Many Hams use wire antennas, and most Hams use them at least for the HF Bands below 10 MHz. I have used Dipoles and ZS6BKW antennas for several years. I try to design the antennas for use with my Rig-internal Antenna Tuners, which mostly are able to match SWR mismatches up to 3:1. While it is great to have resonant antennas, this is just not possible over full lower HF Bands if you want to operate in both the CW/Digital and Phone portions of the Bands using wires. So I try to keep my SWR below 3:1, knowing that the Rig will be able to perform the match without relying on my External Tuner. There are some antennas that just can’t fit this design requirement, and I do have External Tuners for those cases.
Lar K7SV, a friend in the Potomac Valley Radio Club (PVRC), mentioned a “Broadband 80 meter dipole” in an email exchange that I saw. I had to find out about this antenna. He gave me some design information and cited a couple articles about this antenna. It uses only a wire as the radiating element, no traps or tuning capacitors, and it can cover the entire Band with less than 3:1 SWR. One of the first articles about this antenna was from 1993, published in an ARRL article entitled “A Simple Broadband Dipole for 80 Meters.” The antenna uses a “normal” 80 meter dipole element, fed with a combination of a 1/4 wave long 75-ohm coax section and a multiple of 1/2 wavelength section of 50-ohm coax. While the minimum SWR may be a bit higher than that of a standard dipole with a random coax feedline length, the SWR over the entire Band is kept below 3:1. Depending upon the length of the 50-ohm section, it can be kept below 2:1 over nearly the entire Band.
I had an 80 meter dipole already, so I just needed to obtain the correct feedlines in 50 and 75 ohms, and hook it all together. To get the 75-ohm 1/4 wave coax, I took advantage of a product offered by DX Engineering. If you specify the frequency, they will custom cut a section that is electrically 1/4 wave long at that frequency. As a nice side benefit, there is an option to include a number of ferrite beads on one end, which helps reduce EMI that may be coming down the coax as common mode radiation.
I used my Rig Expert ZOOM-230 Antenna Analyzer to measure a 1/2-wave 50-ohm piece of coax. I made some measurements of the Dipole before making this change, then again after replacing the coax feedline with this 75-ohm/50-ohm coax combination. It seemed to perform just as advertised. My dipole structure is flat and at an elevation of about 40 feet. I have heard that the results may change with different elevation and if the dipole is more of an inverted Vee with sloping elements.
I have been using this antenna for more than a year now. Other than getting temporarily downed by a nearby tornado, it has been trouble free. Now when I am working a contest and I have to go to 80 meters, it just plays with no issues. I would highly recommend this antenna. It is simple and gives you access to the entire 80 meter Band with an SWR that is easily handled with most modern rigs and external antenna tuners.
Dipole SWR Curves Before and After Broadbanding
I have been meaning to upload SWR curves for this project. Here they are. First is the 80 meter dipole with just a 50-ohm coax feedline. The second and third curves show the SWR across the 80 meter Band with the 75-ohm section added, and the 50-ohm section cut to 1/2 wavelength at 3750 kHz.
Clarification of the Title
Just to clarify what is actually being done here – the end effect of this Dipole/Feedline structure is that you can use it with reduced SWR at the Rig across the 80 meter Band, compared to a standard 80 meter dipole. The combination of the two feed line sections, is called a Transmission Line Resonator (TLR). Like an antenna Tuner, this feedline structure transforms the SWR that is seen at the Rig, but nothing can change the impedance characteristics at the feed point of the Dipole element. This is something we should always keep in mind when using antenna Tuners or other Matching approaches.