I went to the Radio shack site and looked up RG8 (16 AWG) and got these
numbers and calculated the loss for the rounded distance numbers:

Freq loss per foot @ 10 foot @ 30 foot
100 MHz .037 dB .37 1.11
400 MHz .080 dB .8 2.4

3 dB is half your power so over 400 MHz your higher antenna better pick
up twice the signal to make the longer run worth it. The cable loss is
fairly linier so you could plot the two points in excel sheet and figure
the loss for any frequency around these points.

In radio work 3 db is not really that much for most applications. It can be
going from 1 watt to 2 watts or 500 watts to 1000 watts. Also you will not
be getting 3 db differance as to go the first 10 feet you loose .8 db, to go
20 feet more (the first 10 plus the next 20) it is only 1.6 db differance.