After having success with my resonant end fed antennas I decided that I wanted to build a more traditional resonant half-wave antenna that was also considerably shorter than normal. The plan for this antenna was to build a lightweight 80/40 meter antenna for field use (as part of my Go Kit) that wouldn’t overload my 21 foot telescoping fiberglass mast. The antenna also needed to be capable of handling 50 watts at 100% duty cycle for digital operation as well as 100 watts of SSB.
Similar to my 80/40 resonant end fed antenna, the goal for this antenna was to achieve resonance on both 80 and 40 meters by using loading coils large enough to isolate the 40 meter element of the antenna while simultaneously greatly shortening the space required for 80 meter operation. Several vendors sell antennas of this design (MFJ, Alpha Delta, etc.), however, I always prefer to build my own since I can build the antenna exactly how I want, save money, and learn something in the process.
There are a lot of good resources regarding how to build this type of antenna. K7MEM has a loaded dipole calculator that lets you play with different parameters to determine how big the loading coils should be and how far they should be placed from the feedpoint. This works best for single band designs, but it also serves as a good way to double check antenna dimensions. I also found this design, as well as an article in the April 1961 issue of QST that both provide a great starting place for antenna dimensions and what size loading coils to use. The coils used tend to be in the 80uH to 130uH range. Larger coils allow for a shorter antenna, however, they also reduce the available bandwidth. I went for somewhat of a middle-ground with 111uH coils. Due to the antenna’s limited bandwidth I planned to use extension stubs to shift the antenna’s resonance from the top of 80 meters for voice work to the bottom for digital operations.
In order to keep the antenna as light as possible I used 18AWG stranded copper wire. The coils were wound using 22AWG enamel wire. Each 111uH coil was made using 65 turns of the enamel wire on a 1.25″ PVC form (I used K7MEM’s coil designer to figure out the details). I used stainless steel screws and 8-32 hardware to secure the enamel wire and provide a connection point for the antenna wires. I then coated each coil with two coats of polyurethane for weather sealing and to secure the coil to the PVC.
For the 40 meter elements, I first connected the two antenna halves to the center balun (I used a Unadilla W2DU 1:1 balun that I had laying around). Then I connected the other end of the wires to the loading coils. The short 80 meter elements were then wired to the other side of the loading coils. The 40 meter elements were trimmed for resonance at 7.1MHz which resulted in a span of about 70 feet for the 40 meter section.
I then began trimming the 80 meter elements. While there is minimal interaction between the 40 and 80 meter sections of the antenna due to the choking effect of the loading coils, when the 80 meter section is trimmed is does slightly effect the 40 meter section’s resonance. For this reason the 40 meter section was left a little long so that when the 80 meter section is the correct length, the 40 meter section resonates on the desired frequency.
After trimming, the 80 meter elements were about 3.75 feet long for a total antenna span of about 78 feet (including the coils). This resulted in resonant frequencies at 7.15MHz and 3.977MHz. I found that by adding 18.5 inch stubs (using Anderson powerpoles) to the end of the antenna resulted in a resonant frequency of 3.583MHz. The 40 meter 2:1 SWR bandwidth effectively covers the entire band. On 80 meters the antenna has about 40kHz of 2:1 bandwidth and 60kHz of 3:1 bandwidth. One major advantage of this antenna over my resonant end fed is that it does not use any complex matching system, only a 1:1 balun. This allows for more aggressive use of an antenna tuner without the risk of damaging the matching system, which increases the usable bandwidth of the antenna. Using the internal tuner in my Yaesu FT-450D I can increase the antenna’s 80 meter 2:1 SWR bandwidth to about 130KHz.
This antenna turned out about as well as I had hoped it would. With the winder it weighs only 3lbs, 1lb less than my 80/40 end fed. It is also a very good match for my Go Kit’s fiberglass mast as this combination held up well even when loaded down with some ice and snow and with wind gusts over 30mph. The loaded dipole makes a nice balance between size and performance and will be my Go Kit’s primary HF antenna going forward.