# 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.

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.

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.

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.

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.