A Broadband 3.5/4.0 MHz Antenna

Updated January 2008

For the last several years I have been using an inverted dipole for 80 metre CW (resonant at 3.525 MHz) with reasonable success. (Worked 31 countries mostly on CW) Note: As of February 2008 I have worked 61 countries on this band

In early 2003 I was asked to act as a net controller for the ONTARS net. It soon became apparent that the SWR at 3.755 MHz the ONTARS frequency was causing My Yaesu FT847 to reduce power when I was transmitting. Of course this problem could have been corrected with the use of an antenna tuner if I wanted to retain the ability to operate ant the low end of the band. I could also have shortened the antenna but this would have compromised the performance at the low end of the band. Being from the old school that says an antenna tuner should only be used as a last resort I started looking for a solution using the antenna modelling program EZNEC v 3. As I had previously developed a vertical antenna that covered all the ham bands from 40 to 6 metres using a single 50 ohm feed line it seemed reasonable that the same principal applied to the 80 metre dipole should work. Initially I was looking for an antenna that would be efficient from 3.5 MHz to 3.8 MHz. In the end, probably by accident, I ended up with an antenna that covers from 3.5 MHz to 4 MHz.

In February of 2008 I finally was able to model the antenna using a recently purchased version of EZNEC v 5.0. and obtain results that compared to the antenna's actual performance. The critical parameters (which I had built, by pure luck, into the antenna) are as follows.

(1) The transmission line that I used was 75 ohm hardline with a velocity factor of about .81 and is about 65 feet long. I assume that other types of 75 ohm coax could be used if the velocity factor is taken into account.

(2)The spacing of the lower wire is fairly critical and may require some adjustment depending on the soil conditions below. In my case I believe the actual ground that the antenna sees is very close to the surface as I know that the water table is not far below. I have noticed a slight change in the SWR at the low end of the band during the winter which I believe is caused by the water table dropping.

(3)A good starting point in the construction of the antenna is to resonate the upper wire before adding the lower wire at about 3.525 Mhz. ( The actual length of the upper wire will be dependant on how high the ends are above the electrical ground) After this has been acomplished add the second wire and adjust the length to give a low SWR over the whole band. (In the inital antenna I used a twisted pair with the lengths slightly different) It is my intention to rebuild the antenna with a single upper wire when the weather is better.

The measured SWR using a MJF 259 antenna analyser is as shown.

3.5 MHz 1.4:1

3.55 MHz 1.8:1

3.6 MHZ 1.15:1

3.65 MHz 1.105:1

3.7 MHz 1.24:1

3.8 MHz 1.6:1

3.85 MHz 1.7:1

4.0 MHz 2.0:1

The dimensions given for the antenna at approximate and depend on how close to the ground the ends of the dipole are. The configuration is shown below.

In July 2005 I removed the 1:1 balun with the following V.S.W.R. results. (Measured at the transmitter antenna connection)

3.5 MHz........... 1.6:1

3.6 MHz............1.2:1.

3.7 MHz............1.2:1

3.8 MHz............1.2:1

3.9 MHz............1.2:1

4.0 MHz............1.2:1

I did have to increase the length of the longer wire of the driven by about 18 inches at each end to achieve these results. All other elements remained the same.