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I made my MB antenna out of 3/4" wide copper foil stuck on the inside of
the tail cone on the right-hand side. It extends from the quarter-panel
window back past the bulkhead in front of the horizontal stab.

Mine is 77" long, center fed with RG-58 coax. Although I have heard it is
not really necessary on a MB antenna, I installed a quarter-wave
bazooka-type balun. This is simply a copper sleeve which fits over the coax
as it leaves the dipole, soldered to the coax shield-braid on the end away
from the antenna. My balun is 26" long or so...

Disclaimer: I have not tested the antenna in any way, since I have no MB
receiver installed at this time. Your mileage may vary...

(The marker beacon carrier is 75Mhz)

>From the ARRL Handbook:

length of a dipole in free space (in inches) = (5902 x K) / frequency (in MHz)

(K depends on the length to diameter ratio of the antenna which in my case
was 100:1. You need some charts to come up with the exact K factor, but a
good rule of thumb is to use K=0.95 to 0.96)

Rule of thumb:
length (in inches) = 5607 / frequency (in MHz), (or about 75" for a MB
antenna)

The reason my balun is shorter than 1/4 wavelength is that you also have to
take into account the "velocity factor" of the coax you use, and shorten
the balun accordingly. The velocity factor for RG-58 is 0.66.

Final note: My MB antenna is slightly longer than the above formulas would
come up with. To tell the truth, I can't remember why I did it that way...I
musta' had a good reason, right? Maybe is was to move the resonant
frequency away from TV channel 5 (76-82 Mhz) or something.