[Elecraft] Harmonics from K3

[Robert Friess]

Hello Poul-Erik,

If your K3 is putting out 100 watts, it is very unlikely that it is
generating the harmonics that you are measuring, especially in the case of
the even harmonics.  Your measured results may be due to some non-linear
element in the near field such as a corroded or loose connection or the
difference in path loss, antenna pattern and gain (loss) of both the loop
and the your short probe on the spectrum analyzer.  Also, it appears that
the balun is a significant contributor as shown by the difference in
measured results when using one balun verses the other.

You can verify that the K3 is operating properly by connecting a suitable
attenuator between the K3 and the spectrum analyzer and making a conducted
harmonic measurement.  If you do this, be careful that the signal level into
the spectrum analyzer is low enough so that harmonics are not generated in
the spectrum analyzer itself.  The harmonics from the K3 will be in the
range of -60dBc if operating normally.

73,
Bob, N6CM

On Thu, Oct 27, 2011 at 4:52 AM, Poul Erik Karlshøj (PKA) <pka at tele.gl>wrote:

> I have been measuring the level of harmonic spurious from my K3 #3674 and
> came up with some interesting results.
>
> I measured with the K3 connected to a dummy load and to my antennas (loop
> for 80-40-30 and 17m dipole for higher bands, both antennas fed by 450 ohm
> feeder and Elecraft 4:1 baluns). Since I have one of the older baluns with
> just one ferrite core and one with two cores, I measured on each antenna
> with each of the baluns.
>
> In all measurements I used the K3 ATU and the SWR was "1 bar segment"
>
> I used a Rohde&Schwarz FSH8 portable Spectrum Analyzer using a short wire
> in the input connector center pin  to pick up the signal from the antennas.
> The analyzer was in my shack, next to the rig. But since I also used the
> pick-up antenna to measure the leaking signal from the dummy load (and the
> harmonics were all very low) the higher level harmonics measured when
> connected to the antenna supposedly did not come from stray signals in the
> shack.
>
> So, what did I see with K3 transmitting 100W?
> 1,8     >38 dB
> 3,5     37
> 7       >55
> 10      43
> 14      40 (2nd harm)
> 18      44
> 21      53
> 24      49
> 28      59
> This seems pretty good to me.
>
> With K3 transmitting 100W and 2-core balun on loop (160-40) and 1-core
> balun on dipole (20-10)
> 1,8     38 dB
> 3,5     40
> 7       55
> 10      28
> 14      16 (2nd harm), 3rd harm at 30 dB
> 18      29
> 21      28
> 24      28
> 28      37
> Especially the 14 MHz value was a surprise, but generally much higher
> harmonics on 30-10m than on the dummy load
>
>
> With K3 transmitting 100W and 1-core balun on loop (160-40) and 2-core
> balun on dipole (20-10)
> 1,8     16 dB
> 3,5     45
> 7       31
> 10      33
> 14      17 (2nd harm)
> 18      35
> 21      45
> 24      36
> 28      42
> The 14 MHz value still pretty bad and the high bands much better now with
> 2-core balun. Low bands some worse and some better with 1-core balun!
>
> Finally - assuming that some of the harmonics might origin from
> non-linearities in the balun core - I measured on 14 MHz with the 2-core
> balun at different power levels:
> 14 MHz, 2-core balun and dipole antenna
> 100W    18 dB
> 80      17
> 60      16
> 40      16
> 20      18
> 10      21
> 5       21
> It seems that the power value has very little influence on the harmonic
> level, so my theory of harmonics being created in the balun may not be
> correct.
> When the ATU shows SWR ~1 does it mean that the current waveform in the PA
> transistors are independent of whether the load is a dummy load (and the ATU
> practically bypasses) or the load is a complex impedance requiring some L
> and C to reach 50 ohms?
>
> Now I have the following questions:
> 1) is my K3#3674 specifically having a problem concerning harmonics on 14
> MHz or would I measure the same on any other K3
> 2) what is the reason that I measure much higher harmonic levels when
> connected to the antenna than on dummy load?
> 3) when the ATU shows SWR ~1 does it mean that the current waveform in the
> PA transistors is independent of whether the load is a dummy load (and the
> ATU practically bypasses) or the load is a complex impedance requiring some
> L and C to reach 50 ohms?
> 4) have I overlooked some fundamental measurement error in using a small
> piece (3 cm) of wire to pick up the RF signals?
>
> 73 de
> Poul-Erik
> OZ4UN
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