W4TWW 1/4 WAVE BROAD BANDED COAXIAL INVERTED L ANTENNA FOR 160 METERS
Article and Website By KN4LF (Now NZ4O) in 2007
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.
Note, this antenna was introduced to me by Coleman Rowland W4TWW of Charleston, S.C., now a silent key. I make no claim to have invented or even improved the antenna in any way. I'm only passing along information about it because it works very well. This antenna appeared in CQ Magazine in August 1984 on page 72.
My antenna has been slightly modified from the W4TWW design which used 450 ohm window line for the non stub portion of the antenna. I found that use of the window line structurally weakened the antenna with time due to wind resistance. In my design the non stub portion of the antenna is made up of 50 ohm coax.
As follows is information on construction of a 160 meter version of this antenna. This antenna can also be adapted to 80-10 meters too and I do also have up an 80 meter coaxial inverted L antenna.
First we have to discuss what type of coax to use. The larger the diameter of the coax used the more broad banded the antenna is. We also have to cover velocity factor because it's used in calculating the "matching stub" length.
As it's been a long time since I thought about it I don't remember all of the theory involved in how the antenna works. I use the description of "matching stub" for the longest piece of coax and that might be an inaccurate description of it's function. From memory though basically the matching stub acts to raise radiation resistance of the feed point from the theoretical 36 ohms and actual real world of approximately 18 ohms, to 50 ohms which is more efficient. Also the large diameter of the coax acts to counteract reactance as you QSY around the band, giving you broadband transmit coverage.
On February 6, 2009 I built and installed another coaxial inverted L for 160 meters. This time I used high quality and very strong Belden 88241/RG-59. It is a 72 ohm coaxial cable with a velocity factor of 66%. The stub is 87' 6" feet long and the rest of the antenna 38' 6" feet long for a total of 126 feet, which is a 1/4 wave on 1857.142 kc kc. However the resonant frequency is 1910 kc due to interaction with other nearby antennas. It is fed with 75 feet of RG-213 coax.
I laid down twenty 1/8 wave radials (64 feet) using #14 stranded bare wire. I also have nine eight foot ground rods, my house copper water pipe system and city water system tied in with the radials.
The vertical section is 40 feet high then goes out 86 feet in the flat top.
As follows is the VSWR curve with one 1/8 radial:
As follows is the VSWR curve with twenty 1/8 wave radials:
As you can see by the following information about the VSWR curve of the antenna, the more radials that you add the more narrow the band width becomes. I stopped at twenty radials because the antenna works like gang busters with that number. If I want to operate below 1830 kc or above 1968 kc I use an old but well maintained MFJ-989 tee network tuner to fool the amplifier.
One thing that I observed that was expected was a drop in received strength of the local QRN. On the coaxial inverted L the local QRN level is S 4-5. As a comparison my 160 meter 1/2 wave dipole has a QRN level of S9 to S9+5 db.
As follows are four views of the feed point of
the coaxial inverted L. The gray watertight PVC electrical box contains the
reversed feed point connections that makes the antenna magic work. The
radial plate was purchased from DX engineering and is of high quality
stainless steel.
http://www.dxengineering.com/Sections.asp?ID=109&DeptID=32#Top .
|
|
Click To Enlarge
|
|
Click To Enlarge
|
|
Click To Enlarge
By the way the antenna also works on 60, 30 and 6 meters as they are all odd harmonics. It will not work on even harmonics, even with an antenna tuner.
Visits Since 09/01/2007