The frequency assignment is determined by the lookup table further below. It is set as multiples of 9Khz spacing,
from 531Khz to 1638KHz.
The PA output of the transmitter is very rugged, utilising 2 high power MOSFETS which are therefore significantly de-
rated. The output power is maximised by adjusting the output capacitors via the set of links. A table shows the
settings for maximum matching/output. There are adjustments on the matching coils for low band operation,
together with adjustments to various capacitances. The design uses air-spaced output coils for the matching, as their
inductance can be altered marginally by means of squeezing the turns together or spreading apart to maximise
output power.
A link is used on L2 which is used for band matching (between high and low frequencies) as shown in the table (LK2)
by linking either left or right. LK1 is the output capacitor and links are made as per table 1 to maximise output level.
The output is designed to match a 50-ohm load. Therefore, a suitable aerial arrangement is required to match your
aerial for 50 ohms.
It is expected that the output power across the Medium Waveband will vary due to the matching arrangement
design compromises, but broadly speaking the output averages around 20 watts RMS into 50 ohms, that’s a healthy
80 Watts PEP.
Table 1. Tuning adjustments and setup.
NB. The frequencies used in the above table are an example of settings required for frequencies of nearby
frequencies. Although a certain amount of experimentation might improve output level.
SETUP and Tuning
Method: Select quiet AM Channel, if aerial is short (i.e. below 25M) best to use top end of AM band. Pre-set links as
in the above table and switch on transmitter. It is very advisable to use a SWR meter between TX output and ATU
input. Aim for lowest SWR but maximum RF out. A field Strength meter is also invaluable in order to realise the
signal strength emitted from the aerial. For stability, keep the AM20 well away from the ATU and high voltage RF
line.
Set up the ATU with your aerial tester for as near as 50 ohms and zero reactance. Set to ‘LO’ power on rear. (this
limits output to roughly 10 Watts). Switch Transmitter on and observe SWR for minimum. If happy, switch to ‘HI’
power and again check SWR/Power output. Apply Audio signal and check all is good!
