Very loosely speaking, you should expect the radio's antenna connector to measure 50 ohms. But that is only true if you are measuring it using an RF signal in the frequency band the radio is expecting, not using the DC output of an ohmmeter, so 18 ohms is entirely reasonable.
The fact that making the shield connection causes you to stop receiving signals is a very good indication that either the antenna is defective or that it is completely unsuitable for the band you are trying to receive.
For example, you'd make exactly the same observations if you were to partially plug in a piece of coax with no antenna on the other end. This is because, without the shield connected, the coax might as well be a plain wire, and any old wire stuck in the antenna port will make a receiving antenna of some effectiveness.