Since this is a Over The Air reception forum and you are asking a OTA question, I will try to give you as clear of a answer as possible.
IN the analog days, there was effectively two transmitters, one for the audio which was broadcast in AM - amplitude modulation at a very high power level.
The audio was broadcast in FM with a much lower power level, due to the fact that FM is not as susceptible to noise and interference as is AM.
It was found that a horizontally polarized signal gave less interference problems than did a vertical polarized signal. FM radio is broadcast with vertical polarization.
Digital signals are much less powerful then the analog signals and the main problem with digital signals is that some if not most stations has moved up into the UHF band. In order to receive UHF - you will need a large UHF style antenna if you are more than 10 miles away from the transmitter and multipath plays a large role in not being able to receive well any if at all a signal in a mobile vehicle. Since UHF is line of sight, your reception would depend on your antenna being pointed in the right direction at all times. At the same time, when you use a multidirectional antenna, multipath - where the receiver receives more than one signal of the same signal at the same time, the two identical signals cancel out each other and you get no signal at all.
In the analog days, multipath would cause ghosting, in digital it is all or nothing.
Yes you could use a UHF antenna - say 4 feet long YAGI type and a DC converter box, but as soon as the vehicle moved in a different direction, it would loose the signal - if you had the antenna pointed north and the signal was south and the antenna was pointed any direction other then south.
So I would abandon any attempts at trying to build a mobile receiver for a automobile.
There is many people right now who are apartment dwellers who lives in the 3rd story of a high rise building, that has other buildings around them that are higher then they are and they are on the wrong side of the building - that has little or no reception with a indoor antenna due to the limitations of UHF.
Unfortunately, there is nothing that can be done about it other then putting up multiple translators which transmits the signal on different channels at different locations throughout a city.
To make digital television work for a mobile application, you would have to put up multiple transmitters - just like cell towers which would transfer you from one tower to the next and maintain some sort of reception as you moved throughout the city. I don't look for that to happen for free anytime in my lifetime.
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