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 72 ohm Belden 88241/RG-59 coax.
As follows is information on construction of a 160 meter version of this antenna. This antenna can also be adapted for self resonant 80-10 meter operation.
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 However the resonant frequency is 1910 kc due to interaction with other nearby antennas.
It is fed with 75 feet of RG-213 coax and 25 feet of 50 ohm Cushcraft Ultralink TL-93605 RG-213 type coax which is used in an RF choke BALUN to reduce local QRN (noise) on receive. The choke is comprised of 27 turns on a piece of white PVC that is 4.25" diameter and 13" long. The PVC is thick walled schedule 40 sewer pipe. In my particular location the RF choke BALUN reduces the local QRN level by three S units.
The antenna ground system consists of twenty five 64 foot radials and one 127 foot radial made up of #14 solid bare copper wire. Also 300 feet of #6 solid bare copper wire buried 3" deep that encircles my house. Attached are six eight foot ground rods, plus the three eight foot ground rods for the power mains, cable TV and telephone. The ground rods do nothing for collection of RF and are tied in per NEC standards for lightning protection.
The vertical section is 50 feet high then goes out 76 feet in the flat top. The 76 foot flat top is broadside N-S.
As follows is the VSWR curve with one 1/8 radial:
As follows is the VSWR curve with twenty five 1/8 wave radials and one 1/4 wave radial:
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 six radials because the antenna works like gang busters with that number. If I want to operate above 1950 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 was already mentioned above the RF choke BALUN on the RG-213U feed line further reduces the local QRN level by another three S units.
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
By the way using an old MFJ-989 antenna tuner the antenna also works on 60, 40, 30, 20, 17, 15 and 12 meters. As 40, 20 and 15 meters are even harmonics to 160 meters I did not expect the antenna to work on these bands even with an antenna tuner. Though 30 meters is and odd harmonic of 160 meters you would think that the antenna would work on that band also but it doesn't.