Now one of the methods that is often used to transform the impedance of an antenna to suit a 50Ω feed line is the shunt match.
Lets explore that with our test jig reconfigured.
Connect up the two line sections in cascade from the analyser, and terminate it with the two 50Ω loads on the tee piece. Don’t worry too much about what we have in terms of implementation, it provides a load to the analyser that presents a similar scenario to shunt matching a loaded short monopole.
So, measure the input impedance around 21MHz.
Above is a scan with the Rigexpert AA-600 from around 21MHz. Ignore the |Z| line, it is irrelevant and confusing but I cannot switch it off, a shortcoming of the software.
What we are exploring is that as we change frequency, the parallel equivalent resistance changes at 21.275MHz above, it equals 50Ω. The full parallel equivalent is 50Ω//-j77.3. So, if we were to make a small inductor of 77.3Ω reactance (L=X/(2*pi*f)=580nH) and connect it in shunt, the resulting impedance will be 50+j0Ω.
I have written this in terms of parallel equivalent circuit as ham instruments in catering for the audience, tend to not provide admittance display but some provide a parallel equivalent impedance. If you were working with admittance, the target for shunt matching is Y=1/Zo+jBS.
In applying this to a real antenna, you would usually measure at the feed point and:
- adjust the antenna length or the loading coil to obtain Zparalell=50.0//jXΩ at the desired operating frequency; and
- note the residual parallel reactance and calculate an inductor or capacitor of opposite sign, equal magnitude reactance to shunt the measurement point (assuming a low loss shunt element);
- connect the shunt element and adjust it to obtain Z=50+j0Ω.
It is very important that you get the sign of reactance correct at step 2.
Achieving this is easier if your analyser presents the admittance or parallel equivalent impedance on screen.
Above is the display screen on the AA-600. The values are a little different to the graph, a result of moving the test jig for the pic. The target for shunt matching is to adjust the antenna until RII=50.0
If you wanted to explore the final step, you could wind a small air space solenoid coil of 6 turns on 20mm diameter, stretched out to 10-20mm length, and connected in shunt at the instrument terminals (eg using one of the tee pieces). Adjust the inductance be squeezing / stretching the coil for a ‘perfect match’.
Did you try that?
Above is the VSWR plot of the shunt matched system.
Watch the blog for continuing postings in the series Exploiting your antenna analyser. See also Exploiting your antenna analyser – contents.