I was alerted to a video this morning via the RockMite reflector. Mike @K6STR had shared the video regarding a recent successful activation by @W6LOR using a RockMite][ modified for 10m. I enjoyed the video and the RockMite seems to be working very well! The video needs sharing to a wider audience so I’ll link it here!
I’d gone out with my RockMite][ 12m yesterday. My primary purpose was to test a QCX-mini 10m, which worked at least as well as expected- i,e, it needs more RF gain in the receiver chain. I’d chucked the barefoot RockMite for 12m in for a bit of fun. I was pleased to work 5 stations with my ~300mW!
I acquired some QRP RockMite][ PCBs so last year I made the RockMite for 12m, which I nicknamed ‘Pinky’. (I fitted a random water clear LED and it happened to be pink!) The case is homebrewed from a bit of aluminium sheet.
Reverse Beacon hits yesterday from Hutton Roof Crags
To finish, I’m glad that other folks are enjoying messing about with these ridiculous radios too, it’s amusing that you can actually successfully activate with such things - I remember being told that it would never be possible by a grumpy old man!
Hi Colin,
Thanks for posting the video. Congrats on your 12m rockmite! Building tiny gear is a lot of fun, and it’s even better when you make a number of contacts.
We were using a 10m EFHW (a little less than 17’) with a 2:10 winding on an FT50-43 as a matching unit.
I built a linked dipole for 10/12/15 & 17m. I am thinking about adding another set of links for 20m. I found that my RockMites don’t really like EFHW antennas.
Do you use a common mode choke? I’m guessing you’ve got some RF coming down the coax that isn’t enough to bother most rigs but is enough to upset the Rockmite. It would be interesting to find out even if you intend to only use dipoles.
I use an EFHW with my Rockmite and it is ok. But then I do always use a CM choke.
When activating with the RM10 recently, we alerted and RBN picked us up. I was worried we would t get any rbn hits at such low power, but propagation was good and it worked fine.
Investors used CM chokes on EFHWs before, but not this time. I do have roughly 18” of RG316 between the radio and the transformer, which serves as my counterpoise. No problems with that antenna so far. The Rockmite is susceptible to interference from nearby stations and we really noticed that on our recent activation - we had 4 operators.
Ha… glitch in the matrix was my radio completely shutting off mid QSO for low voltage. Every day a field day… Anyway, fine business 10M rock mite Lor! Sounded great. Glad to get you both in the log and also good to catch you all on Frazier in the dark later that eve. dit dit.
Full disclosure our summits were only 10 miles apart
Also of note Mark is wearing a custom printed rock mite circuit t-shirt printed by Mook AG6N during a San Diego instance of SoCAL SOTA campout earlier in the year. 3x nerd points Mark. As if you needed more
No, I haven’t tried a choke to be honest. I’ve found RockMites so touchy in the past that I just used dipoles. It’s problems on receive, not transmit. I’m not sure if I made up a choke for my bicycle trip in Scotland, I’ll have to look for it or make a choke to experiment.
I don’t own a RockMite but the receiver problem made me curious to look at the schematic. The T/R switch by shunt diodes and absence of any preselector are two areas that I see that could cause interference from out-of-band strong signals. I’d change the T/R switch design…
Exactly my thought. Try climbing a summit and staying below freezing temperatures calling CQ on 20m with less than 5W. I would have 1/10 the points I have now.
The RockMite wasn’t a serious design, it was thrown together as an attendance gift for the 2002 Lobstercon. The idea was to have group build and then attempt to work each across the Lobstercon campsite.
I think the design is way too mature to start re-engineering at this point. The design is a classic and the flaws add to the charm.
The issue I’ve come across is not broadcast/ strong interference, it’s motorboating which manifests when the load presented to the antenna is incorrect or not shielded. If you reduce the AF gain, the motorboating stops. This isn’t a problem with one specific rig, I’ve noticed this issue with many RockMites- I’ve built over 20 of these things!
Motorboating that changes with the antenna and the AF gain is an interesting phenomenon. That is very common with straight receivers but I guess it could happen with a direct conversion receiver. Like you said shielding of the receiver has a big effect, and so can the choice/placement/orientation of the speaker or the earphone, and the proximity between the rig and the antenna (including the feedline).
Yeah from the schematic the intention of the design is pretty clear. Re T/R switch, the simplest way to avoid the diode-related distortion can be done by replacing those two antiparallel diodes with a BJT and a bias resistor to the TX control line, so the component count does not change (or could be reduced if you used a “digital transistor” but avoid MOSFET there.). (But I don’t think T/R sw is the direct cause of motorboating.)
The Rockmite doesn’t have a T/R switch and so the diodes are there to limit the voltage at the receiver front end during transmit.
The original design for this by W7EL has a capacitor with 500 ohm reactance followed by the anti-parallel diodes followed by an inductor with 500 ohm reactance . The idea is that the capacitor isolates the diodes from the transmitter but the inductor cancels outs the capacitive reactance so the receiver doesn’t suffer attenuation while actually receiving. In the Rockmite the crystal acts as the inductor in this design.
So it isn’t a T/R switch but is there to perform a T/R function.
However, I don’t really get what any of this has to do with motorboating.
I wrote T/R switch issue before learning that the problem was motorboating, as you see in this thread above. I thought the problem was interference from super strong off-frequency signals.
Those diodes conduct during transmit, adding the capacitance to the left of them to the LPF shunt capacitance, while isolating the receiver detector (mixer) input. This type of switch is called a shunt switch, one of several forms of T/R switch. The downside of this design using antiparallel diodes is that when the receiver input signal (anywhere between very low frequency up to the cutoff of the LPF) have super strong signals, the diodes may softly conduct during receive, creating a lot of cross products, some of which may fall within the passband of the crystal filter. The simplest way to remove that is to replace the diodes with another switch that is controlled by a separate control, rather than the RF level presented. MOSFETs are not much better because of the body diode. So, the simplest way to do this without distortion is to use a BJT, as I outlined in my previous post. I have actually used that type of T/R switch in one of my QRP transceivers.
Motorboating is caused by feedback. Its troubleshooting is more of an empirical process and it is hard to pinpoint on the schematics.
