System perspective from valve plate to antenna feed point


The recent article Mullard QY4-400 (4-400) x 2 Class C AM plate and screen walked through design of a valve PA for a given power supply, power out etc, arriving at the required resonant load impedance.

This article walks through the design of transmitter pi coupler output, and antenna system (feed line + antenna).

Above is the result from Calculate initial load line of valve RF amplifier.

It is popular today to design the transmitter for a nominal 50+j0Ω load, then use an ‘ATU’ to transform the antenna system load to the transmitter, but in earlier days before VSWR meters were so popular, the pi coupler would simply be adjusted to achieve the desired operating point and power output. This article follows the latter path.

Estimate of pi coupler components

We can calculate the values for the pi network using Calculate RF amplifier pi coupler using the design resonant load impedance and nominal external load impedance as a guide to a ‘centre’ design point.

Note that in practice, one would want a range of adjustment available for load flexibility, in fact we will see that a much larger C2 is needed for the example scenario discussed below.

SimNEC model of the transmitter output and antenna system

For the purposes of discussion the following assumptions are made:

  • antenna feed point impedance 25+j25Ω;
  • 75m of LMR600 coax feed line;
  • initial pi coupler values from calculation above, essentially that Qw=12; and
  • resonant plate load impedance 2500Ω.

Low loss coax is selected as it will have standing waves which may exacerbate line loss.

Now it is difficult visually to design on a Smith chart at the extremes of 2500 and 25 ohms, so to ‘stay on the map’, the F1 element in the SimNEC model below divides its load side impedance by 50, so when its load is 2500Ω, G sees 50Ω (centre of the map) and the matching objective is to get that point at chart centre.

Cp represents the minimum plate capacitance achievable, C1 must not be adjusted below zero.

Above is the SimNEC model which is available for download.

The load impedance and coax length was chosen to illustrate the challenge of low R with +ve X loads.

The load presented to the pi coupler is 31+j31Ω, the result of the actual load L and transformation in the coax feed line. It is this impedance that drives the pi coupler settings.

In this instance, the pi coupler settings are C1=420pF, L=20µH, C2=3959pF (quite high).

A loss analysis is interesting, the loss from plate to load is 0.65dB, of which 0.36dB is in the coax feed line, or put another way 93% of the plate power reaches the output jack, 86% of the plate power reaches the feed point.

Dowload the SimNEC model and play with it, you might find it instructive: PiCoupler4-400×2.7z.

You could even extend the model to include an ATU adjacent to the pi coupler, and / or a matching network at the load and discover the difference in net power to the load, including that for some loads and configurations it may be better and for others it might be worse. “Better or worse” might also consider inconvenience of matching networks exposed to the weather for example.