SOME EXAMPLES OF ERRORS MADE
As said in the opening statement, the primary reason for making a bad choice is done with a view to "regaining the loss caused by the coax".
A prime example is using a 9dB collinear for satellite work. The radiation pattern is flatter than a tea biscuit, so any satellite slightly higher than the horizon is actually out of its effective useful beamwidth and will therefore actually exhibit loss as opposed to, say, a ¼-wave with groundplane (effective max at +45°).
Another classic mistake was personally experienced when issued with a tactical discone antenna for use in military ops in a valley. If I was talking to others in the valley then all would be well (and our comms would be kept in the valley) but it was for comms to mountain units and helicopters (and all this on very limited power from military man-packs!). First, I did rave about the discone, and I still do! As it can operate on such a broad frequency range without any adjustment, the discone really does shine through in a military setting. Knowing the discone's radiation pattern, a simple rig was constructed to mount the discone upside-down. Success!
In both the above examples it can be seen how a ¼-wave with groundplane would function far better than the antennas with gain and could quite easily be said to have "gain" over the other antennas. Visiting Antenna Gain Explained one can see that gain is also loss - the more gain an antenna has i.e. the more effective an antenna is, the less angle the antenna has to exhibit such effectiveness. It is, therefore, highly important to not only chose the antenna with regards required gain, but also to chose the antenna that exhibits that gain in the desired direction.
As said in the opening statement, the primary reason for making a bad choice is done with a view to "regaining the loss caused by the coax".
A prime example is using a 9dB collinear for satellite work. The radiation pattern is flatter than a tea biscuit, so any satellite slightly higher than the horizon is actually out of its effective useful beamwidth and will therefore actually exhibit loss as opposed to, say, a ¼-wave with groundplane (effective max at +45°).
Another classic mistake was personally experienced when issued with a tactical discone antenna for use in military ops in a valley. If I was talking to others in the valley then all would be well (and our comms would be kept in the valley) but it was for comms to mountain units and helicopters (and all this on very limited power from military man-packs!). First, I did rave about the discone, and I still do! As it can operate on such a broad frequency range without any adjustment, the discone really does shine through in a military setting. Knowing the discone's radiation pattern, a simple rig was constructed to mount the discone upside-down. Success!
In both the above examples it can be seen how a ¼-wave with groundplane would function far better than the antennas with gain and could quite easily be said to have "gain" over the other antennas. Visiting Antenna Gain Explained one can see that gain is also loss - the more gain an antenna has i.e. the more effective an antenna is, the less angle the antenna has to exhibit such effectiveness. It is, therefore, highly important to not only chose the antenna with regards required gain, but also to chose the antenna that exhibits that gain in the desired direction.