A Multi-Band 33 Foot Dipole

Cecil Moore, W5DXP.com, Rev. 1.2, Feb. 28, 2014

This article describes a simple way to enable a 1/2 wavelength 20m dipole to work well without a tuner on 20m, 17m, 15m, 12m, and 10m. Although it applies to any 33 ft dipole, maximum performance will result when the dipole is made to rotate. The gain is almost as good as a two-element Yagi on 10m (9.5 dBi).

It is an accepted fact that a ladder-line fed 1/2WL dipole can work well on all higher frequency HF bands but it usually requires a wide-range antenna tuner. In a related article on his web page, W5DXP shows how to vary the length of the ladder-line to accomplish no-tuner operation on all of the HF bands using a 130 ft. ladder-line fed dipole.[1] That same basic technique can be utilized to enable 20m-10m operation for a 33 foot dipole. The 1:1 choke-balun from the earlier article will work well with this antenna. Here's a picture of the 1:1 choke-balun.

The transmission line chosen for W5DXP's 33 foot dipole (height=40 feet) is the "300 ohm" transmitting ladder-line (#562) available from TheWireman.com. The estimated Z0 is actually around 288 ohms and the estimated velocity factor is around 0.83. The lengths of ladder-line suggested in this article assume those transmission line characteristics. Other transmission lines, e.g. 450 ohm ladder-line with a velocity factor of 0.9, will naturally result in slightly different optimum lengths and SWR values.

Decades ago, W5DXP wrote a DOS program, IMAXGRAF.EXE, that given the dipole length and velocity factor of the parallel feedline, will display the optimum feedline lengths.[2] The following display was obtained for a 33 foot dipole fed with ladder-line having a velocity factor of 0.83.

What we are looking for is a grouping of dots around a vertical line. We see one such grouping for 17m-10m around a ladder-line length of 108 feet but we are missing the 20m dot at that length. The best we can do is the grouping around 90 feet and that is the one we choose. The ladder-line length for 20m in that group is a no-brainer and is 3/2 wavelength, i.e. 3(29.5')=88.5', which will reproduce the resonant feedpoint impedance of the 33' dipole. W5DXP used EZNEC[3]/AutoEZ[4] simulations for the rest of the lengths, but his real world results are very close to those simulation values.

Here are the SWR curves for each band based on EZNEC/AutoEZ simulations. The following convention has been adopted for identifying dipoles fed with ladder-line; "Dipole-Length/Ladder-Line-Length", for instance: "33'/95'" will identify a 33 ft dipole fed with 95 feet of ladder-line.

One can use the same method for varying the length of the ladder-line as was described in W5DXP's No-Tuner All-HF-Band Dipole article[1]. But since the length of the ladder-line needs to be varied by ten feet, only four jumper lengths are needed,i.e. 2', 2', 2.5', and 3' ladder-line lengths with banana plugs/sockets on each end so changing bands inside the shack to achieve no-tuner operation takes only a few seconds.

Hams with built-in autotuners, like W5DXP's IC-756PRO, can get by with only one five foot jumper. A fixed ladder-line length of 88 feet puts 20m, 17m, and 15m all within range of the autotuner. Adding a single five foot jumper puts 12m and 10m within range of the autotuner. This is how W5DXP usually operates his multi-band rotatable 33 foot dipole.

Hams with wide-range external tuners, like the MFJ-949E, can use a fixed 90 foot length of ladder-line. Here are the impedances looking into an ideal 1:1 choke using 90 feet of 300 ohm transmitting ladder-line to a 33 foot dipole at 40 feet, according to EZNEC:

14.2 MHz, 64+47 ohms; 18.14 MHz, 113+180 ohms; 21.3 MHz, 69-j18 ohms; 24.95 MHz, 64-j159 ohms; 28.5 MHz, 153-j484 ohms

[1]No-Tuner All-HF Dipole

[2]Optimum Feedline Lengths

[3]EZNEC Demo/Info

[4]AutoEZ Demo/Info