70cms filters

There are plenty of designs available for 437 MHz filters:

450MHz Low pass transmit Filter

Minikits in Australia make a really good low pass filter which is ideal for use with the Portsdown or other 70cms DATV transmitters.

As used by G4KLB, G8VPG and others.

Simple inter-digital filter

Design by F1DJO and F6GQM first published in CQ-TV 245

Tuneable pass band filter

Tuneable passband filter design by Ian, G3KKD, with 2 MHz band pass and 30 dB attenuation 4 MHz away. (the full article is available in CQ-TV 228).

Built units

The following suppliers sell built and tested units:

eBay Chinese made 470MHz low pass filter

This small filter is available from here http://www.ebay.co.ukfor just £6 including post from Hong Kong. Insertion loss is only 0.3dB at 437MHz and the 2nd and 3rd harmonics of 70cm are attenuated by 45 and 60 dB respectively. See plot below:

This makes it suitable for use between the ADF4351 synthesiser and the Portsdown modulator to improve the constellation as well as the output of a driver stage up to about 1 watt. It would also be very useful to reduce overload from phone towers or broadcast TV if place in front of a preamp that has a wideband input.

• Antennair - suppliers of the DTX1

• DG0VE - German supplier of reasonably priced filters for most amateur bands