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I am looking to buy my first mobile antenna, for use with a 2m/70cm HT in the car. The HT needs 50 Ω impedence.
I was looking at this antenna, but another ham told me that I need 1/4 wave or 5/8 wave to get 50 Ω impedance. A 1/2 wave antenna will not be 50 Ω so I will need to add an antenna tuner (or risk burning up the radio).
I'm confused - can someone enlighten me on this?
The HT needs 50 ohm impedence.
An antenna of this kind should be designed to have a 50 ohm impedance (when properly installed on top of a ground plane such as a car roof). It probably won't be exactly 50 ohms across both bands, but that's fine as long as the difference (as measured in SWR) is not too high. Usually the trouble is if you're using an antenna not designed for that band.
A 1/2 wave antenna will not be 50 ohm
This is either not a true 1/2 wave antenna (I see it has a loading coil in the middle) or the description is made of lies. 1/2 wave end-fed antennas require specialized matching networks. While this antenna's length is close to 1/2 wave, it also has a loading coil in the middle, which means its electrical behavior is not equal to that of a 1/2 wave wire.
Comet's page says this antenna should have a SWR of 1.5:1 or less across its bands. That's quite good for a mobile antenna.
so I will need to add an antenna tuner
As a general rule, antenna tuners — adjustable impedance-matching networks — are not used for VHF/UHF antenna systems. This is because antenna tuners are needed when you want to use one antenna across different bands and wide bands, or to use an improvised antenna whose exact length and coupling to the environment are unpredictable (for portable operation). Both of these are activities are common on the HF bands (including up to 50 MHz, which is where HF and VHF techniques and antenna designs start to cross over). But for VHF/UHF, it works quite well to design one antenna that is a reasonable compromise across the entirety of those two bands.
For a new mobile installation, the best option is to obtain the use of an antenna analyzer, which can tell you not only what the SWR is across the bands, but also where the antenna's resonant peaks are — allowing adjustment if the antenna provides any room for adjustment. (For example, my VHF antenna came with a slightly long wire element, to be cut down to length after testing.)
But you will almost certainly not need to do that. The antenna is probably just fine as provided, as long as it's installed per the instructions.
(or risk burning up the radio).
HTs are designed to survive being used with a wide range of antenna impedances. This is necessary, because they are designed to be used hand-held in a wide variety of positions, and all the objects around them will affect the antenna impedance. The stock “rubber ducky” antennas they come with are often far from 50 Ω even in perfect conditions.
You would want to take more care with a higher power mobile radio installed in the car, such as checking with an antenna analyzer. Even then, it is unlikely that the radio would be damaged — modern radios have protection circuits to prevent the final amplifier from overheating.
I agree with most of the previous answers. A mobile antenna is designed to give 50 ohms at the feedpoint by having a loading coil, or inductor, at the base of the antenna. And generally the only way to match a short vertical antenna is to physically change the length of the antenna. Usually mobile type antenna whips are a bit long and the only way to get a good match is to cut a small bit off the base of the whip and check. But any radio should be able to deal with a slight mismatch as mobile communications tend to be short so won't produce smoke.
