Radio Control: Vanquish VS4-10 Crawler Build, Easy DIY 2.4 GHz Dipole Antenna, & RC Car Transmitter FHSS / DSSS On A Spectrum Analyzer

Someone mentioned working on a remote control battery on/off switch project. Oh boy! What a darn rabbit hole that sent me down. It seemed quite simple to send a radio controlled signal to a relay, nothing novel really. And of course, I liked the idea of an analog device. Perhaps even operating on 27MHz just for kicks. And for propagation purposes. Once you are in the couple of gigahertz RF frequency range (i.e., WIFI, and most of our remote controlled devices these days), your wavelength travel distance falls off to the hundreds of feet range. But CB radio? On a really really good day, you could probably bounce an 11M signal around the world. And for those of you not familiar with these radio things, 27MHz frequencies are also called the 11M band (due to wavelength size) and are known as the Citizen’s Band, a.k.a CB. Ten-four good buddy? And as any ham radio operator knows, when there are “openings” in propagation of the wonky higher HF frequency bands, such as 10M, 11M, and 12M, extremely long propagation paths are possible. Add to the fact that the allowed radio control power output in the 2.4 GHz range is less than a watt versus 4W on some CB channels and you are talking a serious potential long range control advantage even without any enhanced propagation phenomena. And I know I mentioned analog, but why not just send a digital signal out on 27MHz? Unlike the olden days of RC, there really is no longer a need to change crystals to avoid takeover of your vehicle by the stray radio waves of fellow RC enthusiasts, CB radio operators, or jammers. Patterned strings of ons and offs (the nutshell of the entire can of worms that is referred to as “digital”) can take care of all that monkey business and ensure that a transceiver is “bound” to a receiver. And this company has apparently already thought of all this (absolutely no affiliation, but I am adding the link here so I can find it in the future.)

Ok, but what does ANY of this babbling have to do with a small tiny little truck driving on a pretend glacier? Look, I am just setting the stage. This is the mindset I had when I entered the utterly fascinating world of radio controlled (RC) vehicles. In fact, I built that little truck from a bunch of baggies filled with loose parts. You can see what its guts look like below.

A few weeks ago, I decided I wanted to build a radio-controlled vehicle. Specifically I wanted a truck known as a “crawler” that I could walk on trails with. And use it to scale incredible bouldering problems such as the curb in front of my house. But mostly, I wanted to see if I could learn a little sumthin’ sumthin’ about radio control. I googled local RC hobby shops and found one called RC Excitement in Fitchburg MA, about an hour’s drive from my Southie location.

I recruited (dragged) AA1F to go with me. Once I had gone into a roadside CB radio shop I no longer remember the name of. It was a claustrophobic cave of towering rows of stacked radios. Small, dark, not actually – but seemingly – dusty, full of imaginary spiderwebs (there weren’t any), and caving racks of collapsing electronics. Much like every hamfest flea market I have ever been to, except for the not actually having dust and spiders part. The proprietor sold me a tremendously overpriced 11M rig, well, because I felt bad for him. He pointed to a wall full of broken radios he was repairing for customers, telling me how behind he was. I left feeling odd, like I had entered and escaped the lair of an anxious obsessive out-of-control electronics hoarder. Recalling this event now more than a decade later, and with a home lab that I’ll sheepishly admit is starting to look a bit similar, I am not sure my reaction would be the same. From the lens of my current vantage, he was just a typical radio guy!

Point is, I expected RC hobby shops to be just the same…And woah! was I wrong!

I don’t intend on this post being a store review, but I just have to make a few comments. Mostly because I think the ham community can learn a few things here. First off, clearly RC Excitement intends to create a community. From the parking lot alone, you can see a track set up for RC crawling as well as an outdoor racetrack. Once you walk into the store, the aesthetic is polar opposites to that of the CB shop. First off, there are a ton of staff members. I suspect that most of them are pure RC enthusiasts who have transferred there passions into a bit of pocket change. The store is neat, clean, extremely well lit, uncluttered, and precisely organized. It is also fairly large. There is an instant sensation that the target customer is, well, everyone. The products all look like toys in fun, loud colors. Or they are perfect little replicas of their street legal counterparts. There are cars, trucks, planes, and helicopters to delight both the real children as well as the inner ones. And dare I say it? RC Excitement has a buzz of excitement. The promise of fun. Imagination and tinkering.

By contrast, ham radio is mastery of tests and secret languages. A walled-off collection of fiefdom-like clubs, grandiose traditions, and the Newington Kingdom. Proprietary-oriented corporate mindsets. Lists of rules, regulations, plans, and practices. Federal licenses. Engineering and overengineering. Judgement, gatekeeping, and grandstanding. With a tall barrier for entry, not a pursuit suitable for the masses. Don’t get me wrong; I love ham radio. My six year journey through this remarkable hobby has opened my mind to a world of possibility I had never known existed. I fit in well as an amateur radio operator, carving my path. But I never am able to forget that it is a hobby that takes itself very seriously. Unlike the very act of piloting around a tiny little “toy” truck…

But RC cars are toys, aren’t they? Why bother bookmarking the word with sarcastic apostrophes? Well, my little truck seems kind of like a toy. Reminiscent of Hot Wheels and Matchbox cars as a kid. Perhaps even a bit of the LARPing and cosplay community when adding in scale – i.e. realistic but smaller – elements. And there is really not a lot of point in taking it for a walk down a trail with me, other than pure amusement (I tell people it’s the dog’s toy since Georgie the One-Eyed Pirate Puppy likes to chase it a bit. And eat pieces of kibble out of the back bed when I call it the “Food Truck”. I wouldn’t want people to think it was my toy truck, now would I?) I am going to circle back around about this idea of RC vehicles as toys later on…

And what is this idea of a RC crawler anyway? As a comparison, AA1F bought an ARRMA Senton RTR (ready-to-run for the uninitiated) basher. It is what I think of as a typical RC car. You put it in a field and smash it around at full throttle, thus the RC subtype “basher”. And full throttle is 50 MPH! AA1F bought it so Georgie could chase it at the farm, because apparently Georgie is a cheetah. A crawler, on the other hand, goes fairly slowly, human walking or running pace, and climbs up things. It is meant to drive up trails and tackle little bouldering problems much like its full-size off-road vehicle counterparts.

I wanted to build my crawler as a kit so I would get a thorough understanding of how the little machine worked. A very helpful salesman at RC Excitement helped me pick out everything I needed for what I have now come to appreciate as a very high performing crawler. When you peruse over my parts list below, pause for a minute on the idea of motors, servos, chassis, lithium batteries, transmitters, receivers, and consider if I told you that instead of a little toy truck that I was building a robot. Would you believe it?

[As my forever disclaimer, I do this website for me. I bought all of these things with my hard earned cash. I don’t make a dime off of this website, its just a part of my hobby. I just like talking about my things, and well, in real life, very few people are interested in listening to it…]

• Vanquish Products VS4-10 Straight Axle 4×4 1/10 Rock Crawler Builders Kit
• 13 piece Racers Edge metric tool kit (extremely nice to have the right tools!)
• EcoPower WP250T Waterproof MEGA Torque Metal Gear/Metal Case Servo
• Hobbywing Quicrun Fusion Pro Brushless 2-in-1 System for 1:10 Crawler 60A 2300kv motor with combined electronic speed controller (and male XT60 connector)
• ProTek RC Graphene Plus 4800mAh 3-cell (3S) 11.4V 130C LIHV battery (with female deans connector)
• Spektrum S155 1 x 55W AC Smart G2 battery charger (with both XT60 and EC5 male deans connectors)
• Racers Edge LiPo Safety Bag (to pretend this would actually contain a lithium polymer battery fire)
• Proline Racing Trencher 1.9″ Predator (Super Soft) Rock Terrain Truck Tires
• Treal Hobby Type D 1.9″ 12-Spoke Beadlock Wheels (black/red)
• Spektrum DX6C 6-channel 2.4GHz DSMR® Radio System (transmitter)
• Spektrum SR515 Sport Surface Receiver
• Pro-Line 1/10 1972 Chevy K-10 12.3″ Rock Crawler Body (Clear)
• Deans Male to EC5 Female power adapter (to connect LIHV battery to charger)
• Deans Male to XT60 Female power adapter (to connect LIHV battery to motor)
• ProTek RC grey high-tack double-sided servo tape (PTK2241)

In total, I spent about 50 hours building this RC crawler. I highly recommend buying the Racers Edge tool kit or one like it just for the ergonomics of the hex wrenches alone if you plan on building RC kits. It also contains an indispensable RC vehicle specific tool called shock pliers, as well as the parts needed to cut and punch holes in Lexan RC bodies. I did need to supplement with a standard adjustable wrench, needle nose pliers, and L-shaped allen wrenches for use in tight spaces. If I build another one, I will get an electric screwdriver — I actually got blisters from securing so many bolts!

Above is the majority of the frame pieced together with the servo screwed in at the front. Below is the assembled front axle.

In general, the instruction book for the Vanquish VS4-10 chassis is excellent. Where things get a little bit dicey is how to add in the electronic components. The Vanquish instruction manual gives you a general idea of what to do with the servo and motor, but for the most part you are on your own when it comes to the receiver and intercomponent wiring. And if you had an ESC that was separate from the motor, then you would be on your own with that too.

Below is a block diagram of how the components get wired. The motor has a power cord fitted with a male XT60 connector that needs an adapter to match the female deans connector of the lipo battery. The motor then has a combined signal and power cable that connects to the receiver, for speed control and to power the receiver respectively. The receiver then sends both power and a signal to the servo to handle steering.

Below are the cable connections from the servo (“STR” for steering) and motor (“THR” for throttle) to the receiver. You can see the darkly colored ground wire of the motor on the left, the positive lead in the middle, and the white signal wire on the right. The connections are simply male header pins within the receiver housing and a block of three female header sockets inserted on them. There is a trick though. If you look closely at the receiver, you can see triangular tabs on the top corners of each of the ports. The servo and motor connectors I had did not have tabs cut out of the female sockets to match these; they were simply normal squared off header sockets. I needed to file down these corners in order to get them to fit into the receiver. Once I got them to fit, they worked perfectly.

And finally we need to discuss the transmitter. The Spektrum DX6C transmits on 2.4GHz and uses a proprietary digital signaling protocol called Digital Spectrum Modulation (DSM) and in this case with the added “Racing” protocol (DSMR). The protocol involves both direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS). I found this video and this video from Sunny Classroom to offer excellent explanations of these modulation techniques, much better than my knowledge on these topics would allow me to do here.

I figured though that I could build a little antenna and test the transmitter out on my new-to-me Advantest U3641 spectrum analyzer.

I remembered I had a run of coax that Georgie bit once while up on a summit playing SOTA, a run of ABR195W-UF. The defect in the outer insulation is shown at the arrow below. I figured it would make for a quick dipole build, even though the signal loss at uhf frequencies is tremendous (from their datasheet, there is 20.1 dB loss of power for every 100 foot of coax at 2 GHz!) I thought a small short antenna would still do the trick though.

I cut off a few inches of cable along with its pl259 connector. The coax is actually white, and that black portion is just additional heat shrink tubing forming a collar around the connector. I cut off the outer insulation all of the way back to the heat shrink. Then I stripped the various components of the coax apart. You can see the cut end of the outer insulation, the now unbraided outer tinned copper braid, the underlying foil shield, the white dielectric, and the stranded copper center conductor. Interestingly, the version of ABR195 I am using is the “UF” type, for ultra flex I assume. The difference is that it has a stranded center conductor versus the solid non-UF type, in order to be more flexible. The hair-like quality of the outer braid probably also allow for more cable flexibility as well.

I removed the foil shield, the outer insulator, and the dielectric, leaving only the unbraided braid and the center conductor. I have twisted and soldered the strands of both. If you look carefully, you can also see I left enough dielectric between the braid and the center conductor to avoid a short circuit between the two. Use the continuity function of a multimeter to assure that the braid and the center conductor are not touching, and also to double check that the outer portion of the connector is in continuity with the braid, and the center pin of the connector is in continuity with the center conductor, and not with each other.

Finally, the dipole needs to be trimmed to the correct lengths. If you want to learn a bit more about the dipole from my perspective, try this post of mine. One way to look at it is that the positive side of a transmission line needs to form one leg of the dipole antenna, while the negative side of a transmission line needs to form the other leg of the dipole antenna. In this case, the transmission line is the coax, the center conductor is the positive leg, and the braid is the negative leg. The dipole needs to be a specific length and have legs with equal specific lengths in order to work (or at least work as a dipole resonator). And by work, I mean resonate at a specific frequency. In my case I would like the antenna to be resonant at the 2.4GHz band which spans from 2.4 GHz to 2.4835 GHz, i.e., a 83.5 MHz bandwidth.

The easiest way to figure out the lengths of dipole legs is to use an internet calculator and let the web do the work. I used this dipole antenna calculator, and found that each dipole leg should measure 29.72mm (1.17″) for a half-wave 2.4GHz dipole. I used calipers to measure each leg, cut appropriately, and then re-soldered the ends.

The result? An adorable 2.4GHz band dipole antenna! Of course, I really do not have any test instruments that can sweep this antenna to see if it actually works, so I needed to just try it out on my spectrum analyzer.

Below is one of the first peeks at my Advantest U3641 spectrum analyzer on this blog. Although I have had it for a few months now, this is my first time putting it to use actually testing anything! Totally exciting! One of my hesitations with this piece of equipment is that I am a bit petrified to blow up its front end. Spectrum analyzers are apparently exceptionally sensitive to any DC input. In fact, in order to avoid even the possibility of loading DC into my analyzer, I purchased a used Weinschel Engineering Model 7003 DC block. I am embarrassed to admit I haven’t thought of a way to make sure it actually works yet, despite stuffing it onto my SA. The next thing to realize is the SA can only handle 27dBm of power…i.e., a half a watt max! Thirdly, I really don’t know how to use it, at least not that well. Regardless, I believe the Spektrum DX6C only outputs 100mW, so I figured all would probably be okay.

Above you see the antenna, connected to a SO239 to N adapter, and then to the DC block into the spectrum analyzer. I was able to figure out how to set the end points of the SA so that the sweep began at 2.4 GHz and ended at 2.4835 GHz, i.e., the entire 2.4GHz band. Below is a single sweep of the spectrum analyzer before I put the antenna in place, and this appears representative of what the continuous sweep looks like. No dominant peaks are seen.

When I add the antenna, two humps (for a lack of a better word) of signals are now seen as part of the background noise that the SA is sensing.

Finally, when I turn on and press the throttle of the RC transmitter while the antenna is on the SA, I can now see various signals peaking over the noise. I have to perform a single sweep with the SA and “catch” the peaks as they pop up. I do not know at this point if there is a more effective way to do this.

Below are two more peaks I have captured.

I wanted to see if I could detect the RC transmitter signals without the antenna on the analyzer. Below you can see with the antenna removed, I could still detect signals, just at a lower amplitude. At this point, I do not know this piece of test equipment well enough to say what the values it is reading actually mean.

While operating the RC transmitter, I am picking up transient narrow-band signals in the 2.4GHz band on various frequencies (2.40680 GHz, 2.42899 GHz, 2.43817 GHz, 2.44366 GHz, 2.44831 GHz, 2.45678 GHz, 2.46704 GHz). This is not all of them, just the ones I have been able to capture by randomly pushing the single sweep function, as they come and go too fast when the SA is in continuous sweep mode. Like I said, I do not know yet if this is the best way on this SA to do this, as I am just starting my journey with it. In general, I think what I am seeing is very much in line with the FHSS and DSSS protocols that the Spektrum DX6C transmitter is supposed to utilize. I do not have another piece of test equipment available, such as a oscilloscope or logic analyzer, that functions at that high of a frequency, so unfortunately this is all I can do for the time being test-wise.

So this is my little truck. The Lexan has been cut out, but nothing else (except for a few bite marks thanks to Georgie) has been done to the body. I have taken it out and about a few times, but the real treasure is all that I have learned so far about radio control.

With this long essay as my jumping off point into the exciting arena of RC, I am left to wonder why this faction is not more prominent in the hobbyist world of amateur radio. Hams have access to more frequencies, more modalities, more power, and probably in some ways a lot more knowledge about radio waves than the typical non-ham RC hobbyist does. How is it that these two worlds are not strongly aligned? Is it because RC vehicles are nothing more than “toys”? Do they have an appeal to the more mechanically inclined than to those drawn to the so-called black arts of RF? Is it that the digital nature of the signal control is off-putting to those who prefer the glow of tubes and time-honored historically-accurate traditions? Does RC come across as fun amusement rather than somber service? Hams, look over here! I think we have all been missing out!

Measuring waves in millimeters,

KM1NDY