How long do I cut the limbs for PMR 446?

[CharlesHarris]

At UHF frequencies, the type of coaxial cable used, and the type of connectors used make a big difference. The so-called "UHF" connectors PL259 male, and SO239 female, are not constant impedence above about 150 Mhz, and result in significant losses. Also, the common RG8 foam coax typically used for CB radio, has excessive line losses for UHF. I'm not sure which cables are available yo you in UK, but in the US we run Times Microwave LMR400 cable for anything above 150MHz or the equivalent Belden 9913, and would prefer to run only N-type connectors, if possible. On modern UHF portable transceivers the SMA type connectors also work well.

If your antenna is in your loft, the presence of any metal within about a wavelength of the antenna can cause coupling which affects its resonant frequency and disrupts the radiation pattern of the antenna. If you can find a local radio amateur with both SWR and a field strength meter, it may be helpful to do some trouble shooting.

The US amateur 70cm band includes your 446 MHz portion. It is commonly used here for FM simplex and repeater operation. It shouldn't be difficult to find a US-band 70cm antenna which is resonant on your PMR frequencies. Most commercially made antennas will have a gamma-match stub to allow specific tuning for best SWR on your working frequency, but also have sufficient band width to not exceed a 2:1 VWSR over the 5 MHz repeater splits used on the US FM phone band. An end-mounted 4-element yagi with a boom length of 1 wavelength should provide about 8dB of forward gain if properly set up.