Just buying a Vehicular repeater off eBay isn't advised. You might be implying that you just need the equipment, but such equipment must be approved for such use by the FCC and the frequencies may require an FCC radio station license. A vehicular repeater designed solely for HAM radio (amateur radio service) may not be type-accepted for use on public safety radio systems. However, the Pyramid radio repeaters are used by many public safety agencies. Cross-banding with a VHF mobile to UHF mobile and portable may also be possible in the UHF radio with certain models of both Motorola and Kenwood Mobile radios.
Consider that you are modifying a radio "system", not just the radios in your car. Are you wanting the vehicle repeater only as a personal use? Is a vehicle repeater system already authorized? Or, are you checking into feasibility for upgrading the capability of all the radios in a local fire department? The FCC license modification costs are usually going to be more than an individual could afford.
Other major precautions and considerations for a vehicular repeater design:
Since you are using the vehicle repeater for other than a law enforcement activity (which has some exemptions involving less than 2 watt transmit power), you will need to use only those frequencies that are authorized for repeater use under Part 90 of the FCC rules and licensed by the FCC for the particular geographic area (15 mile vicinity, county-wide, etc) and emergency service (police, fire, EMS, emergency management, rescue squad?) and be certain that mobile radio and portable transmitter power is limited. For example, if you have a five watt VHF portable, you will usually have to reduce the transmit power into the repeater frequency to one watt and have the output power of the repeater limited ~ 2.5 watts. Also, it depends if you will need a one-way (portable radio to car) or two way (portable to car: car to portable) repeater. Frequencies for the latter application are more abundant in UHF and 700/800 MHz radio spectrum than in VHF - mostly because those spectrum had a shorter range from portable to base station, repeater systems were considered in the radio spectrum assignment. For VHF high band, repeater systems were often an "afterthought", because that part of the spectrum was assigned mostly for simplex radio systems among public safety agencies.
A cross-band repeater, using a UHF portable for a VHF mobile radio, was the typical repeater solution because the portable and car repeater frequencies had to be separated by several MHz in the bandwidth if they were in the same band. (For example, 154 MHz fire frequencies usually needed another repeater frequency in the either the 151 or 159 range.) It also depends if the local VHF radio system is simplex (mobile and base station on the same frequency), or semi-duplex or duplex (separate base and mobile transmit frequencies). Using lower power on the vehicle repeater often means that the frequencies may be closer together, allowing a VHF repeater on a VHF mobile, and allowing you to have the VHF portable radio programmed for both the repeater and normal radio system uses. (Such uses are associated with power reduced to 750 milliwatts or .75 watts on repeater output, and that limits car to portable receiver distance.)
If you local radio system is not licensed for vehicle repeater systems, I strongly suggest you check with your State Frequency Coordinator for the Association of Public Safety Communications Officers (APCO) or the FCC designated Fire/EMS Frequency Coordinator for your area. (In Tennessee, our Tennessee Emergency Management Agency provides this assistance.) The coordinators can advise or provide a list of available repeater frequencies for emergency service use in your area and whether an FCC license change will be necessary.
Also, you may have some other options.
If you need to increase the signal strength of your portable radio to the base station, consider that the portable may need a tune-up or a different portable radio antenna might improve the signal. (The short rubber ducky might need to be replaced.) Or, a newer portable radio might help (if you only have a 2 watt portable, consider whether changing to a 5 or 6 watt handheld radio may be a more cost effective solution that a vehicular repeater.)
