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

Adrian Fewster

Yes, but the 20% loss would not be detected by the rx station, so not worth the bother.


Rich K3VAT

Thanks for the post.
The issue here is calibration - an accurate (true) measurement of the parameters of interest.
Neither the FTDX101 nor the MNFJ 989D have been calibrated for your experiment and without calibrated equipments, then the math is really mote.
GL 73 Rich, K3VAT