Example Report 11: L-Band Tapered Helix, RHCP
Description: This mystery antenna was purchased on eBay to serve as a circularly polarized test example. The markings "49956 ASSY" and "442926-1 REV 13" trace back to a Raytheon avionics antenna. The five turns of 2-1/4" spacing and 4" diameter suggested this was an L-band antenna (1 to 2 GHz). Sadly, it was mounted to a hard disk drive platter (as a counterpoise?) with a long attached BNC terminated RG59 coax. The "counterpoise" and coax were discarded, and a new aluminum base plate (ground plane) was designed and milled. An SMA feed point was installed on the new ground plane and the antenna feed was tuned with two added copper strips (details shown in photo) to create the best L-band match (VSWR). After initial testing showed the best froward directivity in the 650 to 1600 MHz range, the tuning stubs were re-optimized again for that range, thus maximizing realized absolute gain.
Analysis: After RTCE's restoration, this antenna had the following characteristics. Positive forward gain from 630 to 1600 MHz, with a peak gain of +13 dBiC at 1100 MHz. Return loss was better than 6 dB over the 760 to 1650 MHz range. The axial ratio was better than 3 dB over the range of 850 to 1350 MHz.
As always, since all data is recorded in spread sheet format, additional plots and post analysis is always possible. Full spread sheet data in Excel can be downloaded from the links at the bottom of this page.
All pictures are "click to enlarge".
| Photo of the antenna under test. |
 |
| Graph of Forward Gain vs. Frequency |
 |
| Graph of Return Loss vs. Frequency |
 |
| Graph of Axial Ratio vs. Frequency |
 |
| Polar Plot of Gain at 1100 MHz |
 |
| Excel Report Data Download | Excel File |
| Antenna Shown To Scale With Glenn Robb |
 |
| Antenna Test Axis Diagram / Photo |
 |
| Photo Of Feed Point Modifications (Input Impedance Matching) |
 |
Service Summary
Antenna Gain Patterns
EIRP, TRP, Efficiency
RF Design and Debug
RSE and Harmonics
Failure Mitigation
GSM/CDMA Cellular
