Topband: Stub measurement error

[George Cutsogeorge]

Ken,

 

When using the stub on an amplifier, for example, the male part mounts on
the amplifier, the antenna goes to one of the straight sides and the stub
goes on the other.  When testing with the transmission method the connection
is the same.  So when we cut the stub, it comes out exactly where we want
it.  Two legs of the tee are part of the transmission line to the antenna,
so they don't enter into the null frequency of the stub.  Only one half of
the straight side is part of the stub.

 

When testing with the reflection method, you can test either with the tee or
without it.  Without the tee, the stub is too short.  With the tee, two legs
are in series with the stub and the frequency will read too low.  The extra
leg which is unconnected adds a small capacitance which can be neglected at
HF.  To get the correct frequency with the reflection method, the tee must
be characterized by measuring it open and with one side shorted and then
both sides shorted.  Some simple algebra then will determine how far between
the two readings the actual null frequency is.  This is a little too much
trouble for someone that cuts a lot of stubs, but it does give the correct
frequency.

 

George, W2VJN