As a rule-of-thumb, generally try to make the antenna wire run from the receiver to the antenna pipe as short as possible. Without trimming the lead length, of course. But then, that becomes a compromise in determining how long or short to make the antenna pipe depending on the installation.
Additionally, if possible, locate the receiver away from any spurious signal generator, such as a motor or servo. Caveat, I have mounted receivers atop servos with no adverse operational results. Still, I try not to do it.
Carbon fibre, being a conductor of sorts, manufacturers are sleeving their antenna leads to isolate them from [possible] signal capture and drain off from the CF. Fortunately, even sleeved, the antenna lead still slips into an antenna pipe. Regrettably, it is now difficult to cap off an antenna pipe. Antenna pipe caps tend to prevent poking an eye out when marshalling a car and also offer protection to the antenna lead.
This is becoming moot, as more and more manu's are offering receivers with internal antennas. My preference, albeit old-school, is for an external antenna. Ergo, all the above.
Curious? What are your plans for an external antenna? Why do you want one? Regards. 'AC'