After several weekends on the roof tuning without any success this antenna, I decided to take the choke away. Below is what the interior of the EAntenna balun looks like. It’s effectively a 1:1 current balun (not a voltage balun) also called choke, that is the inductors are in series with the coaxial cable coupled around the magnetic core and exiting at the dipoles’ entry point. After removing the choke, the antenna behaved exactly the same as before. It appeared that the presence of this choke was not efficient. Below is my short story around this.
The problem was that the coaxial cable at the length of the mast was actually influencing seriously the SWR across all bands, an indication that the (current) balun was not effective in eliminating the unbalances from the coaxial cable. That is, the coaxial cable had current flowing on the shield because the balun was unable to cancel this out. This was a serious problem as it didn’t allow me to tune the antenna properly. I am wondering whether EAntenna is aware of this problem or whether there is something very wrong with my setup. But, these are just classic pre-tuned dipole antennas in parallel. Very little can go wrong. And I’ve constructed dozens of single band HF dipoles in the past with success.
All antenna masts, the center and the 2 at the end of the dipoles, were at 6m height. Surprisingly the 80m and 40m bands were the easiest to tune, despite close to the ground. The problems started when I tried to tune the other higher bands. I had to shorten each of the rest of the dipoles as much as 35cm per side, or a total of 70cm across the entire dipole! That was beyond EAntenna’s recommended range of up to 5cm reduction per side. It was this observation that made me to suspect that there was something seriously wrong with this antenna. However, I carried on trimming each dipole hoping that I will be able to tune each dipole on its band. At this point I was not aware of balun’s low efficiency or the fact that coaxial cable was influencing SWR across all bands! Applying the classic tuning procedures for dipole antennas, that is measure SWR, measure fo, then adjust length for desired fo with simple division, couldn’t help me at all! So, step by step I started looking for solutions, and, as described above, I discovered accidentally that the choke balun was actually useless: When I was changing the location of the coaxial cable for the length between the balun and the bottom of mast I noticed that the needle of my SWR antenna analyzer was jumping up and down! What the h$#& was that?!
I removed the EAntenna’s 1:1 current balun and I’ve decided to make my own ‘ugly’ balun as a replacement – a trusted and well accepted construction. To my surprise, this small thought solved all my detuning problems caused by the coaxial cable! So, the 1:1 balun by EAntenna did not provide any balance to unbalance transformation! To make sure about this, I’ve double checked for unwanted short-circuits between turns on the choke balun, but nothing was found.
The EAntenna choke was on a 40mm OD core with about 12 turns on a 1.5 diameter wire. I am not sure what the material type is, but from the appearance of the colour and the look it is definitely of a ferrite type.
If anyone has some experience on EAntenna’s fan dipole designs, please leave your comment below. I would like to know whether someone else out there has been on a similar situation or of give any useful advice that might help face the problem with the balun.