For most hams the antennas they deploy are constranded by budget, minimizing
complaints from neighbors and XYL, and the physical space they have to work
with.
Multiband antennas commonly fit into this scenario. Since the physical
constraints of the antenna will determine the pattern of radiation, the
amateur is pleased to take what he can get. Sometimes one has a poor signal
in one direction due to this, but that is the way the cookie crumbles, and
better to make contacts in other directions than not to make contacts.
The key here is to minimize the amount of power lost in inefficiencies. Power
losses in antenna systems come from feedline loss, balun loss, tuner loss,
and ground loss. It is easy to burn up 3 to 6 dB in losses, sometimes a lot
more.
Often convenience takes precedence over efficiency. We trade efficiency for
the easy routing of coax over parallel feedline, we trade efficiency for
automatic tuning of our antenna systems. That's okay as long as we understand
the tradeoffs involved.
Baluns are designed to work into resistive loads at their design impedance.
They are often used in multiband antennas with huge variations in resistance
and reactance. The reason they seem to help with antenna tuner matching is
because at these problematic frequencies, their losses are higher and
therefore the tuner can now handle the reduced reactances. Like adding a
resistor across the antenna.
The common multiband wire antenna, whether end fed, center fed, or offset
feed, as high and in the clear as possible, over a system of radials, fed
with parallel feedline, and tuned to match the transmitter with a true link
coupled balanced tuner, will minimize the losses, and in conjunction with
tuning the length of the transmission line, reduce common mode effects such
as RF in the shack.
The problem is many hams don't want to go to this much work. Thats fine, just
understand the compromises you have made. You will still work stations, and
maybe even WAS and DXCC. 100 watts to an antenna system with 13 dB of loss
still will radiate 5 watts and QRPers work the world with that. You can
always add more power to overcome the losses, just be sure not to explode any
baluns where flying parts could injure anyone. :-)
Any antenna is still better than no antenna!
73,
Darrell
VA7TO
--
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
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