Here's a quick line loss calculator to use  Note that the simple program used for this web page gives a very close approximation for additional losses due to SWR. 
Set Parameters as Desired
Line Type:
Line Length: Feet Meters
Frequency:  MHz
Load SWR:  : 1
Power In:  W
Results
Matched Loss:  dB
SWR Loss:  dB
Total Loss:  dB
Power Out:  W

Coax Cable Loss / Antenna Gain Calculator

Enter dB Loss Of Cable Per 100 Ft. At The Desired Operating Frequency

Enter Length of Cable in Feet

Enter Power into Cable in Watts

Enter Gain of Antenna in dBd

This program is provided "as-is". It is thought to be accurate but it is the responsibility of the user to verify the accuracy of the calculations when using this program.  

Formulas To Design Your Own Dipole Antenna

INTRODUCTION-The longwire antenna is a very effective antenna for the listener who wants to cover all of the shortwave bands from 530 KHZ to 30 MHZ. However if you have some favourite frequencies that you listen to on a regular basis you may wish to consider a dipole antenna. This antenna is a fairly easy to construct antenna and will give you better reception on the frequency it is cut for. Think of a dipole as a longwire that has a insulator in the middle.

FREQUENCY-A dipole antenna will not only work well on the frequency it is cut for, but also for the multiples of that frequency. For example if you cut a dipole for 7.0 Mhz it will also work well on 14 Mhz, 21 Mhz and 28 Mhz. This way if you can pick and choose your frequency you can make one antenna work on two or three bands.

To find out how long the antenna should be all you have to do is fill in a simple formula:

 234 / design frequency in MHz = each dipole side length in feet.

 OR

 468 / design frequency in MHz = total length in feet.

Most builders cut long and trim the total length for lowest swr on their design frequency.
That is the only formula you need ever know to build a dipole antenna.