Monday, April 3, 2023

Damage to receiver with CW Flea, amplified?

Jo F5NFB recently asked if there might be any harm to his receiver by using a 5-watt amplifier with his CW Flea transmitter.

The CW Flea is a 3-band CW QRPp transmitter for 20, 30 and 40 meters. It has onboard RF connections to a user's receiver-of-choice and to the antenna which is common to both the Flea and the receiver with T/R switching performed by the Flea.

Since any outboard RF amp would be downstream of both the transmitter and the receiver, I didn't see any way that a 5-watt amp might be problematic but I suggested that Jo confirm this with the CW Flea's designer, Harry SM7PNV.

In the meantime, this question gave me an excuse to measure any "leakage" output of the CW Flea that connects to the receiver.

Not shown in the photos is my step attenuator, placed in series with the Flea's "RX" connection and the tinySA used to measure the RF at this output when transmitting. I initially placed 30dB of attenuation at the input to the tinySA - just in case.

This turned out not to be necessary.

RF output (on 20m) going from the CW Flea to the connected receiver is -34.1 dBm. This is S9 + 40dB - a strong signal but not a harmful one.

Barefoot, the CW Flea produces a -34.1 dBm signal to the receiver

Next, I hooked up my Hardrock-50 to the CW Flea.

The Flea's 200mW output drove it to 9 watts on 20 meters. Measurement on the input to a companion receiver was -35.4 dBm.

That signal increases slightly to -35.4 dBM when the Flea is amplified to 9 watts

I then replaced the tinySA with my Belka-DX receiver and used the Flea-Hardrock-Belka combo to have a QSO with N4JAW who was activating a park in Kentucky.

Summary: I see no problem to any component in using a small 5-10 watt amplifier with the CW Flea.



1 comment:

  1. The RX output is actually grounded by a relay while in TX. It's just picking up ambient RF. DE Kevin, KB9RLW