[hfsig] 20m WSPR shield for Raspberry Pi

Keith Wilson keith.wilson.pcs at gmail.com
Sat Mar 23 14:02:23 EDT 2019

Hi Bruce, and thanks for the generous response!

My power measurement was made with a good 50 ohm load so maybe I have a
nice hot MOSFET!

As mentioned in my previous email, the testing was with a USB power bank
battery, and I still have the 120 Hz apparent mixing products.  I tried a
separate antenna (my SOTA antenna) so I was not attached to the station
ground.  (My station ground is bonded to the household power ground) So I
shouldn't have any way for 60 Hz power to get into the Pi or USB battery.
Still puzzled by the presence of these apparent mixing products.  Because
they are down 30 dB, I probably won't get double decodes by receiving

Last question, TAPR is out of the 30 m boards.  Are they still available?

Keith - KE4TH

On Fri, Mar 22, 2019 at 9:14 PM Bruce Raymond <bruce at raymondtech.net> wrote:

> Hi Keith,
> I have to agree with Bryan (well, I suppose I don't *really* have to agree
> with him, I just want to :-). The 20m transmitter is Zoltan's design, but
> very similar to my 30m transmitter. The final is an MMBF170 powered by 5
> volts.
> 1. The power output is approximated by the formula   P = V^2/2*RL. The
> power supply is roughly 5 volts and assuming a 50 ohm load (RL), P = 5^2/(2
> * 50) = 0.25 watts.
> It's reasonable to expect some losses and the safest way to list the
> output power is to say you'll get at least 200 mW. Also, if the supply
> voltage is higher than 5 volts then you'll get more power. It's unlikely
> that it would be *that* much higher; it would take 6  volts to give 360 mW.
> Another possibility is that your antenna impedance is less than 50 ohms. If
> your antenna impedance is, say, 35 ohms, then P = 5^2/(2 * 35) = 360 mW.
> The last (and most probable) thought is that the MOSFET in your transmitter
> is hotter than typical and gets driven harder, producing more output. I've
> played with this on the 30m transmitter and have gotten power outputs in
> this range by biasing the MOSFET on more. The threshold voltage for a
> MMBF170 MOSFET is between 0.8 and 3.0 volts with 2.1 volts being a typical
> value. The 20m transmitter has a voltage divider putting 2.3 volts on the
> gate. If your MOSFET is fairly hot then it would be biased on more and
> likely put out more output. The end result is *yes* the output is real. =>
> Watch for the MOSFET getting hot. If it does, you might want to add a heat
> sink or change the gate bias resistor (R2) from 1.2K to something larger,
> say 1.5K. <=
> 2. Power supplies - in the words of Socrates, suffering an learning are
> two names for the same experience (I don't know that Socrates actually said
> that, but I like to say he did). I have learned through hard experience
> that inadequate power supplies cause a whole bunch of problems, and they're
> usually very difficult to troubleshoot because the problems are either
> intermittent or just not something I'd normally suspect of a power supply.
> The power supplies normally used for the Raspberry Pi are usually
> marginally adequate. I'm very impressed with the job the designers of the
> Pi did, but they cheaped out on the power supply filter on the board
> (electrolytic capacitor).
> Now we compound the problem with trying to run a transmitter off of the
> same power supply in addition to running the Pi. This doesn't help things.
> In the beginning I bought a bunch of cheap 5V/2A power supplies from China
> that worked with my Pi/30m transmitters. I had a bunch of weird problems,
> such as the software getting corrupted during normal operation. At first I
> thought the problem was cheap SD cards or some problem with the operating
> system/software. I now believe the problem was power supply glitches
> causing the Pi to get confused and do bad stuff. I switched to bigger power
> supplies and my problems disappeared.
> My recommendation is to get a 5V/3A power supply and make your
> measurements again. 120 Hz sidebands sounds like AC bleeding through the
> power supply, even if it seems that the power coming off the supply is
> clean. It could also be some sort of interaction between your antenna
> ground and your power supply ground. You might try a different power supply
> and/or an isolation transformer for a test. This might be similar to hum
> problems direct conversion receivers have that are associated with
> grounding.
> 73 Bruce Raymond/ND8I
> Bryan Corkran wrote on 3/22/2019 4:22 PM:
> I had a lot of trouble with power, in the end I bought the “official” 2.5
> amp adapter and had no trouble after that.
> Keith is right the shield is designed for the V1 board hence the little
> slot in the middle for the display port. I had problems with the shield
> fouling on the heat sink I’d added on a 3b board so I used a GPIO extender
> to raise it a small amount.
> Bryan, VK3KEZ
> On 23 Mar 2019, at 5:36 am, Keith Wilson <keith.wilson.pcs at gmail.com>
> wrote:
> I have the 20m WSPR shield working on a new Raspberry Pi 3 B+.  I see
> apparent mixing products in the output, 120 Hz away from fundamental, when
> using a USB power bank to power the Pi.  Since these are not coming from a
> switching power supply, where are they coming from?  These products start
> at about 30 dB below the fundamental.
> Also, with a scope I measure the voltage output at 12V peak to peak into
> quality 50 ohm dummy load.  This is 0.36W, higher than the 20dBm (0.10W)
> specified.  Is this too good to be true?
> Note the shield was not designed for the Raspberry Pi 3 B+ so it can't be
> fully inserted on the 40 pin GPIO plug, but seems stable enough partially
> inserted.  Getting WSPR reports from across the USA and occasional overseas
> reports too.
> Keith - KE4TH
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