A TRI-BAND 2 ELEMENT PORTABLE YAGI
After operating QRP during the 1996 ARRL Sweepstakes Contest I decided that the next
time I entered a QRP contest it was going to be with an antenna that had gain over the dipole
antennas I was using! Operating from a temporary location, a tower and tri-band yagi
constructed of aluminum tubing was not an option. After much thought and a lot of reading, I
decided to build a tri-band 2 element yagi using wire elenments. The results of this experiment
are contained in an article published in the November 2001 issue of QST. A PDF file of the
article as well as the EZNEC antenna files is available at the following links. |
Portable 3-Band Yagi Article
EZNEC zip file
When you are ready to assemble the array, attach the 20 meter reflector and the driven elements first and then hang the array between two supports, (trees etc.) at about 5 feet above the ground. Pull it tight so that the array is fairly flat. It won’t stay flat because the driven elements are heavier than the 20 meter reflector so support one end of the 2 x 2 end supports on a rung of a step ladder or box so that the array lies horizontal. Add the 10 and 15 meter reflectors next using a little less tension than the 20 reflector. Next attach the feed line. The last step is to adjust the V slings as mentioned below so that the antenna is balanced in the horizontal plane. All adjustment for lowest SWR, if needed, must be done with the antenna raised to its typical operating position. Do not attempt to adjust any of the element lengths or the hair-pin match for lower SWR while the array is close to the ground. Check the SWR on each band with the antenna at a height of at least 25 feet before attempting any adjustments.
There are two V slings, one on each end. The secret is that they are not equilateral in shape. The combined weight of the driven elements and feed line is heavier than the reflectors. If the length of the sides of the V sling are equal, the array will turn with the 2 x 2 end supports positioned vertically. Increasing the tension on the side of the V that is attached to the end where the driven elements are attached causes the whole array to turn toward the horizontal plane. Do this by shortening the length of the side of the V that is attached to the driven elements. It is quite easy to adjust in the field. Once the V sling is adjusted, the array stays balanced. You can change direction 180 degrees by pulling on the the feed-line. If you pull it hard enough, the whole array will begin to turn-flip over. Stop it from turning too far by holding on to the feed line once the array has swung over to face the opposite direction.
It is not clear what to do with the coil of coax at the feed point in Fig. 1. This is meant to be a choke balun to choke off any RF from flowing along the outside of the coax shield. It is best to let the coax feed line drop straight down from the centre insulator, attach it to the centre of the hair-pin shorting bar with a plastic tie or string and then make the balun by coiling up the coax 6 to 8 turns with a diameter of 4 inches or so just below this point. The centre of the shorting bar is neutral potential so there is no problem mechanically attaching the feed line to it for support.