VK5FQ | Other Stuff

475kHz 7 pole elliptical low pass filter.

Bare bones Low Pass Filter - interior view.

Need a seriously good high power low pass filter ...?

Having built several amplifiers for the 630m band, the occasion has risen to build a suitable high power filter that will remove the harmonics to the required acceptable levels, when transmitting at significantly higher power levels.

To be really successful, one needs to look at removing or reducing greatly, the second and third harmonics of the output frequency from the RF output of the RF amplifier. The method chosen here, was to use a 7 pole elliptical low pass filter that gives the required attenuation at the frequencies of concern and does this surprisingly well.

The program I used to design, optimize and plot the filter, is called ELSIE and is available from Tonne Software. The link is provided below. The site has a student version (free) which has limited number of elements that can be used in the design. This version is what I used and is more than adequate to perform all the tasks required.

I can highly recommend using this software tool to help you in your filter designs.

The cores used in this design were Amidon T225A-2 powdered iron material and wound with 2.5mm2 (7/0.67) building wire. The core losses can be quite high at this frequency at high power levels. It will be Ok at several hundred watts and additional cooling should be sought when using data modes for extended periods.

Well, the "Proof is in the pudding!" - or so the saying goes. I was pleasantly surprised when the design results closely matched the final built and tested filter results, in that, the required attenuation of the second and third harmonics. Pretty darn good. The component values were not exactly as designed but were suitably matched to be close. Within a few per cent. The effort taken - shows. Job done.