# System perspective from valve plate to antenna feed point

## Introduction

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.