No-Tuner Shortened Folded Dipoles

Folded Dipoles have historically been popular single-band antennas. It's easy to use a 468/f long piece of 450 ohm ladder-line for the folded dipole element. If we feed the antenna with 450 ohm ladder-line, we can install a 4:1 or 6:1 balun at the shack and obtain a pretty good match to our 50 ohm transmitters (sometimes requiring a tuner).

Some hams don't have room for a full-sized folded dipole and a 4:1 or 6:1 balun doesn't always provide a perfect match. What if we could shorten the length of the folded dipole by about 18% and by proper choice of a 450 ohm ladder-line matching section, we could ensure a good match to 50 ohms through a 1:1 choke-balun?

We can do exactly that. The resonant feedpoint impedance of a 1/2 wavelength folded dipole is known to be in the neighborhood of 300 ohms. When fed with 450 ohm ladder-line, the SWR on the twinlead is close to 1.5:1 with very few standing waves on the feedline.

Can the impedance transforming characteristics of 450 ohm ladder-line (with reflections) be used to achieve a perfect match to a 50 ohm transceiver? Yes, if we can cause a 9:1 SWR on 450 ohm ladder-line, the impedance at a current maximum point on the line will be 450/9=50 ohms. So how can we cause a 9:1 SWR on 450 ohm ladder-line when it is driving a folded dipole? The answer is by shortening the length of the folded dipole to less than 1/2 wavelength. The antenna in the above graphic covers the entire 40m band with an SWR less than 2:1.

If we shorten the folded dipole by ~18%, the feedpoint impedance becomes capacitively reactive, causing the SWR to rise to ~9:1 such that the impedance looking into a ladder-line matching section is 50 ohms resistive at a current maximum point on the feedline. The following graphic shows what happens to the SWR on the 450 ohm ladder-line as the 40m folded dipole is shortened. Somewhere around 54.5 feet, the feedpoint impedance of the folded dipole causes an SWR of 9:1 on the ladder-line. That's the length we are looking for indicated by the green lines on the graph.

Here are the formulas for the dipole length and 450 ohm ladder-line length based on EZNEC modeling.

Folded Dipole Length in feet = ~390/f. 450 ohm ladder-line matching section length = ~170/f

Above we have a Smith Chart graphic that explains what is happening with the ladder-line matching section. The antenna feedpoint impedance of 385 - j1107 ohms causes an SWR of 9:1 on the 450 ohm ladder-line. 24 feet from the feedpoint, the ladder-line transforms that antenna feedpoint impedance to a perfect 50 ohms where we can attach a 1:1 choke-balun and run coax the rest of the way to the shack.

This approach will work for any HF band. Here's an example for 40m from the above graphic:

Length of folded dipole = 390/7.15 = ~54.5 feet. Length of 450 ohm matching section = 170/7.15 = ~24 feet. (Used with a tuner, this antenna will also work well on 30m, 12m, and 10m.)

We can also use 300 ohm twinlead and 600 ohm open-wire line but the formulas change somewhat.

For 300 ohm line: Folded Dipole Length = ~415/f and matching section length = ~159/f

For 600 ohm line: Folded Dipole Length = ~368/f and matching section length = ~188/f

Does anyone need a no-tuner shortened single-band folded dipole? Enjoy.