in simple terms a dipole is a two pole antenna which often ( most generally accepted interpretation ) consists of two quarterwave long sections fed at the centre via a coax feed cable the length of the elements being determined by the frequency that you are trying to receive so for example a dipole for Civil airband ( 125Mhz)would have elements 600mm long and one for 300Mhz 250mm long . a simple formula for calculating wavelength in metres is
wavelength = 300 divided by frequency in Mhz
so for 300Mhz the wavelength is 1metre and so a quarter wavelength is therefore 250mm
The elements can be made from tube (copper brass or aluminium are all suitable for loft mounted versions )or wire . if the antenna is for mounting outside aluminium is the preferred material as it is less effected by the weather and is light in weight . external antennas will need to have waterproofing applied to the feedpoint to ensure no water enters the cable . This can be in the form of a waterproof enclosure for either the whole antenna or just the feedpoint . White PVC conduit fittings available at most diy stores is suitable (grey or black pvc contains a filler which may attenuate the signal so is not recommended ) here is one I made earlier
The coax feed is fed through the conduit and soldered to the elements inside the centre T section the horizontal section between the antenna and the short support mast needs to be greater than 1/2 wavelength so that the feeder does not detune the antenna
you will need a male BNC plug on the end of the cable to connect to the female socket on the UBC125xlt antenna connector
A UHF TV antenna will work on a scanner but will not be very effective on frequencies it was not designed to receive and they are generally very directional . It may however, due to the additional height be better than a rubber duck fitted on the RX on some frequencies try it and see it wont cost you anything apart from a short patch lead with suitable adaptors from the TV aerial feed point to your RX .
My earliest antenna experiments consisted of using my steel bed frame as an aerial for radio Luxembourg ( 208m MW ) in the late 50s (which when coupled to a valve radio with 200vdc HT on the aerial socket could have been lethal but I survived )