Ramsey Electronics QRP20 - Manual (Page 5)

QRP-20 • 5
If you're studying this manual before deciding to try this Ramsey "QRP"
transmitter project, perhaps the following discussion will be helpful to you. The
purpose of this manual is to help you construct this Ramsey Electronics 4-
stage, variable-frequency CW transmitter efficiently and successfully, not
necessarily to "sell" you on the merits and fun that have been discovered in
ham QRP operation. The purpose of the following discussion of power levels
and signal reports is simply to assure you that your new Ramsey QRP
transmitter is capable of serious, long-distance communication.
1000 WATTS vs 1 WATT: UNDERSTANDING THE DIFFERENCE
Before we move on, I would like to share with you a practical formula about
transmitter power that I clung to in my younger years when I could not afford
anything other than my original Novice CW transmitter, even well after I got my
Extra Class license. First, some theoretical facts we should know:
•
1. An "S-unit" on a receiver's S-meter or in the R-S-T system consists of a
6 - decibel increase or decrease of output power received from a
transmitter.
•
2. Power needs to be increased four times or 400% to result in a true 1 S-
unit or 6db RST gain.
•
3. Reducing output power down to 25% of previous power should result in
an S- Meter or RST drop of only one 6-db unit.
•
4. A 10 - decibel increase in signal strength requires a power increase of
ten times!
Next, there are three practical facts to remember:
•
1. The R-S-T system was designed originally for the human ear and was
based on typical receiver performance of over 50 years ago.
•
2. Modern receiver design permits signals which are technically "weak" in
measured decibels to sound quite good (ie: 569-579) to the ear.
•
3. The human ear is sensitive enough to appreciate a 1 or 2 db change in
signal strength, which is why moderate changes in output power often
result in more dramatic signal report changes. (In fact, the value of a
decibel was determined to be that increment of sound change which
the ear could detect!)
The following example shows RST reports to be expected, in exact theory, at
various power reduction levels. We will start with the classic 1000 watt station
which gets a report of "10 db. over S-9" measured on the receiving station's S-
meter. Assume identical dipole antennas at both stations. Our chart does not