I'll be meeting up with Guido tomorrow for a couple of days to discuss, among other things, production of a "kit o parts" for a set of Low Pass Filters which can be switched from the Pi plus a small on board Power Amplifier.
You need a Low Pass Filter on the output of the Pi to stop it transmitting the harmonics of the programmed output frequency. The Pi puts out a square wave rich in harmonics. To find out more about this topic see here :-
For a discussion about how to make one you can do no better than here :-
Toroids and capacitors can be bought from a fellow ham here :-
If Guido and I decide to do some kits for a multi band LPF and PA board we will let you know
Hope this helps,
73s Steve G0XAR
--- In Raspberry_Pi_4-Ham_RADIO@...
, Ray Wells wrote:
> LPF = Low Pass Filter. It's purpose is to allow the wanted frequency
> pass to the antenna but attenuate all harmonics and other unwanted
> rubbish in the spectrum above the wanted frequency.
> Ray vk2tv
> On 21/03/13 12:25, Ronny Julian wrote:
> > As one that wants to try this copying the mail, whats and LPF?
> > On Wed, Mar 20, 2013 at 11:18 AM, Guido
> > threeme3@...>> wrote:
> > There is now a modified version of Dan's, WsprryPi that should be
> > capable transmitting WSPR on HF and VHF bands, just with a LPF and
> > antenna connected to GPIO4.
> > For experimentation, my code and binary can be found here:
> > https://github.com/threeme3/WsprryPi
> > 73, Guido
> > PE1NNZ
> > On Sun, Mar 17, 2013 at 8:11 PM, Daniel Ankers
> > md1clv@...>> wrote:
> > > you will see that it is possible to use the Pi as an FM Tx
> > without any
> > > additional components other than a piece of wire for an aerial.
> > >
> > There is also https://github.com/DanAnkers/WsprryPi
> > the same
> > technique (or to be more accurate, reuses the code!) to
> > produce WSPR on LF
> > with only a low pass filter and an antenna.
> > Regards,
> > Dan MD1CLV