80/75/40-meter antenna for Field Day

Posted by Fred C (W6BSD) on Jun 08 2021

The state of California and San Mateo County have eased COVID restrictions. It is why the San Mateo Amateur Radio Club has decided to have their Field Day operation at the usual location, Beresford Park.

SWR on 40-meter branch
Dipole 40-meter branch

The club's Field Day committee1 decided to build a new multi-band antenna for 80 and 40 meters for the occasion. The new antenna had to have enough bandwidth to use the entire 80-meter band without using an antenna tuner, or if we used a tuner, the SWR couldn't be too bad to avoid excessive feedline losses.

I used the mmana-gal software to experiment with different antenna models and get an idea of the radiation pattern we could expect, but more importantly, to calculate its total bandwidth.

I started by designing a fan dipole for 80 and 40 meters. The idea was to have the 80-meter dipole installed straight at 70 feet above the ground and the 40-meter branches of the dipole as an inverted V configuration.

SWR on 80-meter branch
Dipole 80-meter branch

The modeling software gave me an excellent SWR across the entire 40-meter band. Unfortunately, on 80-meter, the SWR was good on the CW portion of the band but could go as high as 10:1 on the phone part of the band.

I continued to iterate on the fan dipole idea, and I modeled a new antenna with three branches for 80, 75, and 40m. The 40-meter part of the antenna was not affected by the new branch. However, it was impossible to make the antenna resonant on 80 (3.55 MHz) and 75 (3.85 MHz). The frequencies are too close, and the two branches behave as a single one.

A technique often used to widen the bandwidth of an antenna is to increase the diameter of the element. I tried to simulate a large-diameter wire using the modeling software by building a cage dipole. The cage dipole had to have eight wires and more than 3 feet in diameter to get a result close to what we wanted. It would make that antenna heavy, hard to build, and hard to install the morning before Field Day.

SWR across 80-meter
SWR across the 80-meter band
with a parasitic element

I tried to model a biconical antenna, also known as a bowtie antenna. I couldn't get the bandwidth wide enough to cover the entire 80-meter band. The overall shape of the antenna was making it hard to build and deploy for Field Day.

Then, I experimented with an 80-meter dipole and a parasitic element cut for the 75-meter band. I wanted to see how far away the parasitic element had to be, and I wanted to know if I could get a second deep at around 3.85 MHz. After fiddling with the modeling software for a couple of hours, I had an antenna with an almost flat SWR across the entire band. The first deep was below 3.5 MHz, and the second deep above 3.9 MHz.

Fan Dipole 80/75/40
Fan Dipole 80/40 with parasitic element on 75

As you can see on the smith chart, the antenna's impedance is at around 100Ω. Using a 60 feet CATV 75Ω coaxial cable allows me to transform that impedance and have an SWR bellow 2:1 on the entire band.

SWR across 80-meter
SWR across the entire 80-meter band

SWR across 80-meter
SWR across the entire 80-meter band
with the 75Ω impedance transformer

Radiation pattern

The radiation pattern of the antenna was nothing exceptional. It looked like any dipole pattern. When installed at 65 feet height, the elevation angle (take-off angle of the signal) was 34º on 40-meter and 71º on 80-meter. I wish we could have installed that antenna higher to have a lower take-off angle. I still think the antenna should perform relatively well because our 3 dB attenuation is at 26º of elevation.

Radiation pattern
Radiation pattern on 80/75-meter and 40-meter

Testing

George (WA6NKM) made plexiglass separators and added the 75-meter element on an existing 80/40-meter dipole. The last weekend, with the rest of the team1, we installed the antenna in the park. Our preliminary test shows encouraging results. I will soon post a new article with pictures of the antenna, the actual number we are getting from the antenna analyzer, and how it performs during Field Day.


  1. Georges (WA6NKM), Mike (N6IMY), Brian (AB6D), Kevin (K3AM) 


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