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