[Elecraft] [OT]: Antenna Tuners - one more reason

[Tom W8JI]

Using a tuner to get rid of BCB problems often works quite 
well. That's a good suggestion because most of the time it 
works, although there are cases where it might not work as 
well or can even make things worse.

The typical T-network tuner is a highpass configuration  and 
will generally reject lower frequency signals at a more 
rapidly increasing attenuation than above the tuner 
frequency. Most link coupled tuners are actually a 
"bandpass" circuit, not a highpass.

In all cases we should remember the exact suppression 
characteristics depend on the impedances presented by the 
radio and the antenna system at the undesired frequency to 
the network. There are cases where a tuner (of nearly any 
type) can actually increase the level of an undesired 
out-of-band signal, even though that same tuner looks good 
into a 50-ohm resistor from a 50-ohm source.

It is the characteristics of the antenna, tuner, and radio 
as a combination of impedances at the **unwanted** signal 
frequency that determine the real attenuation.....not how it 
acts into 50-ohm loads from infinite bandwidth 50-ohm 
sources found in models.

<<Anyway, the advantage I'm talking about is that when 
properly adjusted, a link-coupled tuner acts as a high-pass 
filter. This means that it provides additional rejection of 
AM broadcast stations in the 530 - 1710 kHz band. This is 
really important to many hams, particularly when they live 
close to one of these high-powered transmitters which are 
always on and have plenty of potential for causing problems 
in the receiver.>>