Wednesday, July 18, 2012

Simple Antennas for the Amateur Radio Operator--a continuing series

KH6JRM's Amateur Radio Blog

Post # 170

A Simple 20-meter loop antenna

I've always had a fascination with full-wave loop antennas for the amateur radio bands.  While loops take up a lot of space, they are easy to make and generally quieter antennas than verticals.  Most of the materials for loop antennas can be found at your nearest hardware store or in your garage.  If you're a radio packrat such as I, you probably have extra wire and coax stashed somewhere near your shack. 

While there was a break in the rain showers that have soaked Hawaii Island for the past few days, I ventured into my flooded backyard to examine my antennas for signs of damage or loose connections.  Apparently, a tree branch struck the 20-meter vertical dipole in back of the garage, necessitating lowering of the fiberglass mast and the removal of the wire elements.  I decided to restring the 20-meter antenna as a full-wave loop fed by approximately 40 feet of RG-6 coax.  I cut three, 23-foot lengths of AWG 22 gauge wire from a stock of wire in the garage to form a loop.  I made the loop slightly larger than the normal 66-feet for a full-wave 20-meter antenna.  I attached the wire to the apex of a 32-foot fiberglass mast and spread out the loop to form a fairly uniform delta shape measuring 23-feet on a side.  I fed the loop at a lower corner and ran the coax into the Drake MN-4 tuner.  The swr was no more than 1.5 to 1, a mismatch easily handled by the tuner.  Once the antenna was attached to a HI-QUE dipole connector and weatherized with tape and several layers of old plastic shopping bags, I had a good temporary 20-meter antenna for my afternoon contacts.  I will later replace the RG-6 with 450-ohm ladder line and a 4:1 balun in order to operate between 20 and 10 meters.  So far, the improvised delta loop works well and gets me many contacts.  Since I had the materials on hand, I didn't need to buy anything for this project.

My other full-wave loop is cut for 40-meters and is attached to the underside of my house.  The house is about 5-feet off the ground on a post and pier system, so there was plenty of room to lay out the antenna.  The antenna is fed with 450-ohm ladder line and can be used from 40 through 10 meters.  As mentioned in another post, this is my NVIS (near vertical incident skywave) antenna and is used primarily for local nets.  The loop also serves as an antenna for my Hallicrafters SX-62-A general coverage receiver.

There are several sources available that can help you design effective antennas for restricted spaces.  Among them is a site started by Rod Dinkins (AC6V), now a silent key.  You can find many antenna ideas by visiting ac6v.com/antprojects.htm.

Good luck in your antenna design efforts.

Aloha es 73 de KH6JRM--BK29jx15