[Elecraft] Using the Antenna Tuner in the XPA100

[Fred Jensen]

Well, not really, Mike.  The magnitude of the impedance at the end of a 
wire can vary from low to very high [several thousand ohms] depending on 
its electrical length.  As David pointed out, the general Elecraft ATU 
design is an L-network using relays to combine various fixed values of 
inductance and capacitance.  I believe my KX1 had 64 possible 
combinations, the KAT2 has more, and the KAT3 has even more I think.  
While the ATU may not be able to directly match extremely high or low 
impedances, a simple transformer will usually get the value into a range 
that can be matched.  My end-fed WOOF has a transformer at the feed 
point ... don't know if it's 4:1 [16:1 Z ratio], or 9:1 [81:1 Z ratio], 
but the KAT3 and KAT2 both find matches in the 1.0:1 - 1.3:1 range on 
all bands.  It is located on the fence, about 10 ft from the shack 
entrance weatherhead, but is fed through 100 ft of RG-8 right now ... 
long story involving my wife. 😉  While I can get a match on 6 m and 
occasionally hear signals, no one has ever answered my calls.  10 - 160 
work fine.

I don't know what algorithm Elecraft uses to search for a match, it's 
probably proprietary, but I'd guess its based on some variant of a 
steepest descent design.  Before microprocessors, some hams used an 
analog ATU design, often in mobile installations.  A coupler sensed both 
the voltage and current in the feedline at the antenna and a phase 
comparator operated a motor driving a variable capacitor in an L-network 
to the zero phase difference point.  Small loops often use this design 
today.

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County


On 02/07/2022 23:13, Mister Mike wrote:

"The “automatic” antenna tuner is a completely new concept for me.  They 
appear to be designed to tune unbalanced coaxial feedlines and not an 
end-fed wire."




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