Insertion loss of amps and scopes

Barry D. Jacobson

I normally use my FTDX101D with a Heathkit SB220 amp. I also have two scopes, a Kenwood SM220 and a Heathkit SB610. Because Field Day rules say a 1D station cannot use more than 150 watts, I shut off the amp. I turned up power on rig all the way to 100 watts, while I normally drive amp with 50. My wattmeter on MFJ 989D tuner was showing between 80 and 85 watts on 20 and 40 meters. I was very puzzled and thought I had a malfunction in rig or some kind of weird foldback even with a dummy load. But I looked up manual, and Kenwood scope specs say insertion loss can be up to 0.9 dB (mainly on VHF bands}. Heath manual did not specify. Amp may also consume something even when off. I decided to bypass the amp and scopes with a jumper coax directly to tuner. Power went back up to exactly 100 watts on tuner. The lesson is that if you do the math, a 1 dB insertion loss can eat up 20% or more of your power. Be careful what you put inline.

Barry Jacobson

Join to automatically receive all group messages.