR Valley | 392 |
Now I placed a 50-turn coil in line with the Antenna
| Point Type | Freq. (MHz) | SWR |
| Target Freq. | 144.85 | 31.36 |
| Target Freq. | 440.20 | 2.67 |
| SWR Valley | 278.5 |
Then I unwrapped 10 (40 left) of the coils (Yes, I know this is wrong. If you didn't catch it wait for the end of the section.)
| Point Type | Freq. (MHz) | SWR |
| Target Freq. | 144.85 | 13.8 |
| Target Freq. | 440.20 | 2.22 |
| SWR Valley | 153 | |
| SWR Valley | 176 | |
| SWR Valley | 379 |
Ten Nine more out (31 now. Not only did I realize I miss counted but it occurred to me that I didn't take into account the length of the coil section or the leads on either side. From this point on, I just eye balled the cuts and noted the valleys.)
| Point Type | Freq. (MHz) | SWR |
| SWR Valley | 365.95 |
30 Turns
| Point Type | Freq. (MHz) | SWR |
| SWR Valley | 379.15 |
Results
| Number of Truns | Key Freq. (MHz) | Wavelength of Antenna |
| 50 | 278.5 | 1.077m |
| 31 | 365.95 | .820m |
| 30 | 379.15 | 791m |
- 40 turn data point was removed due to excessive length of the coil leads and inconsistent angle of the cable during SWR measurements.
Second Test
Now that I have my excitement of the first run over, I can appreciate a picture hanging in the Highland Road Park Observatory.
The second test will be more complete test. With constants and photos, well one.
This go I took 2 variables out of the equation. One, the length of each coil and leads are the same 5". Two, the antenna configuration was set up like a dipole. this would limit variations of the ground.
Now here is the photo.
There were 3 coils made one with 10 turns, 15 turns, and 25 turns. Each coil was put in the dipole and the SWR was measured.
| Turns | Freq. (MHz) *lowest SWR | SWR |
| 10 | 308.63 | |
| 10 | 465.4* | 1.09 |
| 15 | 55.43 | 1.83 |
| 15 | 264.32 | 1.78 |
| 15 | 352.94 | 1.25 |
| 15 | 420.46* | 1.07 |
| 25 | 367.71 | 1.61 |
| 25 | 395.14* | 1.15 |
The high-tech graphical data output. They don't match the data above exactly as I ran them separately. VNAs move around a lot, but the goal was to view the trend, and I feel it did that well.
Results
| Number of Truns | Key Freq. (MHz) | Wavelength of Antenna |
| 25 | 395.14 | .759m |
| 15 | 420.46 | .714m |
| 10 | 465.32 | .645m |
I have an LCR meter on order. Once Received I will measure the inductance of each coil and add the data to the list.
Next Step
The plan is to measure the inductance of the coil and then work out the math to match the measured data. From that point I will feel comfortable with estimating sizes of loading coils to build future antennas.