148/90', 3.81 MHz SWR = 1.3:1, 7.12 MHz SWR = 1.4:1, optimized for mid-band
140'/88', 4.0 MHz SWR = 1.6:1, 7.3 MHz SWR = 1.6:1, optimized for high-band phone
The author has for many years been telling the amateur radio community that a 130 foot dipole will work on all HF bands without an antenna tuner by simply varying the length of the 450 ohm ladder-line feedline. W5DXP's No-Tuner All-HF-Band Antenna System
One of the nagging problems with varying the length of the ladder-line is that the knife switches (or relays) must be changed when changing bands. However, the author has stumbled upon a partial solution to that problem for 80m, 40m, and 17m, the author’s favorite bands. The HEDZ is designed for operation at 7.15 MHz and the other bands simply fall out from that choice.
The results of EZNEC modeling will be used here although the actual dimensions at the author’s actual QTH are very close to the results predicted by EZNEC. A free demo version of EZNEC is available at: www.eznec.com
A one-wavelength Double Zepp dipole has about 1.5 dB gain over a ½ wavelength dipole in free space. The ladder-line feedline length needs to be a ¼ wavelength section plus a number of ½ wavelength sections. Such a dipole would be 130 feet on 7.15 MHz and would require approximately 92 feet of 450 ohm ladder-line. That makes the antenna system resonant at 4.065 MHz on 80m, i.e. too high and out of band.
A 5/4 wavelength Extended Double Zepp dipole has about 3 dB gain over a ½ wavelength dipole in free space. The ladder-line feedline length needs to be a ~0.2 wavelength section plus a number of ½ wavelength sections. Such a dipole would be 166 feet on 7.15 MHz and would require approximately 85.5 feet of 450 ohm ladder-line. That makes the antenna system resonant at 3.65 MHz on 80m, i.e. lower than most hams would want.
EPIPHANY TIME!!! If a 40m ladder-line fed DZ is also resonant on 4.07 MHz and if a 40m ladder-line fed EDZ is also resonant on 3.65 MHz, wouldn’t somewhere in between be resonant on 3.8 MHz? - thus the birth of the Half-Extended Double Zepp. Halfway between 130 feet and 166 feet is 148 feet. Let’s see what a 148 foot ladder-line fed dipole will do on 40m and 80m.
A 148 foot dipole fed with ~90 feet of 450 ohm ladder-line has a gain of about 2 dB over a ½ wavelength dipole on 7.15 MHz. But its real claim to fame is that it is also resonant on ~3.85 MHz with a 3:1 SWR bandwidth of approximately 200 kHz. That means that this single antenna system is resonant on both ~7.15 MHz and ~3.85 MHz at the same time. This is something for which a lot of amateur radio operators have been wishing. Using my IC-756PRO’s internal tuner allows me to tune the entire 40m band plus 200 kHz of 80m without changing anything.
An additional bonus is that the 17m band is also within the tuning range of my IC-756PRO’s built-in autotuner. So I have seamless switching among three bands, 80m, 40m, and 17m which are my favorite bands. I couldn’t be happier with this 148 foot dipole fed with approximately 90 feet of 450 ohm ladder-line. Of course, a good 1:1 choke-balun is required at the ladder-line/coax junction.
Here’s the procedure for tuning one’s own 148 foot dipole for 80m, 40m, and hopefully also 17m.
Install the 148 foot dipole fed with 100 feet of 450 ohm ladder-line. Check the SWR on 7.15 MHz which will be too high. Start trimming the ladder-line one foot at a time until a minimum SWR is achieved - (It will not be 1:1 unless you are extremely lucky.) Now you have a Half-Extended Double Zepp resonant on 7.15 MHz.
Check the resonant frequency on 80m. It should be close to 3.85 MHz. If it is not the desired frequency, lengthening the dipole and shortening the ladder-line will lower the resonant frequency on 80m. Shortening the dipole and lengthening the ladder-line will raise the resonant frequency on 80m – all while keeping the antenna resonant on 7.15 MHz. Somewhere between a 135 foot dipole and a 175 foot dipole will allow one to pick one’s favorite 80m resonant frequency. EZNEC says a 135 foot dipole fed with 92 feet of ladder-line will be resonant on both 7.15 MHz and 3.99 MHz. A 145 foot dipole fed with 90 feet of ladder-line will be resonant on both 7.15 MHz and 3.85 MHz. A 155 foot dipole fed with 88 feet of ladder- line will be resonant on both 7.15 MHz and 3.74 MHz. A 165 foot dipole fed with 86 feet of ladder-line will be resonant on both 7.15 MHz and 3.66 MHz. The trend is obvious. Ten feet dipole increments shifts the 75m resonant frequency by about 100 kHz while maintaining the 7.15 MHz resonant reference frequency.
Hopefully, this will solve the dual-band 80m/40m antenna system problem that so many amateur radio operators seem to experience. As can be seen from the radiation patterns below, the 80m pattern is essentially a ½ wavelength dipole pattern and the 40m pattern is akin to an EDZ pattern. On 17m and 15m, the antenna will have a multi-lobbed "cloverleaf" pattern with gain over average ground in excess of 10 dBi and a take-off-angle of ~16 degrees.
Since the article was written, W5DXP has taken a look at the ladder-line lengths required for all HF operation of the 148 foot dipole at 40 feet as indicated by EZNEC. The 90 foot length of ladder-line used in this article should be close to system resonance on 3.8, 7.15, 17m, and 21.4 MHz. ~82.5 feet should work for 30m, 12m, and 28.4 MHz. Finally, ~75.5 feet is required for resonance on 14.2 MHz according to EZNEC. Of course, local conditions can and will affect those typical lengths.