According to wikipedia
https://en.wikipedia.org/wiki/PMR446 the maximum radiated energy (ERP) is commonly limited to 500mW in most countries.
An antenna tuned to the 70cm band won't drop off like a vertical cliff, but will also be quite useable at frequencies close to its center freq.
- you will most likely see a miniscule increased SWR at worst -
https://en.wikipedia.org/wiki/Standing_wave_ratio - but depending on your cabling and antenna mount position, the last two factors often plays the better of getting good antenna Rx/Tx signals.
HF frequencies often utilize a single antenna and frequencies spanning multiple megahertz - and if the power is high, an antenna tuner to ensure the transmitter is not blown up by the returning power (SWR). A high SWR ratio won't usually effect Rx operation, besides making the receiver a little more deaf due to the destructive effect of the received/reflected wave/energy ratio between antenna 'output' impedance and the radios input impedance.
Antenna filters can often be made by self wound inductors (wire) and salvaged capacitors from old/broken electronics.
Ebay is also a good place to get stuff:
http://www.ebay.co.uk/bhp/pmr-antenna (Notice these are also sold as 70cm antennas)