Iron Slug Coil Antenna Matching for 27MHz Transmitter (Model Rocket Transroc) - Page 1
Forum section: electronics / rf-microwave

author: fuzzoli
author_rank: Regular Contributor
timestamp: March 16, 2021, 01:47:14 am
content: I'm working on recreating a 1970's 27MHz model rocket transmitter and documenting the buildhere. I'm at the point where everything works, but I need some advice from the RF experts to help me understand the circuit's behavior as I'm a digital person.The large coil (T2 in the schematic) is the antenna matching coil, and the signal strength as received on a walkie-talkie is greatest when the slug is almost completely out of the coil form. I'm not sure why this is. The coils were wound to match the original device, including number of turns, diameter of the form, and height of the coil windings. I do admit that the specifications were determined by both measuring the original device, as well as taking measurements from photos and scaling accordingly.What could cause the "best" performance being when the slug is more or less not even in the coil?For reference, the slug samples areTH48-0106FandTH48-2002from MicroMetals. I'm currently using the TH-0106F slug (slugs have different threads, so I haven't re-tapped the coil form yet to fit the second slug).Thanks!

author: MartinL
author_rank: Regular Contributor
timestamp: March 16, 2021, 06:46:54 pm
content: The coil is for matching the transmitter impedance to the antenna impedance, right? So how confident are you of each of those being right?Presumably if the transmitter and antenna were both as designed for, then the best performance would be found somewhere in the middle of the range of slug movement.If either impedance is far from what the matching coil was designed for, then it would not be surprising if the point found to give the highest output would be all the way at one end of the range.Presumably the antenna itself is a piece of wire - are you testing with a piece of the expected length, suspended in the air?You mention in your other thread that the output voltage from your modulator is lower than you expect.I'm not familiar with the topology in the RF section but it looks like the modulator output directly controls the DC operating points of the transistors used. The AC source impedance of any transistor will vary with the DC bias conditions applied to it.So my wild guess, without getting into any of the specifics of the design, is that your low modulator output might be the cause. Can you test the RF section with a test signal you can vary the level of, rather than your modulator output? You may find that a different drive level gives a matching point more in the middle of the slug range, and power output closer to the expected level.

author: A.Z.
author_rank: Super Contributor
timestamp: March 16, 2021, 07:56:44 pm
content: I'm no rocket scientistbut then, even if that rocket has a reentry parachute, the landing won't be nice, so, in my humble opinion, whatever you'll design, ensure it can survive acceleration and reentry

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 16, 2021, 08:07:51 pm
content: Tuning won't mean much until you have everything packed up together. That's an electrically short antenna, I assume a dipole between the electronics (circuit board and battery) and whatever the antenna-as-such element is.If everything is laid out loose, that might imply lower capacitance, suggesting higher inductance is needed. Which would be the other way around, shrug. Maybe the coil dimensions aren't quite perfect, either? (Shouldn't be too far out, if they measure the same +/- a few 0.1mm, same turns.)Tim

author: floobydust
author_rank: Super Contributor
timestamp: March 16, 2021, 08:25:04 pm
content: I find a breadboard increases capacitance, worse on a metal workbench or ESD mat. I would prop the little board up and see if tuning changes, to know if it's the setup that is too different from the original.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2021, 08:16:31 am
content: Yes, figure an additional ~4pF between adjacent positions, whatever effect that might have on the circuit.It looks like the antenna and coil is off to the side, on the PCB, while the two transistors are breadboarded. The 33pF is the smallest capacitance there, and it doesn't look like a few extra here or there would have too much effect, tending more to detune the crystal (so it'd be off by maybe a kHz) if anything. Hard to say. Also hard to say what all that stray wiring does; it's still fractional wavelengths and not huge compared to the nearby inductors (they'll be in the ballpark of ~100nH).Also, did you get the "authentic" transistors, or are they subbed with something more common? Capacitance might vary there, too.Tim

author: fuzzoli
author_rank: Regular Contributor
timestamp: March 17, 2021, 11:33:24 am
content: I did figure on things not working 100% as expected due to the breadboard; at this point I'm just happy things work as well as they do. From what I remember, the original Transroc was temperamental at best, so I consider the current status a big success.I did manage to figure out the power problem (I need to update the build log), so I'm getting the full power out now.Regarding parts, one goal of the project was to recreate the original as much as possible, so I even found some NOS transistors in the original TO-98 and TO-106 packages. The biggest deviation from the original design is replacing the DZ805 diodes with 1N914's (which does not impact the RF section).I just noticed that the manual did state a length for the antenna wire (30"), and I was a bit short. I lengthened it to the proper size, but the maximum output was still with the slug out of the form.Everything T3sl4co1l said about "Tuning won't mean much until everything is packed up" is also called out in the manual (which I didn't notice until after T3sl4co1l mentioned it here). Therefore, I think I'll call the current performance satisfactory and begin populating the board correctly. Perhaps then it will work a bit better.Thanks for all the help! :-)-Frank