2.4GHz Cavity Filter

The small introduction. I am radio amateur operator transmitting signals at 2.4GHz band into the geostationary amateur satellite QO-100. I have got two stations. Base station which use a good quality SG-Labs 20W amplifier and all works perfect, and then a portable station, which use a bad quality Chinese PA from AliExpress.

The problem is that the Chinese amplifier produces a lot of crap parasite signals and as result when I transmit I cannot correctly hear my own signal. So why do I use the Chinese PA? Because my portable station has only 40cm antenna dish and I need a lot of power to get into the satellite in the UK. I need 20W in peaks. Closer to equator you do not need so much RF power!

Just to make sure my Pluto is not causing any problems first of all I also tried to filter the signal between Pluto and PA using “amazing” quality filter:

However, despite ordering two of them I found out that both don’t work at all!!! My NanoVNA tells me they are technically broken as the signal is not coming through at any frequency. Also later on I found that some people confirming that they are just a junk. Also, even if they work fine, the maximum RF power they accept is 10W at max. 1 min so I could use them only between PA and Pluto.

Anyway, I checked out my copper junk in the loft, I collected all parts and within 2h I made a cavity filter tuned to 2.400.000GHz.

The cavity filter reduced 95% of the problem but there is still a small amount on noise when I transmit. Just to make sure the receiver is not getting any crap, I ordered another filter which looks the same as 2.4GHz filter so probably it will be a rubbish too, but I have not received it yet. I will publish an update when I get it.

Edit: I am still waiting for the FBP-750 filter, but as I got a bit impatient, I decided to make my own 739MHz bandpass filter – see article HERE.

Assembly drawing, pictures and results of my 2.4GHz filter:

Click HERE to get a full size PDF drawing.

Almost 40dB at 739MHz 🙂

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