Introduction: My First Yagi, Log-Periodic Array and my Tri-Band Wire Yagi|
As a Ham Radio Operator, antennas have always been of interest to me. I have used many different antenna types over the years, DIPOLES, QUADS and VERTICALS, but my first Yagi, an old TA-3 Tri-bander mounted on a home made tilt over wood tower on top of my home was a real eye opener.
When we moved to North Vancouver, I decided to build a wire Log Period Dipole Arry, (LPDA) called the Telerana which is pictured below and also featured in the LPDA menu heading above.
A Tri-Band 2 Element Portable Yagi
After operating QRP/portable 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 Wire Yagi Article EZNEC Triband File at 30 Feet.
(Copyright ARRL. All rights reserved, used with permission of the ARRL.)
Additional Construction Notes:
Many have requested information regarding adding the WARC bands. This is very difficult as the 17 and 12 meter bands are very close to the 20, 15 and 10 meter bands and because of interaction between the elements I was not able to obtain a satisfactory SWR on all bands. Click on More-Yagis in the menu bar above for my WARC band portable yagi design.
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.
while John was operating portable in Afghansitan as YA/KX7YT.
November 17, 2009 update.
Mimmo, IZ0GIF recently sent the following email:
Hi Markus, I am Mimmo IZ0GIF from Italy. I would like to thank you for your portable yagi project that I transformed in rotatable yagi. I used fiberglass fishing poles like supports, an alluminium tube for the boom. SWR is less then 1.4 on all bands, it weights only 6 Kg and reports are very good from USA and Middle East. With a cost of about 60 Euros I think it is the cheapest rotatable yagi I have ever seen, HI. Thank you again and best 73 de IZ0GIF - Mimmo.
You may contact Mimmo via his email address at: email@example.com for further details.
There are hundreds of these antennas being used in countries all over the world since the orginal article was published in November 2001 issue of QST and in the ARRL ANTENNA HANDBOOK since 2002.