[Elecraft] Harmonics from K3

[Jack Smith]

Poul-Erik:

You have an implicit assumption that the spectrum analyzer to transmit 
antenna has a flat response with respect to frequency.

That's unlikely to be the case.

Another assumption is that the spectrum analyzer antenna is in the "far 
field" of the antenna. This is also unlikely to be the case.

And another is that the K3 itself does not radiate due to imperfect 
shielding which may be a problem when the pickup antenna is close to the K3.

The normal method to measure harmonic suppression is to use either a 
power attenuator, such as 30 dB attenuation rated at 100 watts, so that 
the spectrum analyzer only detects signals from the transmitter. Or, if 
a suitable power attenuator is not available, a standard dummy load and 
a resistive "tap" or a directional coupler of known frequency 
characteristics can be used to obtain a low level signal sample that 
does not vary with frequency.

With respect to measuring radiated harmonics, the most accurate method 
is to locate the measuring equipment sufficiently far from the transmit 
antenna such that the signal is far field. 10x the transmitting 
wavelength is a safe number, but a somewhat shorter distance may be OK.  
The spectrum analyzer or field strength meter uses an antenna with a 
known antenna factor at the fundamental frequency and all the harmonics 
so that the actual radiated field strength can be computed and from that 
the harmonic level. Since the transmitting antenna almost certainly has 
directional characteristics -- particularly at harmonic frequencies -- 
several sets of measurements should be made at varying azimuths from the 
transmitting antenna.

So, to make these measurements accurately in the case of radiated 
harmonics is a non-trivial undertaking.

Jack K8ZOA


On 10/27/2011 7:52 AM, Poul Erik Karlshøj (PKA) 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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