Shunt matching a loaded HF whip with just a VSWR meter gave a direct answer and supporting explanation to an online poster’s question about optimising an 80m loaded mobile vertical with shunt matching, specifically the inductor needed and an adjustment procedure.
The original poster clearly had the impression that this improvement of the original VSWR=1.3 would make a large difference.
The only other option for me is to remove the shunt and set my swr back to 1.3:1 and not be able to communicate.
Power delivered to the antenna
It is difficult to predict the difference in power delivered to to the antenna when perfectly matched (including the effect of matching loss) and that delivered to the antenna as is.
Before you jump to the conclusion that the classic formula for MismatchLoss applies, it applies only where the source is well represented as a Thevenin equivalent linear circuit, and typical ham transmitters for 80m are not well represented by that model, so the model is not applicable.
The difference can be measured in the actual implementation by using a directional wattmeter calibrate for any real Zo to measure:
- the forward and reverse power to the poorly matched antenna and finding the difference which is the power delivered to the antenna system (including matching network); and
- the power delivered to a matched load.
The basis of this is sound, see Power in a mismatched transmission line for proof.
It is worth noting that a practical transceiver specified to operate into a nominal load of 50+j0Ω may well deliver maximum power into a slightly different load.
Other benefits of load within specification
Transmitters are designed to deliver a given output power into a nominal load with some limits on distortion, heating, operation of active devices within ratings etc.
Whilst it is unwise to operate a transmitter into a load outside of its ratings, the original poster’s case of VSWR=1.3 would be within the ratings of most practical transmitters and the transmitter is likely to work substantially as specified.
Disadvantages of this matching arrangement
The shunt matching solution does improve VSWR, but it also has narrower bandwidth than the ‘unmatched’ vertical which is already much narrower on 80m than is convenient to most operation.
A broadband autotransformer would be a better solution, albeit probably a little less efficient and potentially less efficient than the ‘unmatched’ antenna.
Expected performance of a practical loaded mobile whip on 80m
A practical loaded mobile whip on 80m is likely to have a radiation efficiency in the region of a tenth of that of a good half wave dipole antenna system, so expectation of results needs to be tempered by that.
An observation: VSWR=1.3 is a little lower than would commonly be the case for this type of antenna, and might well indicate a problem.
- Any degradation of power output into the original poster’s VSWR=1.3 antenna is likely to be very small and insignificant, the transmitter is likely to be operating substantially as specified in every respect.
- The shunt match narrows the VSWR bandwidth of in this case an already narrow antenna.
- Other matching arrangements may have advantages, but also potentially less efficient than the ‘unmatched’ antenna which in this case is not a particularly poor match.