Posted for "Tom Gourley" <tom.gourley@verizon.net>:

 RG58 and RG400 are both 50 ohm coax so, assuming both cables meet their
specifications, mixing cable types, in and of itself, will not cause any
reflections or loss due to the different cables.  There will be some small
loss due to the coupling, and here I would use a female BNC on the end of one
cable and a male BNC on the other.  That way there is only one connection
instead of two that would result from putting male BNCs on both cables and
joining them with a barrel connection.  I've talked with techs at avionics
shops who told me that putting an extra connection or two in a comm cable is
not an issue.  (GPS antenna cables are a different story.)
 
 RG58C/U from Pasternack, for example, has a rated loss of 4.9dB/100ft at
100MHz.  RG400/U is rated at 3.9dB/100 ft.  It takes 3dB of loss to reduce the
signal by half so unless you're planning to run an awful lot of cable the
extra loss of the RG58 is negligible.  However, RG400 is a much higher quality
cable with better shielding and rated for a much wider temperature range; -55
to 200 C vs -40 to 80 for RG58.  RG400 uses flourinated ethylene propylene for
a jacket where RG58 uses PVC.  I've noticed the avionics shops around here use
RG142B instead of RG400.  They're the same physical size with the same
electrical properties but RG142B is slightly less costly.  By means of
comparison the costs from one on-line vendor are:
 
 RG58C/U - $0.23/ft
 RG142B/U - $2.01/ft
 RG400/U - $2.61/ft
 
 So RG58 costs a lot less, but how much are you going to use?  The extra cost
of RG142B or RG400 is nothing compared to the cost of your radios.
 
 Tom Gourley
 Legacy kit #122