[hfsig] soliciting advice regarding 30m HAT

[KB3CS - Chris]

the analysis does help, yes, thank you. looking at the situation from a
maximum power permissible for a spurious output is a useful legerdemain.

my antenna might not be as frequency selective as your model. i'm using a
window line fed doublet here. i suspect it isn't quite resonant on 40m,
either.

i have enough information to dispel most concerns about radiating a gang of
harmonics.  :-D  i was planning an order from QRP-Labs anyway.

tip for everyone: the Raspi supplied (via Element14) power supply (mine is
a "Stontronics" 5.1V 2.5A) puts hum sidebands on the output of the 30m HAT.
You can hear the hum on the transmitted signal. WSPR managed to decode my
signal both at X and at X +/- 60 Hz. My Ravpower 4-port USB phone charger
turns out to be a much better supply - the transmitted signal is, to my ear
(and on the panadaptor display) a pure tone now.

 - 0x49 -

On Mon, Feb 11, 2019 at 8:51 AM Bruce Raymond <bruce at raymondtech.net> wrote:

> Hi Chris,
>
> I picked a 7 pole lpf to ensure I met the FCC/Part 97  -43 dBC
> requirement. According to Elsie 2.72, the lpf response is -46 dBc for the
> second harmonic on 30m. The MOSFET is acting sorta like a switch, so the
> output should be primarily odd order harmonics, although the measurement
> for 40m shows this isn't completely true :-)
>
> I'm also assuming that the antenna isn't as good at radiating a second
> harmonic compared to the fundamental. I made a 40m dipole model and
> analyzed it with 4NEC (I'm new to this process and don't know that I've
> necessarily done it correctly). The return loss is -15 dB at 7 MHz and
> essentially 0 dB at 14 MHz. I'm assuming that this means for my particular
> model the antenna essentially does a 15 dB worse job of radiating the 40m
> second harmonic than it does at the fundamental, so with the existing 30m
> lpf (-23 dB reduction of second harmonic) the actual radiation of the
> second harmonic would be down very roughly -38 dBc (23 + 15). This is still
> greater than -43 dBc requirement, but we're dealing with power levels very
> much less than allowed, so if it's okay to radiate -43 dBc with 1500 watts,
> we're allowed to put out 75 mW. The WSPR transmitter with a 30m lpf is way
> below that even at 40m.
>
> Hope this helps.
>
> 73 Bruce
>
> [...]
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