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XBM80-2 An Experimental 80m CW Transceiver


An 80m transceiver using just 14 parts plus crystal and earpiece

This is an experimental CW transceiver for 80m, although the same idea will work on all HF bands. So far I have only used it on the bench and not on the air. I have called it the XBM80-2 (80m two transistors - very original!). It has something of my XBM80 and designs by PY2OHH and K4TWJ.

The circuit uses a simple Colpitts VXO producing around 120mW when the key is pressed. On receive, audio is derived across the 15k emitter resistor (could be higher to reduce backwave more?) and this is amplified in a single stage of audio gain. Output is to a small high impedance crystal earpiece. Sensitivity has not been accurately measured and will depend on the earpiece itself, but I was able to hear the output of my RF generator on its lowest setting and with an additional 10dB pad in the coax lead. The offset between RX and TX is around 400Hz. This is a bit too small, so more can be obtained by shorting the trimmer capacitor in series with the crystal. C1 is a fixed capacitor chosen to give 400-800Hz between switch S1 open/closed. In my case 33p was best and gave around 600Hz offset.

Update 3.2.10: I've changed the values in the collector circuit and this has reduced the AM breakthrough from medium wave a lot (still get some from 75m BC band). Also, power out is now 120mW at 12V (was 60mW) and the RX is a little more sensitive too. The receiver is picking up a lot of stations now on 3.560MHz (G, GW, LA, DL etc).  Making R3 82 ohms increases power still further, but I may stick with 100 ohms finally.

So far I've only had local QSOs with the rig (599 both ways and able to copy a 3mW TX at 3kms OK on RX) but I was able to hear the TX (at 60mW) 430kms away on the on-line SDR receiver in Twente in the Netherlands at RST529 the other evening. It is only a matter of time before I work some reasonable UK distances with this transceiver. For serious use it would need a low pass filter unless the ATU used had some decent selectivity.

On the left is a suggested PCB layout for this transceiver. It fits on a small 1 inch (25mm) square single sided PCB. Please note I have NOT built the transceiver on this board; this was an exercise to see how easily it would fit on a small board. With resistors "on end" rather than laying flat it could be made even smaller - probably 20 x 25mm. With SMT parts it would be possible to make this in 12 x 25mm - tiny!

Recently I received this nice email from Mike Rainey, AA1TJ:

Hi Roger,
 
I posted a quick comment on you blog last evening on the topic of your XBM80-2 transceiver.
 
Mine worked exactly as advertised! I couldn't improve upon your choice of Colpitts feedback divider caps. I raised my receive emitter resistor to 15k to reduce the backwave a bit more, but your choice of 10k is FB as well. Instead of a 10uH collector inductor I used an LC tank (3.5-6uH slug-tuned inductor/390pF) with link coupling to a bandpass filter (similar to the one I used in my Reggie). The output power approached 100mW with a 12.6Vdc supply potential. I backed the supply voltage down to 11Vdc, again, in order to hold the back-wave down. The transmitter puts out a clean 80mW at this supply voltage, with a backwave of 360uW.  
 
I didn't use an AF amplifier stage. Instead, I connected a step-down AF transformer (in series with an electrolytic cap) across the 15kOhm resistor in order to use my 600Ohm magnetic headset directly. The MDS is nearly identical to my "gain-less" Reggie receiver.  
 
My third CQ brought a reply from K1PUG, down in Connecticut; a distance of 158 miles. I gave him a 579 and he replied with a 549. Within minutes of signing I was engaged in a rag chew with W1GUE (112miles). He replied to my call with 100w, but as soon as he heard that I was QRPp he dropped down to 5w. He was 579 running QRO, and 549 QRP. He gave me a solid 589. It turned out to be a very pleasant, half-hour chat on the topic of QRP. Of course, I asked him to please take a peek at the XBM80-2 on your blog.
 
I think it's interesting to compare your XBM80 with G3IEE's (circa-1954) one transistor transceiver design. The lineage is striking.
 
 
Your XBM80-2 is a beauty, Roger! I think one would be very hard-pressed to come up with a simpler transceiver design. Thank you for a very enjoyable QRPp evening. Well done, OM!
 
73/72,
Mike, AA1TJ

Well it's good to hear that Mike has had success with the XBM-80-2, although in truth with his changes I think Mike could give it a new name, HI.

Alan Yates VK2ZAY has also been having a go at this circuit (see Alan's website) and has come up with some changes which make the transceiver more useable but at the expense of added complexity.


30M CW TRANSCEIVER FROM GJ7RWT

Andy GJ7RWT has sent me his schematic for a 30m QRP CW transceiver which has some similarity to the XBM80-2 and the transceiver from

K4TWJ. He gets 150mW out and the receiver can copy signals down to around 10uV.



He has made the circuit on strip board (veroboard)
and the layout he used is attached.





POSTSCRIPT

Playing with this circuit makes me wonder how simple can you get and still have a useful transceiver? For example, is it possible to make a functional HF transceiver with fewer than 10 parts? This would be a real challenge but is anyone willing to try? Well, I hear tonight that Mike AA1TJ has got it down to 7 parts! He'll be trying this tonight (May 13th). This was his email to the QRP-L list.

Fellers,

I spent the morning "Muntzing" G3XBM's, fine little XBM80-2 transceiver for this evening's MAS event.

http://sites.google.com/site/g3xbmqrp/Home/80_2t

It's presently down to seven components; not counting the harmonic filter, key, headphones and 12V battery.

1) 2N3904 transistor
2) 100k resistor
3) 80m crystal
4) RF inductor
5) 360pF capacitor
6) 20k potentiometer
7) 4.7uF capacitor

The headphone coupling cap doubles as one of the Colpitts divider caps (the base-emitter junction supplies the other one). 550Hz of RIT is provided "naturally," by the change in operating conditions (key-up to key-down). It's full QSK and putting out around 70mW. I haven't yet measured the receive MDS but I have a sense that it's in the ballpark of my Reggie receiver.

I spent the last couple of hours trying (in vain) to extract one more component. I'm starting to think seven components really is bare-bones! Dunno if I'll make any contacts but it sure was fun deep-sixing those "extraneous" parts :o)

The transmit frequencies of the two crystals that I'm likely to use this evening are 3535.1kHz and 3569.7kHz.

One more thing; I've presently logged four QSOs with my 28mW transmitter that's made from a 1962-vintage JFET (plus 2-transistor reflex-regen receiver). I moved them both up to 20m late last evening but haven't yet made a contact on that band. The 40m stations worked thus far are

K1GOW, Providence, RI, 569/339, 167miles
KA2PQY, Milmay, NJ, 569/439, 338miles
N2AYI, Carney's Point, NJ, 579/539, 332miles
KA2KGP, Forestville, NY, 569/339, 340miles

I hope to connect-up with some of you fellows this evening.

72 es GL,
Mike, AA1TJ






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