If you are space restricted and cannot use a full size half-wave dipole, you have to make it shorter. One way is to use coils (and create an inductively loaded shortened antenna), the other way is to change antenna geometry. The geometry can be changed in different ways:
by bending the ends downwards,
by folding the arms back and forth (like the Cobra antenna),
by forming an open square loop (like the Cobweb antenna).
I would like to suggest yet another way.
The picture below presents the concept. I had to call this antenna somehow. Because its shape reminds me of the letter E connected with a digit 3, I called it Epsilon-3 or E-3 in short. The table below shows the dimensions and performance parameters of the antenna designed for 20 m band.
The E-3 is shorter than a half-wave dipole by about 50%. Generally, the larger the distance b between the wires, the shorter the whole antenna. Its resonance resistance is close to 12.5 ohms, what makes it relatively easy to match with a 1:4 balun. The E-3 gain is only slightly reduced (by about 0.1 dB) when compared with a full-size dipole. Its bandwidth is sufficient for 40 - 12 m bands if you accept somewhat increased SWR at band edges. Contemporary transceivers with built-in antenna tuners should have no problem to tune their outputs to the E-3 antenna,
It can not cover full 80 m band nor 10 m band.
Click here to download this antenna models.
Interesting modifications of the basic antenna are possible. See below the folded version. This antenna can be fed either with a 75 ohms coax or a 50 ohms coax.
The folded E-3 has greater resonance resistance and greater bandwidth. Due to losses in the connected coax, you may expect SWR<2:1 at the transmitter output (for typical feeder lengths).
However, the folded version is longer than the basic version, what explain its greater SWR bandwidth.
Click here to download this antenna models.
Folded E-3 in the inverted V configuration requires only one mast (with a cross arm at its top). It occupies 5.08 m along the Y axis.
Click here to download this antenna models.
After changing a little bit the geometry of the basic E-3, you can create a compact square antenna. This antenna has a dipole-like radiation pattern, the same as the basic E-3.
Click here to download this antenna models.
If one extra wire is added, the antenna feed point impedance increases as well as its bandwidth broadens.
Click here to download this antenna models.
This antenna is smaller than the E-3 Squares. As could be expected, its SWR bandwidth is also smaller, but still sufficient for 20 m band if you have an antenna tuner in your transceiver. You can mount it as shown in the figure below, or rotate it by 90 degrees and use it in the vertical plane.
Click here to download this antenna models.
This antenna is not flat as the previous designs, but three-dimensional (dimension D is its height). It has a nice feed point impedance close to 50 ohms. Its footprint is smaller than the E-3 Square and it has wider bandwidth. As all previous E-3 design, this antenna has a dipole-like radiation pattern with maximum gain along the X axis.
Click here to download this antenna models.
The Epsilon-3 idea can be used in a quad loop antennas too. Such a quad is smaller than its full size version. Its side is about 60% of the full-size quad side. It has slightly greater gain than a dipole but, of course, smaller than the full size quad loop.
Click here to download this antenna models.
Like in the full-size loops, the quad loop has its brother - diamond loop, in which the wires are tilted 45 degrees. Many hams considered this variant to be more practical.
Click here to download this antenna models.