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The information in the following
table is based on recommendations for helix design in the RSGB VHF/UHF
Manual, 4th edition. The axial-mode helix is quite broad-band and
relatively non-critical in its dimensions. Therefore the
frequencies calculated are for the center of each band. For these
bands the helix conductor can be 3 - 6 mm diameter (1/8 - 1/4 inch)
material. Copper wire or tubing is recommended, and connections
can then be readily soldered. In the quad array shown above, the lines from the
helix feedpoints to the common feed point for the coaxial line
must all be of the same length.
If designed for 1100 MHz, the
array should work well for both the 902-928 and 1240-1300 MHz bands,
IF you do not introduce some frequency-specific element in your design,
such as a 'quarter-wavelength matching section in the feedline'.
Documentation for the helical antenna indicates that it is capable of covering to
plus and minus 25% of the design center frequency.
| Frequency MHz |
Wavelength (L) cm |
Diameter cm |
Pitch cm |
Reflector cm |
| 915 |
32.8 |
10.5 |
7.2 |
82 X 82 |
| 1100 |
27.3 |
8.7 |
6.0 |
68 X 68 |
| 1270 |
23.6 |
7.6 |
5.2 |
59 X 59 |
| General Case |
30000/F |
0.32L |
0.22L |
2.5L Square |
VE3BYT
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