top of page
SMD and T37 Low Pass filter components for WSPR LP1, Mini and Mezzanine card

SMD and T37 Low Pass filter components for WSPR LP1, Mini and Mezzanine card

This is a bag with four SMD capacitors and three Toroidal Iron powder cores and magnet wire to wind on the cores.

One bag enables you to build a 7-pole low pass filter that removes the overtones on a low power transmitter for a specific frequency band from 137kHz up to 70MHz.

These are suitable for all my WSPR tranmitters like the Mini, LP1 and the Mezzanine LP4 card.
It can also be used for other home made transmitters with up to a few Watts of output power.

A low pass filter built with these components will typically have less than 1dB loss at the intended pass frequency and about 45dB suppression on the second and third overtone frequencies. 
When used with the WSPR-LP1 all overtones will be below 50dBC which meets or exceeds the regulatory standards for overtone suppression in most countries.


The Toroidal cores are T37 (0.37 inch in diameter) by Amidon.


The Capacitors are from different but well know manufactures like Kemet, Yageo, Samsung etc and are of the NP0 type for lowest loss.


The magnet wire included is 0.4mm (AWG26) and the coating is heat strippable.
To prepare ends for soldering burn the ends with a flame and scrape of the residue with a knife.

See table for value of Capacitors and material of the Toroid.

On the two lowest bands - 2190m and 630m - the Toroids are Ferrite material.

On the other bands it is iron powder.


Filters were deigned by the late George Dobbs G3RJV of the British GQRP club

Full information in this PDF file

    bottom of page