Hi all,
I'm a complete noob when it comes to CB's. Plans are to mount a 4' firestik on the passenger side mirror. Any tips for routing the coax so I don't have to drill a hole? I can drill a hole no problem if necessary, but I'd like to avoid it if possible.
Also, is 18' coax really necessary? 9' would easily reach in my situation.
Thanks,
The length of the coax is really not critical if you can tune your antenna.
The thing that makes coax length funny is when there is a mismatch between the 50 ohm coax and the antenna. For instance, a monopole generally has an impedance somewhere around 35 ohms. This introduces reflections (this is where you can measure an SWR). Due to the nature of radio wave (time-harmonic sources), as you move down the coax, the amplitude of the reflection actually changes. That is, the impedance looking into the end of the coax changes.
Now, it is entirely possible to tune your antenna for one frequency using this knowledge, it's called "transmission line tuning". But it's only good at one frequency. DSB-TC (AM radio waves) will occupy about 6 kHz of bandwidth, so this means that you're dealing with a whole bunch of frequencies, so transmission line tuning may not be that effective.
However, realizing this... there is one important length to *avoid*. That is an odd multiple of 1/4 wave of coax. A 1/4-wave length of coax is called an "impedance inverter". It will take that 35 ohm load and turn it into a 71 ohm load (doesn't sound like an inversion... but if you're interested I can write a thread about Smith Charts). The reason why a 1/4 wave should be avoided is due to it's open-circuit properties. The impedance transformation of an open-circuit (with your antenna removed) is a short circuit! CB is approximately 11 meters wavelength. 1/4 wave is just about 9'. But that's assuming 100% velocity of propagation. Cheap RG-58 generally has a propagation factor of 66% or so, which means your 1/4 section to avoid is actually about 6' or so (notice that 3*6 = 18'... what many think is a 1/2 wave length of coax could possibly be a 3/4 wave length, depending on the properties of the coax).
Ideally you want your coax to be short as possible, to eliminate losses. The antennas on my Civic are fed with about 4' of LMR-300 low loss coax. The reason why you want to eliminate as much loss as possible, is the loss actually introduces 'noise' to your system. The noise figure of your system is dominated by the noise performance of the part closest to the source (which is why you put a preamp on the TV antenna outside, and not at your TV). Shorter coax also reduces the likelihood of the coax picking up RFI, or becoming part of the antenna system.
