135 MHz Portable Beam
– May 2007 QST by James Hanson W1TRC
Opened Up and Ready for Use.
Presented here, is a simple, but effective tool to assist in location of RFI sources that frequently plague ham radio operators.
I live in a rural area that has very old power lines, and frequently suffer from noise problems. Using this antenna, in conjunction with my Ultrasonic Power Line Arc Detector, helps to pinpoint troublesome RFI sources. My power company has been very responsive to fixing problems, especially when I can help them identify the troublesome poles.
This Project is based upon an article in the May 2007 QST Magazine (Pg 28) by James Hansen, W1TRC. Original article can be found here. It is designed for use on 135 MHz aircraft frequencies, since noise sources are easier to detect on AM. James has also written an excellent article on an Ultrasonic Power Line Arc Detector, which I have also built, and is described HERE.
When connected to your VHF Mobile rig (I use a Yaesu FT-7800R) or Handi-Talkie, you have very useful portable RFI detection tool. I use the antenna’s forward gain and directivity, to triangulate the RFI. After getting close to the source, the very deep null off the back of the antenna, is used to isolate the problem pole. I highly suggest using a step attenuator between the antenna and your radio to prevent overload. Adjust the attenuator to reduce the noise level to a lower level, making it easier to detect audible level changes when trying to isolate the noise. Start with little or no attenuation and add more as you get closer to the actual interference source.
The elements are very flexible, making the antenna very easy to get in and out of the car door. The elements just pop back into shape after they come in contact with anything. When not in use, the antenna breaks down into small pieces for easy storage. Very Cool!
The construction details are very straight forward, and all of the components can be obtained for under $10 USD, at your local hardware store. Consult James’s article in the May 2007 QST for element and boom dimensions.
Good Luck, and happy hunting!
73’s Frank, K7SFN
Disassembled for storage
Elements Folded for Transport
Driven Element Attachment showing Coaxial Balun and Hairpin Match.