Lowpassfilter for DATV - DTX1

[Pd0hni]

Hello everyone,

I recently discovered that my 436MHz DVBs ATV transmitter emits a lot of harmonics.
Now I'm trying to find a small filter with low power, but that's not easy.
The power of the transmitter is 0.27mW.
Does anyone know a nice fitter for this ATV frequency?
It may be a low-pass filter or a band-pass filter.
I tried to find something at Aliexpress but to no avail.

Jan / Pd0hni

[g0mjw]

Just build one from inductors and capacitors. Not hard, especially at this power level. It isn't necessary to buy everything ready made.

You can make the coils from wire, 2T on 4mm and 4T on 4mm respectively and standard value 7.5pF will be close enough for 7.3pF.

Screenshot 2024-07-12 155224.png (145.21 KiB) Viewed 4578 times

Mike

[Pd0hni]

Thank you g0mjw,

I will look in to it.

Jan / Pd0hni

[M0DTS]

The LPF433 and BPF433 filters seen on Aliexpress and other places should work fine, they are not that narrow band.

Rob

[Pd0hni]

s-l1600.jpg (30.93 KiB) Viewed 4106 times

Hi Rob,
I had several BPF and LPF filters sent to me.
meanwhile 8 different models. But they are just not good enough. I am now working on adjusting 2 fitters so that they are on a good frequency. Hope that works.

[radiogareth]

Those filters are ok up to a few watts, but CHECK the passband. My first one was OK, subsequent ones had a 420MHz cut-off. The poor passband units had inductors that were mounted at a slight angle to the PCB unlike the picture above.
Gareth

[Pd0hni]

Hi Gareth
Thanks for the tip.
There are two versions, see photo.
I am now looking at which one is best adapted for 436MHz DVBs. I was also advised to place it as close to the transmitter as possible for the best results. The transmitter does 0.27mW

[G1BRB]

Hi Jan,

I have got two of the LP filters like the upper one in your picture. Both had a low cut off frequency and also approximately 2db through loss when checked with my NanoVNA.
My examples had five turns on the coils, pictures on the website they were bought from showed four turns. For one I bent the end turns over ninety degrees, that raised the cut off frequency but the through loss was little changed. For the other filter I wound three new coils, of four turns each on a one millimetre drill, using some similar sized wire found in my components drawer. The cut off frequency was increased and through loss was about 0.9db at 437Mhz measured on my NanoVNA.

Adrian G1BRB.

[Pd0hni]

Hi Adrian,

Thank you for your help.
This is what I wanted to hear.
I'm going to have a fellow amateur try this out because he has the right measuring equipment.
And I can hardly see it with my bad eyes.

Thanks again

Kind regards Jan / Pd0hni

[Pd0hni]

Hello all,

Had the filters measured today.
They can be used both.
One is slightly better than the other, just look at the images and markers.
Measured between 400 and 600MHz.
Now just make a housing.

Jan / Pd0hni
Thank you all for your support.