The Slopo evolved from a desire for a portable antenna of some merit. Extensive use of EZnec modeling revealed a basic problem with verticals. They are just not up to snuff when compared with classic dipoles. A simple dipole or better yet, a fan dipole of careful design, mounted at a realistic height, significantly outperforms any reasonable vertical, regardless of the type of ground or how fancy the ground plane is. On this EZnec is most definite. Many attempts to convince EZnec to improve on the vertical led far a field into such strange creations as vertical extended double Zepp's over 50 feet tall at 10 meters.
Finally I tried an idea. The sloper is a dipole that only needs one support. I had tried inverted V's with disappointing results. Well the sloper was disappointing as well, not at all surprising. But I had another idea. The pole had to be there, why not put it to work? So I came up with the sloper/pole combination, the SLOPO. The slopo is designed to incorporate the metal support pole into the pattern. The pole was made a bit longer than a half wave. When this was insulated from ground and allowed to float a fantastic pattern jumped out, right when the right pole length was obtained.
Further fiddling showed that various radial systems were a detriment. Grounding the pole was a disaster. But adding some reflector wires was not. One needs a few more guy wires anyway, why not put them to work? Surprisingly the front to back ratio increased greatly as the guy wires were made closer to the pole.
The slopo is practical on 20-6 meters, though the use of a center pole more than 1/2 wave tall limits its practicality on bands much lower than 20 meter. The design is self supporting and can be set up to favor any direction. It has good low angle radiation and a very useful front to back ratio. One is tempted to make the design symmetrical and then switch the various identical slopers to change directions, but there is considerable loss of front to back ratio and gain with a symmetrical design.
So after considerable fiddling I came up with this formula:
Building it on 15 meters:
The sloper element is at an angle of 60 degrees down from horizontal. It is 258 inches long and fed in the middle with 70 ohm coax or transmitter twin lead.
The pole is 282 inches high. It has a metal section 271 inches long which goes from 9 inches above the ground to 280 inches above the ground. It is insulated on both ends, it floats. It is an active reflector and has a high induced current on it. What I had in mind was the type of pole stock sold in 5 and 10 foot lengths by Radio Shack.
The guy wires are each 271 inches long, and insulated at the top, that is the conductive part is 271 inches long. They come within 1 inch of the top of the pole. They are sloped down a full 80 degrees from horizontal. This extreme down slope makes for a much improved front to back ratio with virtually no effect on the forward gain. These are all active reflectors, though the current in the rear one is minimal. However it is worth having. It adds to the forward gain slightly and to the front to back ratio. You have to have a guy wire there anyway.
The diagram shows the currents in purple and the wires in green. The central wire is the pole itself. But the really great news is the front to back ratio of this antenna and its significant forward gain, which is in a very broad front lobe. No fancy phasing harnesses. But there is a surprise in the SWR plot.
So here it is, the SLOPO:
The pattern below is an elevation. The cursor is at 15 degrees, a good angle to have gain if you want to work DX. The antenna can be scaled for other bands.
This pattern is stable over various types of ground. There is some slight gain changes and some slight change in front to back ratio but not extreme at all, in fact quite minimal.