[TowerTalk] Tuners

[Jim Lux]

From: "jerryc" <jerryc at clinchrivercorp.com>
>  Bill
>  If the swr meter is between the antenna and the tuner then you are not
>  reading the  swr the radio or amplifier sees.  The proper place for the
> swr meter is  after the amplifier and before the tuner.  Tune the tuner
> with the amplifier in standby and then  bring the amp online.
> The tuner isn't really to fool the radio but to act as an impedence
matcher
> to transform the impedence from the antenna (whatever the value is) to the
> 50 ohms our radios and amplifiers need to safetly produce full power.
> Hope this makes sence.
> 73s
> JerryC
> KC8TES
> > ----- Original Message -----
> > From: "Bill Hawkins" <w5ec at bellsouth.net>


As Jerry says, there's no fooling going on.  The tuner is an impedance
matching device.  If the antenna is non-reactive (aka resonant), and the
impedance isn't 50ohms, the tuner transforms what ever it is to 50ohms (much
like a 4:1 or 9:1 transformer does, just over a narrower band of
frequencies).  If the antenna is reactive then the tuner supplies reactive
power to cancel the reactive component (and probably also does an impedance
transformation too).

If your installation is:

Amp > Tuner > SWR meter> 50 ohm transmission line > Antenna

your meter is measuring the reactive power that is expected to flow back and
forth in the transmission line.  There's also the issue of whether the
calibration of the meter is correct when it's not terminated into 50 ohms on
both ports (that is, if it's measuring current or voltage, rather than
actual power, there's an assumption of a particular characteristic
impedance... )

The ideal situation is:

Amp> 50 ohm transmission line > tuner>antenna.

This puts the tuning at the antenna where it should be, and gets rid of all
those issues about loss in mismatched lines, etc.

Al Lorona, W6WQC, has a great explanation of tuners, meters, etc. starting
at:
http://www.west.net/~jax/archive/basic01.html
(or http://www.west.net/~jax/archive/antmenu.html )


Jim, W6RMK