Kg3v Ham Radio Blog

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A Dipole that Covers the Entire 80 Meter Band with SWR less than 3:1

Posted: 23 Dec 2020 07:20 PM PST
https://kg3v.com/2020/12/23/a-dipole...-less-than-31/



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. 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 cant 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, 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 minumum SWR is 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
50-ohm/75-ohm coax. It seemed to perform just as advertised. My dipole 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 would be easy to handle
with most modern rigs.




I will soon add my plots for the Dipole before and after conversion to the
Broadband version, and give you a few details about my specific build
process.