"Inverted L" - Antenna Construction Tutorial
by Brian Ingoldsby K7ZRZ
|This tutorial describes a method of erecting an Inverted L antenna in a temporary or portable location. I am now retired and have taken up volunteering for the Oregon State Parks and Recreation Department, serving as campground host or caretaker at a number of day-use and heritage sites - 11 months of each year. I serve no more than 2 months at a time in one location - after which I must pull down the antenna installation and move it all to another location. The methods I have developed are perfect for hanging the wires and laying out the radials in a very short time and with a minimum of trouble. Each location is different, but I have been fortunate to have lots of great trees to work with each place I've been in the last three years. My typical wire length is about 145 feet... some times longer, and occasionally somewhat shorter. (depends on the location of the trees) I have a full kilowatt amplifier that I drive into this antenna every day, and have had no problems with arcing - in any kind of weather.|
Last update of information 12-24-2010
|Click on the images for full-size pictures
This Inverted L - end-fed antenna is quick and easy to erect, with no tree climbing necessary, and using basic materials in addition to an antenn tuner, a length of 50 ohm coax, and a feedline transformer (UNUN)... or a remote antenna tuning unit at the feedpoint. The "leader" string, which is launched and mounted over tree branches in one of a number of possible methods (explained below), can be 50 lb kite string or 50 lb monofilament fishing leader. After that, I pull up 200 lb. woven kite line. That 200 lb line is plenty strong to keep the antenna aloft for the short period of time during portable operations... even a month or two.
I have used wire from 24 gauge bare copper to 18 gauge insulated stranded. The size only seems to affect the strength of the wire, and not the overall electrical performance characteristics. 18 gauge insulated stranded copper clad steel wire from The Wireman (the # 532) seems the best, in that if the wire has to touch branch material anywhere along its run, it will be electrically isolated in those places, and the strength to weight ratio is VERY good. Anything larger can be difficult to haul up and hold taut with just the kite string.
At the location where the wire transforms from the vertical section to the horizontal, I have placed a pulley for the wire to pass through, thereby easing the stress at the bend of the wire. As the trees sway (and they will in the wind), that bend could otherwise suffer some destructive fatigue. If I have to just thread the wire over the branches and not try to suspend it, that seems to work just fine - even shooting a thousand watts into it. On the lower bands, the effects of the wire proximity to the trees (over the branches) is minimal to none, really.
I've tried using a sling-shot for launching the weight/string, but have not had any success with that at all. The best method of all is pictured above. Click on the picture elements to see larger images of the launcher components... and HERE for instructions on how to put one of these together - based on the model shown above.
Launching/Mounting the String
The method I have generally used involves winding a 500' roll of 50 lb twisted KITE STRING onto a 6" spool - designed as fishing tackle and used extensively in kite flying. A loop is tied onto the end of the string and a 3 oz. fishing weight connected to the loop. Standing next to the spool, I twirl the string and weight, by my side, and when sufficient speed is gained, let it go (at just the right instant) up and towards the intended branch. The kite string will "pay off" the spool freely as it goes.
My most recent acquisition for line-launching is a pneumatic "cannon" or "potato gun - pictured at left - made of common ABS and PVC plumbing parts. A very good friend, Steve AD7VL, has designed and assembled several of these units for friends and radio club members. The launcher, pictured at left, consists of the pressure tank of 4" ABS pipe and end cap pieces, then transforms into PVC parts, consisting of the ball valve and barrel of 1 1/4 inch - schedule 40 pipe. The pressure tank is fitted with a threaded "tire fill valve" and a pressure gauge. The shuttle (projectile) is made from more PVC pipe parts - a short pipe section and two end-caps - of the size to very closely fit down the barrel of the shooter. With 30 lbs of pressure, the 4.3 oz projectile will be launched approximately 80 feet in the air, pulling a line of monofilament or kite string behind it. The shuttle weight is important in that it has to be able to carry the line back down all the way to the ground so that it can be retreived and used to pull the antenna wire up.
Antenna Line Launcher - Construction Details (PDF file)
Some of the parts
Shown at left is the spool with string mounted, the 3 oz fishing weight (available at hardware and variety stores everywhere), and the Longwire Unun... this one from Balun Designs LLC.
The spool is a 6" GATOR YOYO Fishing Reel. Here is one source on the Internet.
The 200 lb line can be found at places like Catch the Wind - kite supplies website.
The Unun is a 4:1 feed-line transformer used to connect an unbalanced coax feed-line to an unbalanced antenna system - which this antenna is. The unit I am currently using is capable of handling the high power that I frequently run from my station, but a lower power (lower cost) unit is also available from the same manufacturer. The bigger unit is fully weather proof, whereas the smaller one is not. Of course coax connectors have to be weatherproofed also.
A word about the RADIAL FIELD
The successful implementation of this kind of antenna - or any end fed antenna for that matter - is critically dependant on the use of a very good ground and ground radial system. Each time I setup my antenna, I also drive a good ground rod next to the antenna feed point, and install a network of radials which I have constructed. I have a radial plate, to which I have connected 6 - 50 foot wires and 5 - 18 foot wires, and usually one 120' wire just for good measure - and generally as close to directly under the horizontal wire above as I can make it. I spread these out as straight and evenly about the feetpoint base as I can. More radial wires and some longer ones might be better for me, but I have provided a minum for my needs. I have to take up the whole antenna system at the end of each month or two and move it all to another location. (I'm doing campground hosting for the state parks of Oregon in my retirement.) One of the reasons for the 50' radial wire length is because my first (and current) selection for the wire was some 50' packages of clothesline wire - which is quite flexible, resistant to tangling and rolls up easy, and is a nice green color which blends in with the grass and weeds/bushes it runs about.
And, the ANTENNA TUNER
Until recently I had been using an antenna tuning unit inside the station - an older Dentron MT-3000A, capable of handling plenty of power - which is a must for this type of antenna, especially when using it on other bands than for which it might be resonant. I have now finished the construction of an external, remotely controlled antenna tuning unit, placed at the feed point of the antenna. There doesn't seem to be such a commercial product on the market which has been made for Amateur Radio use, and military units would likely cost more than an Army Toilet Seat. I loosely followed the plans of a unit which I have found after asking about such things on one of the ham forums.
Jim AB2CD and his wife Shelly AB1CD have not only built a beautiful
station (have a look), but also have built a very clever remote antenna tuning unit from parts, following plans from a QST magazine article.
There's no question that the tuner placed at the feed point of the antenna
provides superior matching and RF transfer to the antenna. Here
is a picture of mine. I have worked up a page about its
construction details, which you can find by going back and clicking on