[Antennas] How do you use an antenna tuner?

2002年3月10日 20:31:41 -0600

Yup, that is a good 2 cents worth and probably worth much
more. Max current is an indication that power is being
transferred to the antenna.
The thing that most amateurs don't understand is that in
order to make an antenna work well ( any antenna ), you have
to get power into it. That only happens when the
transmitter AND the antenna is matched to the transmission
line.
That is pretty easy to do if you use coax. Simply connect a
50 ohm coaxial feedline to the transmitter, then connect the
other end of the feedline to an antenna that matches to 50
ohms. No need for a tuner here. In fact, a tuner in this
situation will not help and may degrade performance.
Most novices start this way and have pretty good luck with
simple, single band dipoles flat-topped at about half
wavelength above ground.
The trouble begins when someone suggests using an antenna
tuner to turn the single band dipole into a multiband
system. The unsuspecting usually get a cheap transmatch or
'line-flattener' and install it between the radio and the
coax going to the antenna. Then they wonder why it does not
work so well on all the other bands even though the swr is
very low at the radio. They have a hard time understanding
that their troubles are caused by the impedance mismatch at
the point where the antenna attaches to the feedline. 
Antenna current meters don't help the mismatch and the
transmatch at the radio does not help it either. Such a
setup guarantees that the radio will deliver a maximum
amount of power to the feedline but most of that power never
gets to the antenna, it just heats the feedline because of
the mismatch at the feedline to the antenna. 
One solution is to install a remote tuner to match the
feedline to the antenna. Another solution is run open wire
line to a balanced tuner that will tune the feeders AND
antenna as one balanced system. The third and best solution
is to stay with the original single band system that works.
Putting up a second single band system to cover an
additional band is usually lots cheaper than getting an
antenna tuner.
Lastly, consider this.....even with a decent all band
system, you can only work one band at a time. With seperate
single band antenna systems it is possible to work more than
one band at a time if you have the radios to do so. Works
well for contesting.
Frank Kamp
K5DKZ
Pat W wrote:
>> My 2 cents worth.
> Tuners can tune things they really have no business
> tuning in the first place, and since a t-net tuner has
> infinite settings for a match(only 1 being the correct
> setting), it is difficult to know when the tuner is
> doing any good.
> THe simplest method for tuning(from Walt Maxwell's
> 'Reflections') is to add a current sensing indicator on
> the ouput side of the tuner. This is a very simple
> device consisting of small powdered iron toroid(eg,
> T50-2), a germanium diode, capacitor, 50k ohm carbon
> potentiometer, and a good meter. The antenna center
> output is wound 1 turn thru the toroid, and about 20
> turns of small diameter wire is wound for the
> secondary. Tuning for maximum output on the meter is
> all that is required. WHen the tuner settings are at
> their optimum, the meter is showing the most antenna
> current.
>> Without an output indicator, tuning in receive mode
> for maximum received signal will indicate optimum
> setting also.
>> Pat W0OPW
>> __________________________________________________
> Do You Yahoo!?
> Try FREE Yahoo! Mail - the world's greatest free email!
> http://mail.yahoo.com/
> - - -
>> Your moderator for this list is:
> Larry Wilson KE1HZ [email protected]
> _______________________________________________
> Antennas mailing list
> [email protected]
> http://mailman.qth.net/mailman/listinfo/antennas