For some time now, I have been noticing a deterioration in reception conditions. There is virtually constant interference, which I notice most on the 80-meter and 10-meter bands. This kind of QRM is a limiting factor for amateur radio contacts, including those during the OM Activity Contest.
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TIP: balun / unun / ferrite choke
Connecting an unbalanced feeder, such as coaxial cable, to a balanced antenna element—such as a dipole, inverted V, or similar—causes a number of problems. The coaxial cable thus becomes part of the antenna, which is not its intended purpose. We want the coaxial cable to transmit RF energy, not to be part of the antenna. We probably wouldn’t want to have an antenna just a few decimeters from the house or next to other cables, would we?
That’s exactly the point. We need to ensure that the antenna remains an antenna and the coaxial cable remains a coaxial cable. Unless a circuit (balun/unun) is used in the design to ensure this, the most common method is to use a ferrite choke.
In practice, two designs are primarily used: a current balun on a toroid and a series of ferrite toroids slipped onto the coaxial cable’s outer jacket. I use both types. The ferrite material is crucial to the result. Example of a current balun on an FT-240-43 based on the W1JR design
W1JR balun on FT-240-43
The balun’s performance can be easily verified using a nanoVNA. This unit has excellent performance in the lower bands, where it achieves an attenuation of approximately -40 dB at 5 MHz. However, at frequencies above 12 MHz, the attenuation decreases, reaching -13 to 21 dB.
A series of ferrite toroids slid onto the coaxial cable jacket necessarily requires a larger number of them. This is because, while a current balun like the W1JR increases inductance through multiple turns, sliding a ferrite ring onto the coaxial cable’s shield counts as only one turn.
I use ferrites from Peter DK1RP https://www.dx-wire.de/ferriteringkerne/ferrite-fuer-mantelwellensperren/. They are relatively expensive, but they also come with a guarantee that they will work. Peter also provides practical measurement data at various frequencies. For an 80-meter inverted-V antenna, I use 20 ferrites; for a 40-meter antenna, it’s smaller—14 ferrites. Watch out for the weight
TIP: How can you tell if the balun is working?
The first sign is that the antenna is out of tune. Since the coaxial cable was previously part of the antenna, the antenna with the balun tends to resonate at a different frequency.
The second sign is that the noise and interference picked up by the antenna will decrease.
The third sign is noticeable during transmission. If the ham radio operator’s transmission was affecting any electrical equipment, the problem may disappear.
Demonstration of the receiving antenna’s effect
I selected recordings from two contacts during the final round of the OM Activity Contest. The antenna is temporarily mounted at a height of approximately 5 meters, pointing northwest. However, it does not have a particularly high front-to-back ratio. There isn’t much time for testing during the contest, but hopefully I managed to capture the difference. The receiving antenna has less noise.
Jožko OM4DU’s signal
Janko OK2GER’s signal
September round of the OM Activity Contest
I’m starting with a contact with Martin OK1VHH, because he represents a relatively rare multiplier H
I’m keeping an eye on the ionogram. It shows that reflections are occurring at a relatively high altitude compared to the summer rounds. It stayed that way practically the entire time, which is also unusual. I start slowly and gradually work through the band repeatedly. Contacts are coming in at a steady, albeit slow, pace. While calling a single OK station, Laco OM5LD, who’s operating in the same QRP category, beats me to it. He’s even giving a higher frequency. A few minutes later, I find Laco on 3775 kHz
Laco gives me a number significantly higher than the one I’m transmitting to him. So it’s time to try harder! There’s no time for SO2V today, since I’m trying to switch the receiving antenna. But I’m able to find call-sign frequencies relatively easily for brief moments. Meanwhile, I’m repeatedly trying to make contact with OM3KEG, who’s showing S9+5dB on the S-meter. But I couldn’t get through to Marián.
Ratio of OK / OM stations in the OM Activity Contest
Number of connections per minute in the OM Activity Contest
I know that Jaro OM1EE and Ivan OM0AD were also on the band, but I only heard them once while they were in contact with another station. Gradually, the number of contacts increased, and I ended up with 54 contacts. Two multipliers are missing, but after the summer rounds, this is once again a more encouraging result. But the most interesting part will probably be seeing how the receiving antenna performs. I’ve noted that I switched it during contacts with OM8ABC, OM7AT, OK1AY, OM4DU, OM4AY, OK1UKY, OK2GER, OK7AW, and OM0AJ. Try listening for them!