160 & 80 METER 1/4 WAVE FAN INVERTED L ANTENNA
Article and Website By KN4LF (Now NZ4O) in 2008
I decided to publish this website in order to pass on some insights about this antenna that I've garnered through extensive experimentation. A warning though, some of the combined design aspects of the antenna may be unique and unorthodox, a think out of the box antenna design. Note! I do not have a B.S. or M.S. in EE, which makes me a true amateur radio operator not a "professional" amateur radio operator, so some of my antenna theory explanations may be incorrect.
For many hams it is impossible to get a 160 meter dipole high enough in the air (~260 feet) to effectively and consistently work DX, On 80 meters (~130 feet) it can also be difficult. However a wire inverted L antenna is an easy and relatively inexpensive way to get a decent performing RF low takeoff angle antenna up to work DX.
The 1/4 wave inverted L for 160 meters is made up of 130 feet 4 inches of black UV resistant double coated plastic stranded #14 wire. The 1/4 wave inverted L for 80 meters is made up of 70' of black UV resistant double coated plastic stranded #14 wire.
#12 stranded wire works fine too and is stronger but more expensive, heavier and easier to see for those that may be living in antenna restricted communities. The 500 foot spool of #14 wire was purchased at local big box hardware store.
The vertical section of the 160 meter element is 43 feet tall with a horizontal section of 87 feet of which the far end with the insulator points towards the NE. The vertical section of the 80 meter element is 30 feet tall with a horizontal section of 40 feet of which the far end of the insulator points towards the NE.
The elements are suspended from oak trees by 5/16" diameter black UV resistant Dacron rope purchased from http://www.BUXCOMM.com. My oak trees are only 22 years old and the relatively small branches allow for to much movement of the elements, so high quality rope is necessary.
From a mechanical standpoint the setup is as follows. A two foot piece of black UV resistant double coated plastic stranded #14 wire comes out of the gray watertight PVC electrical box in a vertical plane. It is attached to the center screw terminal of a single pole double throw ceramic knife switch. The remaining 128 feet 4 inches of the 160 meter element is attached to the left outer end screw terminal of the knife switch and the 68 feet of the 80 meter element is attached to the right outer end of the screw terminal of the knife switch. The unused element floats above electrical ground when not in use.
After the 2 foot section of wire the 160 meter element goes up another 41 feet vertically and then out 87 feet in the horizontal plane. The 80 meter element goes up another 28 feet vertically and then out 40 feet in the horizontal plane. At the knife switch the two wire elements are spaced 4" apart. Where the two vertical elements turn horizontal they are spaced approximately 15 feet apart. The 160 meter element is perfectly vertical (90 degrees) but the 80 meter element slopes upward at an approximate 70 degree angle from the surface of the ground.
The antenna is fed with 75 feet of high quality low loss RG-213U coaxial cable #CXP213C75 that I purchased from http://www.cablexperts.com/cfdocs/cat.cfm?ItemGroup=2&itmsub=0&bskt=0&USA_ship=1&c=0 . In the shack is another six feet of RG-213U going to the amplifier.
At 160 and 80 meter frequencies signal loss in coaxial cable is minimal, so RG-58U coax can be used if you use no more than 200 watts PEP. However from a stand point of power handling if you run the legal limit of 1500 watts PEP you will want to use RG-213U or better, for 600-1000 watts PEP RG-8X is fine.
The lightning arrestor is the I.C.E. 303/U 8kw PEP model that I purchased from The Wireman http://www.thewireman.com/coaxialimpulsesuppressors.html . I purchase it from The Wireman because I.C.E. has poor customer service.
Using the formula for an 160 meter 1/4 wave antenna of 246 x 95% stranded wire velocity factor equals 234 divided by 1830 kc would give you a length of 127 feet 10 inches. However to resonate the antenna at 1830 kc I needed 130 feet 4 inches.
Using the same formula for an 80 meter 1/4 wave antenna of 246 x 95% stranded wire velocity factor equals 234 divided by 3800 kc would give you a length of 61 feet 6 inches. However to resonate the antenna at 3800 kc I needed 70 feet.
Longer wire elements were needed due to interaction with three other nearby antennas, seven oak trees, my house and lot's of chain link fence all on my 1/3 of an acre.
The VSWR curve on 160 meters is as follows:
2:1 1700 kc
1.5:1 1744 kc
1:1 1814-1829 kc
1.5:1 1865 kc
2:1 1885 kc
The VSWR curve on 80 meters is as follows:
2:1 3713 kc
1.5:1 3746 kc
1.1:1 3795-3805 kc
1.5:1 3859 kc
2:1 3893 kc
If I want to operate above 1870 kc as well as below 3746 kc and above 3859 kc I use an old (built in 1981) but well maintained MFJ-989 tee network tuner to fool the amplifier. VSWR on the RG-213U is still low enough at the lower and higher frequencies so that RF losses are not to high.
As far as my ground system I use a radial plate that I purchased from DX engineering that is made out of high quality stainless steel and comes with twenty sets of stainless steel mounting hardware to attach the radials. You can check it out at http://www.dxengineering.com/Sections.asp?ID=109&DeptID=32#Top . My ground system consists of twenty four 1/8 wave radials (64 feet) made up of black UV resistant double coated plastic stranded #14 wire laid on top of the ground and held down by home brewed staples made out of the little orange flags that you see everywhere along the side of the road marking underground utilities. I bought 100 flags for $7.99 and got 200 5 1/2" long ground radial staples out of the bundle.
I use 64 foot long radials as it is the longest length that I can fit on my 1/3 of an acre lot. Allot has been written about the number and length of radials. Some basic rules of thumb for radials, they need not be any longer than the vertical section of your antenna. Also allot of short ones are better than a few long ones. Check out this article by N6LF at http://www.antennasbyn6lf.com/2008/10/are-the-lengths-of-radials-related-to-the-height-of-a-vertical.html .
According to ON4UN's fourth edition book Low Band DXing, for 1/8 wave radials on 160 and 80 meters you see an increase in diminishing returns after twenty four. Under chapter 9-9 through 32 there is much more information on radials.
As far as the height of the vertical section the rule of thumb is of course the higher the better. Through twenty years of experimentation I have had good success with vertical sections as short as thirty feet and as tall as sixty five feet. Basically the taller the vertical section the more efficient the antenna is with a larger bandwidth. Of course a vertical section of 125-130 feet would be best but then we would have a 1/4 wave vertical not a 1/4 wave inverted L.
Once again in ON4UN's fourth edition book Low Band DXing, in chapter 9-33 through 79 you can find allot more information about vertical section height. By the way you can by ON4UN's book through the ARRL at http://www.arrl.org/catalog/index.php3?category=Antennas%2C+Transmission+Lines+and+Propagation .
Modeling the 160 meter inverted L element with EZNEC 5.0 shows an almost perfect omnidirectional pattern in the azimuth plane with 0.5 db of low angle gain in the opposite direction of the horizontal flat top, which in this case would be towards east Asia. The elevation plane of the highest RF current shows a take off angle of 25 degrees, which is low enough for allot of DX contacts. The antenna also radiates some high takeoff angle RF current broadside (NW-SE) to the flat top which allows for stateside contacts.
Modeling the 80 meter inverted L antenna with EZNEC 5.0 shows an almost perfect omnidirectional pattern in the azimuth plane with 0.5 db of low angle gain in the opposite direction of the horizontal flat top, which in my case would be towards east Asia. The elevation plane of the highest RF current shows a take off angle of 30 degrees, which is low enough for allot of DX contacts. The antenna also radiates some high takeoff angle RF current broadside (NW-SE) to the flat top which allows for stateside contacts.
Note! When it comes to 160 meter vertically polarized antenna's you can get a lower take off angle (TOA) from a full 1/4 wave vertical or electrical 1/4 wave tee vertical of 10-20 deg., versus ~30 deg. with the inverted L. However it's a moot point as the night time E layer MUF blocks 160 meter low angle transmitted radio signals from ever reaching the F layer to be propagated. So unlike with HF propagation, MF propagation success does not require the lowest of take off angles.
Also higher take off angles of 30-45 deg. via the inverted L are better able to take advantage of the low signal loss E valley-F layer propagation duct mechanism, a form of Chordal hop propagation.
Below are a some contacts that I've made so far on each band.
MM/DD/YYYY UTC FREQUENCY STATION COUNTRY MODE RCVD RST/RS POWER
160 Meters-
November 1, 2008 0017 UTC 1846.000 kc MM0SJH Scotland LSB 57 400 watts
October 26, 2008 0124 UTC 1837.500 kc CN2R Morocco LSB 59 400 watts
80 Meters-
December 25, 2008 0159 UTC 3800.000 kc KL7J Alaska LSB 56 400 watts
January 09, 2009 2315 UTC 3789.400 kc PA0GMW Netherlands LSB 57 400 watts
January 15, 2009 0016 UTC 3791.100 kc CT1IZU Netherlands LSB 58 400 watts
As follows is a photograph of the antenna feed point with the radial plate, gray watertight PVC electrical box that protects the feed point connections and the single pole double through ceramic knife switch. The Teflon PL259 coax cable connector is water proofed using high quality electrical tape and also liquid electrical tape.
Though not shown in the picture below, at
the feedpoint of the antenna I have recently placed a choke BALUN made out of RG-213U.
The dimensions are eight turns
(twelve feet) of RG-213U coax on a PVC form 6 1/2" in diameter X 12" long. The choke
BALUN prevents the feedline
from radiating RF and also from picking up unwanted local QRN.
160 & 80 1/4 Wave Fan Inverted L Antenna
Feedpoint Radial Plate & Ceramic Knife Switch
Click To Enlarge