160 METER 1/4 INVERTED L ANTENNA
Article and Website By KN4LF 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. Note! I do not have a B.S. or M.S. in EE, so I'm and amateur radio operator not a "professional" amateur radio operator, so some of my antenna theory explanations may be incorrect.
For 99% of us it is impossible to get a 160 or 80 meter dipole high enough in the air to effectively and consistently work DX. However a wire inverted L antenna is an easy and relatively inexpensive way to get a decent performing low takeoff angle antenna up to work DX.
My current 1/4 wave inverted L is made up of 130 feet of black UV resistant double coated plastic stranded #14 wire. #12 stranded wire works fine too and is stronger but is 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 Home Depot. The vertical section of the antenna is 43 feet tall with a horizontal section of 87 feet which points towards the NNE. Eventually I will get the vertical section 65 feet high. The antenna is suspended from oak trees by 5/16" diameter black UV resistant Dacron rope purchased from http://www.BUXCOMM.com .
It 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 meter frequencies signal loss in coaxial cable is minimal, so RG-58U coax can be used. 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 a 1/4 wave antenna of 246 x 95% velocity factor equals 234 divided by 130 feet gives you a resonant frequency of 1800.000 kc. However in my case with other nearby antennas, trees and my house my antenna is resonant at 1810.000-1840.000 kc. Of course every setup will be different.
The VSWR curve is as follows:
2:1 1697 kc
1.5:1 1739 kc
1:1 1810-1840 kc
1.5:1 1870 kc
2:1 1888 kc
If I want to operate above 1870 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 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 1/8 wave radials (64 feet) 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 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 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 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 87 foot horizontal flat top, which in my case would be towards Australia. The elevation plane 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 broadside (WNW-ESE) to the 87 feet 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.
I have not had an opportunity as of yet to make allot of contacts with the antenna on DX so far this season as there has been allot of lightning QRN on the band from here in Florida. Below are a couple of contacts that I've made so far, so it looks like the antenna will be a winner.
MM/DD/YYYY UTC FREQUENCY STATION COUNTRY MODE RCVD RST/RS POWER
November 1, 2008 0017 UTC 1846.000 kc MM0SJH Scotland LSB 57 600 watts
October 26, 2008 0124 UTC 1837.500 kc CN2R Morocco LSB 59 600 watts
As follows is a photograph of the antenna feed point and radial plate. The gray watertight PVC electrical box contains the feed point connections that make the antenna magic work. 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 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.
1/4 Inverted L Feedpoint & Radial Plate
Click To Enlarge