Review of low-pass filters
Balun OM0AMR, Balun OM0AMR, Balun OM0AMR. Commercially produced shortwave devices have these products suppressed mostly well, but in the case of connecting the final stage, it is necessary to include a low-pass filter with a cut-off frequency of 30MHz behind it.
Schematically, these filters are quite simple - a multipole pi cell. Parts for higher powers are mostly difficult to find, and therefore some radio amateurs will certainly welcome commercially produced filters. You can find three such filters in the review - Kenowood LF-30A, MFJ-704 a Vectronics LP-30. Designers can also take inspiration from the sketched solution.
Kenwood LF-30A is a small filter (244x50x40mm). The manufacturer states the dimensioning for 1kW PEP power and inherent losses lower than 0.5dB at frequencies below 30MHz. Notice the mechanical arrangement in the photo - the capacitors are made up of massive disks, which are isolated from the grounding partition by a thin PTFE film. All coils are thus mutually shielded.
Vectronics LP-30 a MFJ-704 they are mechanically constructed very similarly. In boxes, which are the same size (220x77x71mm), there is no shielding partition. The capacitors are formed by a double-sided printed circuit board and the coils are soldered at different angles. The manufacturer assumes, that the distance between the parts and their orientation are sufficient, so that there is no unwanted bond between them. The load capacity of these filters is 1.5kW PEP and inherent losses lower than 0.5dB at frequencies below 30MHz.
All filters were subjected to measurement using a HP signal generator and a selective voltmeter. The results can be seen in the graph. This is where the advantage of the LF-30A design became evident, which achieves better values in the area of the stop band. While the LP-30 and MFJ-704 achieve about -50dB of rejection, Kenwood more than -66dB, which was the range of the meter. Vectronics and MFJ also have a few at higher frequencies, where the suppression is lower, but also higher.
The winner of the test was the Kenwood LF-30A, which is also the cheapest. The MFJ/Vectronics design is rather an inspiration for the domestic production of such a filter.
