 Hi, my name's Mark, also known as Smitty, or Smitty Halibut on Twitter. My call sign is KR6ZY, crazy with a six in the middle. And yesterday I was doing the T-hunt put on by the Ham Radio Village, and I was walking around with this in the halls and pointing it at things and people and, you know, nothing to see here move along, nothing suspicious, I promise, kind of conversations. And I got stopped so many times from people asking questions about what this was and what I was doing, and, of course, I never made it back to my hotel room, so this was in my backpack sticking out of the top of my backpack for most of the day. And literally I spent about 50% of yesterday talking about this. And so I contacted the Ham Radio Village folks, I'm like, can I do a last minute talk just to talk about it? And they said, sure. So we have a T-hunt running, T-hunt coordinator person. What's your call sign? Do you want me to be excited about you at all? Please don't be. Don't be. Very good. We're running a T-hunt, and by we, I mean him, running a T-hunt at DEF CON, and what is a T-hunt? It's a transmitter hunt. So they hide a transmitter somewhere around the event. They hide a transmitter around the event, and it just starts beaconing and beeping and putting out a signal every once in a while. And your goal as the hunter is to find where that transmitter is. And there are lots of different ways of figuring out where that transmitter is. What I'm going to be doing is talking about the equipment that I use that's practically like cheating, because it makes it so easy. And there are several different techniques, but primarily what you're trying to do is find out what direction the signal is coming to you from. And there are lots of different ways, but kind of the easiest way is probably based on signal strength. Like if you are, if you're standing here and the signal is over here, the transmitter is over here, if you walk toward the transmitter, it's going to get, the signal strength is going to get stronger. And if you walk away from the transmitter, the signal strength will get weaker. I'm just going to go ahead and put this down. I don't need to hold this the whole time. So the kind of the simplest way to do a tee hunt is to just walk around and listen to the signal strength of the transmitter. If it's weak, you'll hear a very staticky signal. And if it's strong, it'll be nice and clear. And that works great to a point. The problem with that is once you get to a certain point close to the transmitter, your radio is at full quieting, which means it's strong. You have a strong signal and there's nothing you can do without some extra equipment to be able to tell how strong it is, right? You get to a certain point and it is strong, strong enough. And you can't tell how strong it is above that. So once you get within a circle around the transmitter, it's hard to do any more direction finding that way. So one way you can do that, to handle that is by attenuating the signal. You can stick an attenuator in that, which is just three resistors and a particular orientation. And it makes the signal weaker. And so when you're far away and you've already got a weak signal, you don't bother with an attenuator. But when you get close and you kind of get inside that circle, then you start adding attenuation. And that works great because that kind of shrinks your circle. And you can kind of keep shrinking the circle until you get to a certain point where even if you take the antenna off of your radio, it's still getting into the radio and it's too strong. And without, with no antenna, the radio itself is picking up enough signal to be full quieting. Right? So there's kind of a small circle around the transmitter where that's the case. And there's really nothing you can do while listening to the transmitter directly to solve that. I have a solution. It's not unique to me. I did not design this. But there is a solution called an offset attenuator, which I'll talk about in a little bit. That can solve that problem, and we'll go into that in a bit. But first, one of the other ways you can figure out where a transmitter is is not just by signal strength based on where you are location wise, but by using a very floppy, in my case, directional antenna. Right? So we all know what yaggies are, directional antennas. This antenna, you can kind of think of it as an arrow, right? The signals that come in or transmit down the beam will be much stronger. It's focusing the signal intensity. So you can think of it like a lens, right? So a lot of the energy coming down that beam will be very strong, but any of the energy coming in the side will be very weak. So that's another way you can kind of attenuate your signal. Not only does this tell you whether you're getting closer or farther away, but it can also tell you what direction it is, right? So let's say the camera is the transmitter. If I'm pointing right at the camera, I'm going to have a very strong signal. But if I'm pointing away from the camera, it's in the null of the pattern of this antenna. So it's very weak coming in here. And so you can kind of spin around and listen for where the signal is the strongest. Oftentimes you actually want to listen to where the signal is the weakest. And it's like, all right, because it's strong, strong, strong, full, quieting, full, quieting, full, quieting. And it's not until I get to hear that it comes away from full quieting and I can kind of go, all right, where's the weakest? It's right about there. So I know that that's the side, right? So sometimes you want to listen for the nulls. Sometimes you want to listen for the strongest signal. So those are kind of the two basic ways that I know to T-hunt. There may be others. And if there are, please let me know afterwards. But you kind of walk around based on signal strength and walk to where the signal is stronger. And then the other one is to use directional antennas and kind of use that to give you a direction to start walking, right? So there are a lot of considerations. One is that if I were walking around with that big arrow antenna, let's say it was even just a three element antenna like this, those elements on that antenna back there, sorry folks on the camera, it's an arrow antenna. If it was one of those elements, they aren't very flexible, right? And if you're doing a T-hunt, a lot of T-hunts are in the woods or in a forest. And as you're kind of trundling around, let's say that's a tree, right? The elements on this antenna bend and flop around very freely. But then when they're done, they just kind of flop back. So that's why I used tape measure for this antenna. Besides, you can get tape measures for free at Harbor Freight. You can get PVC fittings for cheap if you don't already have them in your junk box. This entire thing cost me less than $10, right? And that includes buying everything new. So this was a very inexpensive to build but very effective Yaggy antenna for doing T-hunts. You can just Google search for tape measure Yaggy. Their designs are very common. Whereas that arrow antenna, those are firm aluminum elements. And they're great if you're doing satellite work because it's very precise. But if you're running around a forest with that thing, you're going to get caught up on trees and everything. Or a DEF CON hallway. Or a DEF CON hallway and attendees in line con and all that kind of stuff, right? So those are great antennas for some things but not great for T-hunts. So the tape measure Yaggy is a great design. The feed point for it is a little bit tricky because you do have to solder to the steel tape. And spring steel doesn't solder very well. So you have to get out a torch. I use a propane torch that I use for copper fittings to be able to get it on there. And it's a little tricky to do the feed point. But otherwise, it's very inexpensive and very easy to build. So Google search for tape measure Yaggy. You can find designs on that. So the other neat thing that I have I touched on earlier is called an offset attenuator. And it's a circuit board that I designed and built in here. You can also buy kits online. I think they're 10, 15 bucks. They're not very expensive. Other people have already done the design. But what does it do? So you all know what a mixer does in the RF sense. So it's one of the things you had to study up on to get your license. A mixer, this is not a mixer in the audio sense, which is more accurately described as a combiner. And it's down. A mixer in the RF sense is a device. I'm not going to go into the details of how it works. But take my word for it, magic happens. You have a signal coming in one side. And you have a local oscillator that comes in the other side. And the output of that mixer is two signals. One that is the sum of the frequencies of the two inputs. And one that is the difference of the frequencies of the two inputs. So let me give you an example. Let's say we're listening to a signal on 146 megahertz. That's one of the frequencies that the T's are on, 146.565. Ignore the 565 for a minute. Let's say it's on 146 megahertz. And let's say I have a 4 megahertz local oscillator. So I feed a 146 megahertz signal coming in from the antenna, and a 4 megahertz oscillator coming in from my local oscillator. And I feed it into a mixer. The output of that mixer will be two signals. One on 150 megahertz, 146 plus 4 megahertz. And 142 megahertz, 146 minus 4 megahertz. So we have those two signals coming out. That's what a mixer does. Now why do we care in this particular case? So remember what I said earlier that once you get close enough to a high-powered transmitter, even if you take the antenna off, you're still hearing full quieting on your signal. That's because you're listening to, say, a 50-watt output from the transmitter that you're trying to find, or whatever it is. If you're trying to find a rogue FM broadcast station, it could be a heck of a lot more than 50 watts. But you're listening directly to that frequency. But if you tune off that frequency and you listen somewhere else, you don't have that problem. So the offset attenuator, what it does is it takes the signal you're looking for and moves it somewhere else. So in the handle of this antenna, I can take the 146 megahertz signal coming in from the antenna and move it to 150 megahertz. And I tune my radio to 150 megahertz. And now I don't have the same problem with the radio itself being overloaded from the transmitter signal. That's one of the key features, is that now I'm listening to a frequency that's different than the transmitter, but it is the same signal as the transmitter. The second really cool feature on that is that I can put a potentiometer or an attenuator on the local oscillator. And the signal strength on the output is a function of both the signal strength coming in from the antenna and the signal strength of the local oscillator. So I can pad down the signal strength on the local oscillator, and that will make the output weaker. So even when I've got a super strong signal coming in from the antenna, if I give it a really weak local oscillator, that will make a weak signal coming out of the mixer. And I feed that into my radio. And now I have that weak signal where I can tell the difference between super weak and only kind of weak. Because you want to be listening to the signal in that linear range where you have some static on the signal. If there's no static on the signal, there's no way to tell whether the signal strength is getting stronger or weaker. If you've got nothing but static, then you can't really tell whether there's a signal there. So there's that middle range where it's complete static on one side and full quieting on the other side. You want to be listening to it in that range where it's ramping up and down. And you can tell just by listening to it, oh, that signal is stronger or that signal is weaker. And so what the adjustable amplitude on the local oscillator does is it allows me to adjust the signal strength going into my radio to keep it in that linear range. As I get closer to the transmitter, I have more signal coming in the antenna. I turn down my local oscillator. The output signal from the mixer is the same intensity. And so I can keep it in that linear range. So that's why this is like cheating because it just makes it so easy. So I have a directional antenna. I can point at things and figure out which direction this signal is the strongest. I have an attenuator that I can use to vary the signal strength however I want pretty much continuously. And I have my signal moved from the main frequency off to a different frequency so that the transmitter doesn't get into the body of the radio and overload the receiver in the sense of it's always full quieting. I don't have that problem anymore. So those are the three things that this antenna accomplishes. And I would love to do a demonstration. Really? That only took me 15 minutes? Are the transmitters up and running right now? Yeah, you're not going to hear them from here. I'm not going to hear them from here. But I would love to, if other people are interested, I'd love to go find them and maybe do a demonstration of this. After you're up and running. OK. So first off, before we do that, are there any questions? There's a signal on that offset. So the question is, can I adjust the frequency of the local oscillator in case the mixing products lands on some other signal? In this particular design, no. Because it is a four megahertz crystal oscillator. So it's one of the four pin metal can oscillators that you get from a computer. So I'm just using a very fixed oscillator. But in theory, there's nothing stopping you from creating a variable oscillator. You could use an SI5351 or something like that if you really wanted it to be variable. I went for simplicity. And those things are a quarter or whatever they are. I mean, they're super cheap. Maybe a buck, I don't know. And that was the easiest way to get a fixed frequency oscillator. The good news is that depending on your mixer, you shouldn't be transmitting the mixer products. But depending on how your mixer is designed, and if you don't have any input filtering, you might actually be retransmitting those at very low powers. But for the most part, most of that energy goes into your radio, not outside. So hopefully you're not interfering with anything. And anything that was on 150, so where you're going to run into problems with that is, let me do the math in my head. If there's a signal on 154, so the difference of 154 in my local oscillator goes down to 150. The sum of the transmitter I'm hunting on 146 and my Fulmeriger Hertz oscillator goes up to 150. So you've got a dual image problem there. You definitely have that. You will hear signals that are both on 154 or 146 on the input. So yeah, that is a problem. I've never run into that problem, but conceivably it could be. Any other questions? No? Well, that was easy. Is anyone interested in trying to go find some transmitters? I got a lot of thumbs up and nods. Cool. Let's go do that. Thank you.