Wednesday, June 8, 2011

No-Tuner Shortened Folded Dipoles

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 20% 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 ~20% to ~3/8 wavelength, 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. 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

Install a 1:1 current-choke-balun at the end of the ladder-line and run coax from there back to the shack. Adjust the length of the folded dipole and ladder-line to achieve a perfect 50 ohm match on one's favorite frequency requiring no antenna tuner. How can one get more efficient than that?

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

Length of folded dipole = 390/7.2 = ~55 feet. Length of 450 ohm matching section = 170/7.2 = ~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.

No comments: