For a long time, I have been observing the deterioration of income possibilities. There is practically continuous interference, which I mostly observe on 80m and 10m. Such QRM is the limit for amateur radio connections, including the OM Activity Contest.
You will read:
HINT: balun / flour / choke made of ferrites
By connecting an unbalanced feeder such as a coaxial cable to a symmetrical antenna radiator such as a dipole, inverted V and the like cause a number of problems. The coaxial cable thus becomes part of the antenna. That is not its purpose. We require the transmission of HF energy from the coaxial cable, not be part of the antenna. We probably wouldn't want to have the antenna a few decimeters away from the house or next to other cables, no?
That's what it's about. We need to ensure, so that the antenna is an antenna and the coaxial cable is a coaxial cable. Unless a circuit is used in the design (balun / flour), who will ensure it, so the most common way is to use a choke made of ferrites.
Two constructions are mainly used in practice: a current balun on the toroid and a series of ferrite toroids pushed onto the jacket of the coaxial cable. I use both types. The ferrite material is very important for the result. Example of current balun on FT-240-43 in W1JR design
The operation of the balun can be easily checked using the nanoVNA. This piece has excellent properties on the lower bands, where at 5 MHz achieves a suppression of approx. -40 dB. At frequencies above 12 MHz, but suppression decreases, achieves -13 to 21 dB.
A series of ferrite toroids pushed onto the jacket of a coaxial cable necessarily requires a larger number of them. This is given by, that while a current balun like the W1JR increases the inductance with more turns, thus, sliding the toroid onto the sheath of the coaxial cable represents 1 thread.
I use ferrites from Peter DK1RP https://www.dx-wire.de/ferriteringkerne/ferrite-fuer-mantelwellensperren/ They are quite expensive, but also a guarantee, that they will work. Peter also presents practical measurements at different frequencies. For the antenna I use inverted V on 80m 20 injuredv, for the antenna on 40m it is smaller, 14 injuredv. Pay attention to the weight
HINT: how to know, that the balun works?
The first signal is a detuning of the antenna. team, that the coaxial cable used to be part of the antenna, so the antenna with the balun tends to resonate at a different frequency.
The second signal is, that noise and interference picked up by the antenna will be reduced.
The third signal is detectable during transmission. If the radio amateur affected some electrical equipment by broadcasting, so the problem can disappear.
A sample of the receiving antenna effect
I selected entries from two connections in the last round of the OM Activity Contest. The antenna is placed temporarily at a height of approx. 5 meters directed to the northwest. However, it is not characterized by a large front-to-back ratio. There isn't much time to try in a race, but hopefully i was able to catch the difference. The receiving antenna has less noise.
Jožek's signal OM4DU
Janka's signal OK2GER
September round of the OM Activity Contest
I start by connecting with Martin OK1VHH, because it is a representative of the relatively rare multiplier H
I follow the ionogram by eye. He shows, that the reflections occur at a relatively high height compared to the summer rounds. It stayed like this practically the entire time, which is also not usual. I start slowly and gradually go through the band repeatedly. Connections increase evenly, albeit at a slow pace. When calling one OK station, Laco overtakes me OM5LD, which follows the same QRP category. It also gives a higher number. In a few minutes I find Lac on 3775 kHz
Laco gives me a much higher number than I send him. So it's time to try harder! There is no time for SO2V today, because I'm trying to switch the receiving antenna. But I quite easily find frequencies on call for short periods of time. Meanwhile, I keep trying to connect with OM3KEG, which according to the S-meter is S9+5dB. But I didn't ask for Marian.
I know, that Jaro was also on the band OM1EE and Ivan OM0AD, but I only heard them once in conjunction with another station. Gradually the connections increased and it is at the end 54 contacts. Two multipliers are missing, but after the summer rounds it is again a more encouraging result. But the most interesting thing will probably be to perceive how the receiving antenna behaves. I have it marked, that I switched it during connection with OM8ABC, OM7AT, OK1AY, OM4DU, OM4AY, OK1UKY, OK2GER, OK7AW a OM0AJ. Try to listen to them!
