As the radio spectrum become more and more cluttered with signals, filters are becoming an even more essential part of any radio equipment.
It is absolutely essential that low pass filters are used on the output of the Portsdown and any other wide band DATV transmitter such as DTX1 and DATVexpress to remove harmonics of the wanted signal. Just try tuning your receiver to 439.5 when transmitting on 146.5 MHz!
The same also applies to the input of any ATV or DATV receiver which uses a wide band satellite tuner in the front end (and they all do!) - this is because these tuners were designed to be used in a "closed system" and connected directly to an LNB on a dish pointing at the clear sky.
This is very different to being connected to a "wide band" yagi antennae which will also pick up PMR and paging systems, short range devices and broadcast transmitters many of which are digital and just produce white noise so you cannot tell you are being overloaded by them!
As these 2 pictures of 146 MHz, taken on the Isle of Wight during the BATC December 2016 activity day show, the receive filter needs to be tighter than the transmit filter as you are trying to remove all strong signals that are high and low of the desired frequency.
On transmit, as long the transmitter has been designed correctly and you are not over driving any power amplifiers, a low pass filter to remove all harmonics is adequate.
Reasonable filters are actually quite easy to build with just a medium weight soldering iron and tin snips!
A great place to start if you fancy building your own is W1GHZ paper on making filters from an Altoid's tin (any other metal tin will do) - http://www.w1ghz.org/filter/Altoids_Tin_Filters.pdf
If you want to design one from scratch Doug, VK3UM, now sadly Silent Key, has a calculator http://www.vk3um.com/interdigital%20filter.html.
Filters for each band
There's a lot of filter designs available for DIY build or to buy listed on these pages: