[TowerTalk] Antenna Tuner

[Roger (K8RI) on TT]

On 5/6/2014 1:33 PM, Paul Christensen wrote:
>> "But the antenna tuner does not change the feedpoint impedance of the
>> antenna which in my view would be actually "tuning" the antenna."
>
It does however change what the rig sees as the antenna impedance. (R, 
L, and C). So whether you change the antenna height or length, or stick 
a "tuner" in the line the results are the same, so call it what you 
want, The results are the same.

An antenna does not have to be resonant to radiate otherwise 5/8th 
radiators would not work. It's a radiator and will eventually radiate 
all the power getting to it although some may make the trip up and down 
the feedline a number of times.

With the tuner at the antenna (matching network) the loss is less 
because the trips up and down the coax are fewer.
With low loss coax, the loss is less because there is less loss per trip.

Where ever you put the matching network, antenna tuner, line tuner or 
what ever you want to call it don't forget there is loss in that network 
and for low impedance antennas the loss may be considerable.  A matching 
network, a 4:1 balun, what ever, the low impedance antenna will lose 
power in the matching network.

Put a 160 dipole at 30  or 40 feet for 20 or 30 ohms R and that coil in 
the network can get mighty hot and that is all lost power. You can lose 
a substantial amount of the power going to the antenna in the matching 
network.  That is why, all matching networks are limited in power at low 
impedances. Look at the limits for the (legal limit MFJ 989 series) They 
will not handle the legal limit at low impedances.  You, You may be able 
to match it with the legal limit, but thy get noisy in a hurry because 
that rotary inductor gets hot and will have a short life!

Look up the power limits for high to low impedances for any tuner. Even 
my AT5K tuner is substantially limited at low impedances although it'll 
still run the legal limit.


73

Roger (K8RI)