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This month I arrived at the QRPcc website by accident.
FYM and QRP?? Well, QRP is not my main thing but this QRP club
organizes contests on Ascension Day since 2000.
Charm and challenge of this contest is to use self built QRP equipment with
minimal components. The less components you have, the more points per QSO you get.
In other words, less is more ; -)
Because I attend the Jutberg, enjoy home brewing and contesting this
MAS contest seems a nice exercise at the end of this years Ascension Day.
Plan is to participate in ‘Class B’, only 40m with a FCP vertical,
using N1MM with my Arduino keyer.
Note: nowhere is stated that ‘QRP’ has to be done with a straight key!
Here you can see some pictures and circuit diagrams of equipment from
participants over the last years.
The MAS contest seems ‘hauptsächlich eine Deutsche Sache’.
So, my aim is to change this a little bit ; -)
Googling on various circuit diagrams for QRP equipment I felt that my entry
had to deliver something new. So . . . I started drawing some circuit diagrams
of minimalistic 40m transmitters -made of junk parts- in my mind.
I excluded tubes, so my 40m transmitter had to be made of transistors and/or FETs.
A Hartley oscillator is one of the most minimalistic designs, it reduces to two components.
I built a Hartley with a small FET. It works, but the frequency stability is horrible.
Peter PA3EXL was so kind to lend me a 7030 kHz crystal, which I used to discipline the Hartley.
My initial idea was to build a Hartley power oscillator with an IRF510 or something.
However, these FETs need biasing to oscillate, which means extra components.
I considered a Pierce oscillator as alternative. Also in this case the IRF needs biasing.
So, I went back to my original idea: crystal disciplined Hartley with amplifier.
After some fiddling, trying to minimize the amount of components, the following
circuit diagram crystallized, named FYMAS (see figure 1 below).
Figure 1. FYMAS, 40m QRP transmitter for 2016 MAS contest.
The oscillator coil (2) is wound around a wooden 28mm diam rod. A hole is drilled in the
centre of this rod to accomodate an adjustable ferrite rod to tweak the frequency a little.
According to the MAS rules such ferrite rod is part of the coil, so it eliminates a capacitor : -)
Unloaded output is a beautiful sine wave with 4Vpp, measured on my oscilloscope.
Unfortunately I got bad results interfacing the oscillator electrically to the gate of the IRF510 (7) .
It was my intention to ‘auto bias’ the FET. It worked a little but revealed low output.
Therefore two additional components (5 and 6) were inevitable to achieve around
2W output @13.8V after some tweaking with 4 and 6.
The IRF510 simply doesn’t receive enough drive to switch ‘firmly’.
More output is possible by raising the power voltage, but I’ll bring only one power supply
at the Jutberg. So, considering the circumstances 2W has to be enough.
MAS rules (2016) state (quote):
“Any selective network in the TX output stage will be assumed and counted as a
3 parts PI filter. For a better suppression of harmonics you are free to use
more components – they will not be counted.”
The key word here is ‘any’. Read on . . .
Below my matching circuit is depicted.
Match from RL = 15 + j0 Ω (assuming ca. 5W output) to 50 + j0 Ω @ 7 MHz.
Of course a good match can be obtained by halving the ‘last’ parallel capacitor.
Interpreting the MAS contest rules I deliberately chose NOT to do this.
The last capacitor is intentionally too big. By adding a parallel coil (ca 1.2 uH)
you go ‘back’ in the Smith chart, making this coil part of my selective network.
‘Cold’ side of the coil is connected to +13.8V, requiring a capacitor to make this side low Z.
The capacitor is not included in the above picture but . . . is a mandatory part of the matching circuit !
So . . . my parts to power the IRF510 don’t count! If they will, the MAS rules have to be changed.
Note: the consequence of my reasoning is also losing a DC blocking capacitor at the output.
This capacitor is not part of a ‘selective network’. Because I will use an ‘open’ antenna, this isn’t an issue.
Initially I wanted to key the transmitter by shortening the oscillator coil tap to ground
to (try to) prevent chirps.
This works with a screw driver, in a sense that the Hartley stops oscillating.
However, it seemed that the IRF510 (7) (see fig. 1 above) in my design was VERY willing to
oscillate around 1 MHz and on its turn was disciplined by the Hartley ; -)
Connecting the (open collector) key output of my keyer to the coil tap stopped the Hartley,
no matter if I keyed or not. Experiments using an ‘intermediate’ BS170 also failed.
Another method of keying the transmitter is to shortcut the IRF510 gate to ground.
This works, but doesn’t stop the Hartley from oscillating, which is unwanted during reception.
(Tip! For those who want to build a transceiver according to my design can benefit from
this by using the Hartley as local oscillator for e.g. a direct conversion receiver)
Also, keying has to be done in an inverted sense, i.e. ‘key open’ = TX.
Luckily my Arduino keyer owns an open collector inverted keying output : -)
Disconnecting the GND side of the coil (2, see fig. 1 above) with a relay (of course) stops the Hartley.
The other part of the DPDT relay is used as RX/TX antenna switch.
Btw, relays do NOT count as parts in the MAS contest.
How does it sound?
Pretty well, showing only a small ‘start up glitch’.
Finally, below a picture of the FYMAS, as used in MAS 2016, is depicted. (click to enlarge)