At AIM4170 – de-embedding the feed line in remote measurement a set of measurements of a monoband antenna looking from the transmitter were analysed to de-embed the feed line and arrive at the indicated feed point impedance.
This article explores a simple series match to improve the load seen by the transmitter.
In the Simsmith model above, the estimated feed point impedance is imported into element L, then a series section of lossless 50Ω line to represent the coax in the common mode choke (balun), then a series section of lossless 75Ω to perform the impedance transformation, then a section of 50Ω lossless line to make up the required length to the transmitter.
In this case, the first step was to find the frequency at which the VSWR(75) looking into the balun was 1.5.
Above, Simsmith is setup to display VSWR(75) at element G and T2 &T3 are made zero length so we are observing VSWR(75) looking into the balun (T1).
Above, element G is set to measure VSWR(50) and length of T2 adjusted for VSWR=1.
Above, the last step is to then build out T3 to the required length.
Note that the example here is used as a vehicle to explain a matching scheme, and the actual line lengths relate to that specific scenario and are not simply extensible to a different antenna. A solution for another antenna starts with measurement of that antenna.
I have modelled all these as lossless sections as I have issues with the loss models in Simsmith. For example, the use of RG6 CCS cable might not reconcile well with Simsmith’s loss model (RG6/U with CCS centre conductor at HF). Nevertheless, the lossless models explain the concepts.
In practice, rather than treating the dipole as a fixed length, you would adjust its length to achieve VSWR(75) looking into the balun at the desired operating frequency. Then you would insert sufficient 75Ω line to measure VSWR(50)=1 looking into that line section. Then connect sufficient 50Ω coax to reach the transmitter.
The astute reader will realise that there are longer length of the 75Ω line that will work, indeed one of these longer length might be conveniently used and T3 dispensed with (ie the 75Ω line terminated at the transmitter jack).