Machine Parts For Reuse Newsgroup Discussion Forum > Large Size Parts
> New
> Electrical
> Los Angeles
> 2 meters 70CMS dual band protable flexible antenna
2 meters 70CMS dual band protable flexible antenna
Dual Band 2/70 Portable Coax Antenna
This is a small antenna for 2m and 70cm, after using it you can roll-it up with the feeder attached (not supplied) and pack it away.
It is based on the 'Townsman' design by B.J.P Howlett G3JAM (Wireless World February 1980) but instead of using aluminium strips it uses a length of coax as the radiating element. It is 1.2 mtrs in length and is terminated with a 50ohm BNC socket at the base.
On 2mtrs it operates as a half wave dipole and on 70cms the center section acts as a coaxial coupler between the upper and lower half wave sections giving 2 x 1/2 wave co-linear. For optimal matching to 50 ohms there is a impedance transformer mounted in the base.
This antenna is as good as a 'slim jim' or 'J' antenna and has the advantage that it works on both 2m and 70cm. It out performs the 'rubber duck' antennas fitted to most hand helds so it is ideal for use in hotel rooms or other accommodation when away from the home station, just attach the end to the ceiling with a pin and let it hang down. For outdoor use just throw a string over a tree branch and pull the antenna up, you can even elevate it with a kite !!. Rolled up it takes no space at all, ideal for holidays.
The antenna has been air tested and the VSWR checked on both bands. The basic antenna is water proof but if you are going to use it outdoors for long periods the feeder connection should be taped up with self amalgamating tape.
A number of tests have been performed on this type of antenna and by coiling the feeder to form a choke just below the connection to the antenna the overall performance can be improved. This is a quote from G3SMW regarding this antenna
" We have added an RG58 "tail", part of which is coiled to form a choke. This choke is to reduce currents on the outside of the feeder that could tilt the main lobes of the vertical radiation patterns (VRP) of both bands up or down. It also avoids possible matching problems at the transceiver. This air-cored choke construction, winding the coax tail inside a 20mm PVC tube (wound with 6 turns of the PVC cable jacket touching), is satisfactory and reliable. We checked VRP's on a large flat open area (Playing Field). Whilst the 70cm VRP is not quite optimum, it does have one of its maxima in the horizontal plane and this is within about a decibel of the others. After initially trimming the top, the VSWR was below 1.5 at the centre of both bands, and below 2 at the band edges. The antenna performance is considered quite acceptable for its purpose on both bands. "
