[Amps] KPA1500 Efficiency vs. RF-Kit B26

[Jim Brown]

On 7/11/2021 9:25 PM, Adrian wrote:
> I would think the better IMD3 performance would be at the higher voltage 
> in the SPE, at the expense of efficiency.

I happened to have measured the bandwidth of both a KPA1500 and an SPE 
1K on the air transmitting FT8 on 6M, using my K3/P3 with the added SVGA 
output that yields additional frequency resolution. The KPA1500 at 500W 
is substantially cleaner than the SPE amp running 300W at its highest 
voltage setting. It was 6 dB worse at the low voltage setting. Both 
stations were within about 13 miles of me. The measurements were 
motivated because a mutual friend running that SPE amp was getting 
complaints, so N5KO paid him a visit to help diagnose the problem, and I 
looked at the signal as they made changes. One of the first things we 
checked was to make certain that AGC was not being used to set TX power. 
It was not.

Observations are based on the strength and nature of the sidebands. 
Sidebands from the SPE were essentially broadband noise extending on 
both sides of the signal. Sidebands of the KPA1500 were at discrete 
frequencies on both sides of the carrier. I didn't make written notes, 
so I'm going from memory, may spaced 150 Hz. The two stations were N5KO 
and W6RN.

Neighbor K6XX and I both run a KPA1500 in one of our SO2R contesting 
setups, and I run an 87A in the other. We're about 5 miles apart, both 
running K3s with the latest synth boards. These stations are clean 
enough that on CW, we can work 500 Hz apart, one running, the other S&P, 
and hear the other as nothing more than another strong signal. Bob is 50 
dB down 235 Hz from his CW signal, 60 dB down at 335 Hz, 70 dB down at 
540 Hz.

Bob, K6XX, is a manufacturing engineer at Elecraft, and about eight 
years ago, we gave a joint presentation of transmitter cleanliness to 
our large contesting club. It's here.

http://k9yc.com/K6XXAmpTalk.pdf

Here's some work I did using one of my contesting radio's P3/SVGA to 
look at the other, and to other rigs loaned for testing.

http://k9yc.com/P3_Spectrum_Measurements.pdf

It's important to understand the power amps are a small fraction of the 
problem. Some rigs are MUCH dirtier than others. Here's a report I 
prepared several years ago that analyzed and compared ARRL Lab test data 
on a selected group of rigs then being sold. ARRL Labs sent me data for 
as many of their measurements as were available in a common form. I 
plugged them into a spreadsheet, and plotted data for all of the rigs on 
the same axes and scale, so that they could be clearly compared. Part of 
the motivation was issues raised about dirty rigs in the WRTC 
competition. That report is here.
http://k9yc.com/TXNoise.pdf

73, Jim K9YC