What does CHAD stand for? . It stands for “Compact Helical Asymmetric Dipole”. This is a new design of compact vertically polarized omni directional antenna. The completed size being only one tenth of it’s operating wavelength in use, the antennas foot print is no larger than my size 8 shoe, no radials. It has a low angle of radiation (20 degrees) and a max gain of 2.5dBi.The 30m antenna is 3m long and the 70cm model is 60mm tall!
This new antenna design has received a UK patent, and process of obtaining a US patent is ongoing.
A commercial company is currently looking into developing this antenna.
Models have been successfully constructed at HF, VHF and UHF frequencies, from 3.5MHz to 1.8GHz, where a directional CHAD has been made with a forward gain of some 5dBi (1.8GHz). The omni directional models have a lower gain characteristic and exhibit a very low angle of radiation. The antennas have been tested and show a low VSWR at resonance over their range of operation. The HF models have been designed to operate on 3.5, 5.0, 7.0, 10. 14, 18, 21, 25 and 28MHz, VHF models have worked on 70, 100 and 145MHz. UHF models have been made for 435 and 1800MHz, and all models have worked very well. The HF models have all been tested and mounted with the feed point some 4m from ground level.
The CHADs have not been designed as the holy grail of antennas, but they have been designed to give as low a visual impact as is possible in an urban environment with good performance.
The very first CHAD operating on the 30m amateur radio band, VSWR over whole band <1.2:1, mid-band 1.05:1. Tested with a transmit power to 100w.
HF tri-band and bi-band antennas: operating with a VSWR <1.5:1 on each band over the whole band. Models include 20/17/12m and 20/17m antennas. Tested transmit power to a maximum of 100w.
The VHF antennas have been tested, working on 4m and 2m amateur radio bands. A selected receive only model was produced for general HF radio, this is some twenty eight inches tall, again - no radials.
Two models have been produced for testing, the 1.8GHz model which has been professionally characterized is for use in mobile communications applications. The second model operating at 435MHz, measures 60mm in height and 25mm in diameter, and has a measured VSWR over the 70cm band of less than 1.3:1 (over the whole 10MHz). The lowest VSWR was shown to be 1.05:1 at 433MHz, and this was further tested with power levels at 5 and 40watts. It didn’t turn into a chard mess, but continued to work.
With the recent purchase of software to simulate the design to verify my field results I am hoping to get antennas in the pipeline soon for manufacture and sale. The project is being kept a little low key at the moment as there are a number of commercial applications.
See the following page for VSWR plots of two antennas.