Originally Posted by
FlyingRon
As pointed out the loss will be of small consequence and it's going to be the connectors rather than the extra foot of RG400. One thing that can be handy if you can swing it with the existing cable slack is to loop the cable down so that the connectors between the original wire and the extension can be reached from the cockpit semi easily. Then you can get another short extension for your handheld and in case the panel dies, you reach up, unplug the extension going to the panel mounted radio and plug in your handheld and get to use the external antenna.
Neat trick Bill Needy used to do on his installations up at HGR.