Lowpassfilter for DATV - DTX1

[Pd0hni]

Hello all,
I have been working with 70cm DATV (DTX1) for a few years now, but a while ago I started having some problems.
Now everything is fine again, but during measurements we found that there are different harmonic frequencies coming from the transmitter.
The idea was to place a "low pass filter" behind this.
But I have looked around here and there but there is very little to be found in this area and especially on these frequencies.
Does anyone have a good idea where I can get these that are suitable for DATV.
For 436MHz and 1242MHz DATV

Jan / pd0hni

[radiogareth]

Jan, is this for TX cleanup or pre-filtering for your RX? If tx, what power level?
https://gm3sek.com/2018/09/29/interdigi ... er-design/ and he also has ideas for coaxial stubs on 70cms.
Gareth

[Pd0hni]

Hello Gareth,

Thanks for your comment.

It is for transmitting on 70cm and 23cm D ATV.
I had the idea to place it in front of the 10watt amplifier. ? ?
Transmitter 0.27mW > preamplifier 1watt/3watt > 16 meters coaxial cable > low-pass filter > 10Watt power amplifier > antenna.

I have no experience with filters at all, so any information is welcome.

Jan

[radiogareth]

Jan, start here...https://wiki.batc.org.uk/Filters
I've built the 2nd one down for 70cms and that does a great job...
Gareth

[Pd0hni]

Ok Gareth,

Thank you, I will look in to it.

The preamplifier is made by https://www.pe1rki.com/amplifiers.html
Amplifier with mmic special for low input signals.

Jan

[Pd0hni]

Hi Gareth,

One question remains.
Where is the best place to place a low-pass filter?

Jan

[radiogareth]

I'm no expert.....but theoretically between the final PA and the antenna. However, with higher power filters with anything resonant in them can flash over. I'd put it before the PA and then see what comes out after the PA. Just check 2nd and 3rd harmonics in case of problems.....
Good luck.
Gareth

[Pd0hni]

Hi Gareth,

Ok, I will do that.

Thank you for your support.

Jan

[Pa3fbx]

I really think you like to have a clean signal into the next stage.
We Know the pluto is giving all harmonics minimal filtered.
So first stage filtering would be behind the pluto and put a clean signal into the preamp.
After the preamp, recheck and if harmonics are higher rethink about the linearity and if the preamp is not overdriven.
Then clean it with additional filters if needed and than go to a final stage and recheck the harmonics.
Antenna's are made for a certain freq. most likely filter some harmonics..

Remember, if you put many different signals into a non linear amplifier some mixing might occur and you have more problems you wish for.
And no amplifier is 100% linear.

Benno

[Pd0hni]

Hi Benno,

I will take your advice in consideration.
Thank you for your input.

Jan / pd0hni