Probably the most basic of antenna questions...

If an antennas length determines what frequency it receives/transmits, how can you have different length antennas for the same frequency?

For example, I just installed DD-WRT on an old wireless router I had so I can use it as a repeater to hit dead wifi space in my house. Now I want to do the same thing to use in our RV, but with an outdoor antenna to pick up wifi signal from a distance. After looking at many options online, I was a little confused as there are several options in length of omni antenna.

I know this is probably simple stuff, but I can't wrap my little brain around it yet.

Same form (rod) can implement different antenna types, need to read the fine print. Generally, more length gives better vertical directivity of an omnidirectional antenna.

Okay, so it's what's inside the form (rod) that matters then?

An antenna needs to be both resonant and large for best efficiency.

Short antennas can be made resonant usually by adding inductance and sometimes capacitance. The inductance can either all be in one place, or distributed along the length of the antenna. For instance, winding a long length of wire around a short insulated rod has this effect.

Short antennas are very convenient, but will generally not work quite as well as a full length antenna.