MULTI-BAND ANTENNAS
Many antennas used will resonate on more than one band. This is good, it saves a lot of space and can make the installation easier.
What are some of the techniques for multi-band antennas?
Some of it can be the nature of the antenna. A 1/2 wave dipole will work well on the third harmonic. For example, a dipole cut to
resonate on 7 Mhz can work also on 21 Mhz. Here you have a two band antenna with little effort. Regretfully these are the only two
amateur HF bands that the third harmonic hits. A 3.5 Mhz dipole will come close to 10 Mhz for the 30 meter band if you are
desperate.
How can you get the other bands you want to use?
One way is the Fan dipole. This is an antenna with a dipole for each band. As an example, consider 80, 40 and 20 meters.
The feed point of each dipole is connected together at one point. The center side of the coax is connected to all three dipoles on
one side of the dipole. The shield side of the coax is connected the same way to the other side of the dipole.
The ends of each dipole will fan out or suspend from the dipole of the next lower frequency antenna. Spacing is not critical, but you
would want several inches of spacing.
Now comes your knowledge of resonance. There will be some interaction to the antennas which will make the size of each dipole
a little different than the formulas predict. Therefore, locate the resonant point for each dipole. If you are using a fancy analyzer,
use the SWR mode only at the end of the coax that you plan to use. The length of the coax will have an affect on your results.
Scan the band you are testing and locate the minimum SWR. Then, adjust the length of the dipole to raise or lower that point where
you want it.
Then go to the next antenna. You may have to do this several times to get all the antennas to resonate where you want. With an 80, 40,
20 set up you can have four band capabilities because 15 meters will work also.
What is the principal. When one antenna is resonant with a signal put into it, the other antennas are so far out of whack that they
will look like an open circuit. Your transmitter or receive will mostly see one resonant antenna.
This is how we accomplish multi-bands with the Isotrons and it is a whole lot easier to work on. (Had to put in a plug.)
Working on an antenna like this will give you good experience on locating the resonant points. This will be beneficial with most
antenna work.
73,
Ralph WD0EJA
SEPT 2018