70 MHz GP Antenna
Short description how to make a 70MHz GP antenna from ordinary hardware store material. Mast tube and mounting brackets are from Biltema. Stainless steel base bracket, stainless steel screws and aluminium tubes are from Bauhaus. Total budget for the antenna is about 50 eur.
Main dimensions of the antenna
Radiator is made of 20mm aluminium tube.
Insulator and the top cap are turned from POM plastic. Use your own creativity if you do not have turning machine. Suitable plastic parts and epoxy glue can be used.
Radials and the radiator are fixed with stainless steel screws.
Feed wire to the radiator is also fixed with stainless steel screw. First measurements also taken.
Feed point water insulation is done with with ordinarry bathroom silicone.
Base bracket is fixed into the mast tube with 35mm wide U-bolts.
Feed cable connection. BNC connector is used in my case. Anything will do. Also feed coaxial cable can be terminated directly into the base plate and the radiator.
Final measutements are done when the antenna is in free space.
Bandwidth and matching looks good.
Tuning tips. First leave the radiator little bit too long. One meter is a good starting value. Find the resonance point and shorten the radiator as long as you find reconance at desired frequency. With radial angle you can addjust the feed point impedance. Lower the radial you get angle higher impedance and vice versa. If the radials are pointing straight down impedance is arround 75 ohm. When radials are at 90-degree angle impedance is around 35 ohms.