About three months ago, I created my first 7MHz Shorten End Fed Half Wave Antenna (SEFHWA) prototype. The main feature of a SEFHWA is its coil. This allows the length of the antenna to be shorter and thus more compact. Allowing me to use it in the field when space is limited. For about three months, the antenna was doing great. It went through a few rain storms, several Santa Ana winds. Unfortunately, the antenna failed at its weakest point – the ends of the coil.
The advantage of using a SEFHWA is its wire length. For a 7 MHz End Fed Half Wave (EFHW) antenna, the wire length is 66 feet or about 20 meters long. Using a SEFHW antenna, the length of the antenna is about 41% shorter! The key for a successful SEFHWA is its loading coil. A loading coil or load coil is an inductor that is inserted into an electronic circuit to increase its inductance (wikipedia). In my case for a 7MHz SEFHWA it was a 35µH loading coil. There are many different designs in creating this type of coil, but the most common build is using magnetic wire wrapped in a PVC Pipe. The magnetic wire is a challenge to work with, but with practice I was able to create my first coil (shown below).
When creating the coil, my first concern was how to connect the lead wire (long wire from the transformer to the bottom of the coil) and the pigtail wire (the shorter wire). I received a lot of recommendations from two Facebook groups on antennas. These suggestions included: adding a clamp, soldering the wires together, or adding teflon screws on the PVC pipe. I felt at the time that adding hardware to the coil would add additional weight.
I decided to use a continuous wire for the coil, the lead wire, and pigtail. By using this design, I didn’t use any hardware for attaching the lead wire or pigtail wire to the coil. I used the sotabeams‘ antenna wire (DEF 61-12 Pt 6) and a PVC pipe (Charlotte PVC Pipe) for the coil to create the antenna. The most challenging aspect in creating the coil was making sure the coil had an inductance value of 35 µH. To do this, I had to use an LCR meter that measures inductance. Second, in theory, the turns of a coil is linearly proportional to its inductance. I measured different lengths of wire. Each wire I wrapped on the PVC pipe, I measured its inductance. Having this empirical data, I created a linear regression to approximate how long the wire would be to create a 35µH coil. It took approximately 5.63 meters of wire on the PVC pipe to create 35µH coil.
Next I cut one continuous wire to create the antenna – starting from the lead wire, creating the coil, and the pigtail. Afterwords, I went to the park to adjust the antenna, which took a lot of trial an error. It was a bit painstaking because I wasn’t sure, how much wire to cut. Eventually, I started to see a pattern. In the end, the antenna was approximately 39 feet or 11.89 meters long. After I adjusted the wire, I hung the antenna on a 33 feet mast at home .
When considering a SEFHWA, I knew there was a lot of mechanical strain on the ends of the coil. What I was more concerned about was the efficiency and effectiveness of the antenna. As well as the overall length of the antenna because of my limited space at home. I knew I would have to go back to the drawing board and create a better way to connect the lead wire and pigtail to the coil. When the wire snapped, I was surprised to see a burn residue on the surface of the PVC pipe and the wire. In addition, the other end of the coil where the pigtail was located, the wire was already weakening and started to fray and tear.
Severed wire from the lead wire side of the coil. 
Bent wire from pigtail side of the coil. Small tear on the PVC wire.
I asked those on Facebook what they thought about the “burn” marks on the wire. These were some of their responses:
- Insulation breakdown due to high voltages
- Flexing of wire caused an arc
- Tight Spacing of the wire on the coil can cause high RF arc on enamel wire insulation
- Multi-stranded wire not good choice for coil
This is what I think happened based on my experiences on observing this antenna. At the base of the coil where the lead wire ends, there was a lot of strain and the integrity of the wire failed. In addition, thats where the 20 meters band ends and electrically, that’s where the high voltages occur. Therefore, an arc may have occurred and severed the wire. I have a parallel prototype to take to the field. About two weeks after my home SEFHWA broke, so did my parallel prototype. It failed at the same spot, but no noticeable “burn” marks. Although, the wire was strained.
I’m already working on my second prototype. I will relieve the strain further by using cable clamps to manage the wire strain – shown below. Although, I feel having a more complicated design, will lower the efficiency of the antenna due to all the folding I have to do to relieve the mechanical strain at the coil. Let’s see what happens.
