739MHz Helical Filter for QO-100

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 article is HERE.

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.

So as I got impatient with the delivery of the filter above (which probably will not work anyway), I decided to make a helical filter for 739MHz – this is frequency which you receive from your LNB when you listed to 10 489 750 MHz 🙂

Idea is from this website: https://www.changpuak.ch/electronics/Helical_Bandpass_Filter_Designer.php

So according to the website above, the filter I need is as shown below:

So, theory above, practical implementation is below:

PDF drawing from the picture above is HERE.

…and then a real life implementation is shown at the pictures below:

I gave up the tuning screws as I tuned everything by moving the coils. Next filter will be made by machining a solid piece of aluminium – it will be much neater.

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