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My Personal STEM Experiment

I’m very much in favor of the effort to interest students of all ages in STEM (science, technology, engineering, math) disciplines. Even if this does not increase the number of STEM professionals long term – and frankly, I have no idea of how many are needed in the future; no one does – I like it for another reason. My hope is that being exposed to STEM will make even non-technologists of the future at least have some appreciation of what it takes to investigate, innovate create.

Let’s be real here: STEM disciplines can be fun and fascinating, but they are also hard and demanding in almost every case. That’s the reality, especially as technology advances and the needed expertise “bar” gets higher and higher. I am incensed whenever someone who knows little or nothing whines and says, “why didn’t they just do it this way?” or “why didn’t they just add this feature?” without any understanding of the countless technical tradeoffs and time-to-market pressures that engineers and others face as they create these magical products. It’s easy to complain, as so many do, when they have no idea at all of what it takes it make it happen.

As part of the STEM effort, I am always looking for projects and experiments which can easily demonstrate basic principles. Yes, there are product teardowns, but they have some major drawbacks. If the product has no moving parts, as is often the case, there’s not much to show except a PC board with all sorts of mysterious components somehow attached. If the product has moving parts such as a printer, there are issues of safety when disassembling while keeping it functional especially if it product runs off the AC line. Even the chemistry-based STEM (and school) projects have scaled down into “microchemistry” when tiny quantities of the reagents are used, for both personal safety and to minimize area-wide risk. While I understand the reasons for that, but simply can’t beat a beaker-full of stuff changing color or flashing into flame for excitement. But in today’s litigious society, you can’t be too cautious and must practice “defensive science,” right?

Nonetheless, I recently inadvertently created a simple, safe, and tangible STEM demonstration. Here’s what happened: I have an over-the-air (OTA) TV antenna in the attic (a http://store.gomohu.com/leaf-50-indoor-amplified-hdtv-antenna.html Mohu Leaf® 50 Indoor Amplified HDTV Antenna), Figure 1, feeding about 75 feet of coaxial cable, then going to a far-end mounted preamp (Figure 2), down to a digital TV converter box (http://www.dsconverter.com/img/DTX9950_Manual_V1.4.pdf Digital Stream DTX9950, Figure 3. The outputs of that DTV box go to an old-fashioned CRT color TV (no laughing, please). It’s an all-coax setup, and no 75Ω/300Ω balun transformers are needed.

Figure 1

The RF signal chain starts with a simple, flat HDTV antenna which is about 12 × 18 inches (30 × 45 cm) and includes a 75-Ω coax connector. (Image source: Mohu)

The RF signal chain starts with a simple, flat HDTV antenna which is about 12 × 18 inches (30 × 45 cm) and includes a 75-Ω coax connector. (Image source: Mohu)

Figure 2

The antenna comes with a USB-powered pre-amplifier which can be optionally used; due to temperature constraints, it was connected at the far end of the transmission line rather than the preferable position right at the antenna. (Image source: Mohu)

The antenna comes with a USB-powered pre-amplifier which can be optionally used; due to temperature constraints, it was connected at the far end of the transmission line rather than the preferable position right at the antenna. (Image source: Mohu)

Figure 3

The antenna coaxial cable connects to the DTV box, with produces setup and RSSI screens for the TV screen. (Image source: Digital Stream)

The antenna coaxial cable connects to the DTV box, with produces setup and RSSI screens for the TV screen. (Image source: Digital Stream)

(I know the preamp would be more beneficial and would yield better SNR if it was located right at the antenna, not at the far end of the coax, but the preamp and its USB power unit cannot withstand the >130o F (55o C) attic temperatures. So, I had to compromise and put the preamp in a less-hostile location at the DTV box located next to the TV.)

Due to its flat configuration, the antenna has two opposing primary lobes. My problem was that signal strength and reception to the North and South lobes was good for some signals and inadequate for others. I especially wanted to improve the reception of a few of the channels to the North and didn’t really care about most of the ones to the South.

So, I turned to the well-known reflector technique. I taped a piece of aluminum foil to a picture frame about the size of the antenna, and then held that frame some distance from (and parallel to) the antenna on the south side, so it would act as a booster to the signal from the North (and reduce it from the South, of course, since it acted as an RF shield). To minimize the effects of hand/body capacitance and detuning, I held the frame using a long wooden stick attached, of course, using duct tape.

The improvement was dramatic, as indicated by the received signal strength indicator (RSSI) bar which you can call up on the DTV converter box, Figure 4, as I was able to get it to go from “weak signal” to “very good signal” indication.

Figure 4

The red/yellow/green RSSI bar and numbers give a relative indication of likely performance for each RF channel as the DTV-box channel-selection setting is changed by the viewer: red is too low and has no or a barely visible image; yellow is 'borderline' and the image often breaks up; green results in a clean, stable TV image. (Image source: Digital Stream)

The red/yellow/green RSSI bar and numbers give a relative indication of likely performance for each RF channel as the DTV-box channel-selection setting is changed by the viewer: red is too low and has no or a barely visible image; yellow is “borderline” and the image often breaks up; green results in a clean, stable TV image. (Image source: Digital Stream)

But the “STEM” part of this mini-experiment came when I moved the aluminum-foil reflector back and forth away from, and towards, the antenna. By varying its position back and forth by even a few inches, I was able to produce a tangible change in the RSSI. The range was from not-good red zone (about 20-30 on the relative scale) to mid-range yellow (30-80) and reaching into full-strength green (80-100). The effect was also periodic with distance, as expected. As I kept moving further away, the RSSI cycled up and down, although at further distances its maximum was less, undoubtedly since the small reflector was less effective at longer separation. Still, the change was visible and consistent.

That’s my tangible, safe, easy-to-do STEM experiment to show the reality of wavelengths and reflections. My next step will be to beta-test it on some willing students. Have you devised or done any low-cost, reliable, tangible STEM experiments which resonated with the audience and demonstrated advanced ideas?

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Give Me Back My External Wi-Fi Antenna Connector, Please

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Analog Circuits Give Boost to STEM Reality

1 comment on “My Personal STEM Experiment

  1. jonharris0
    October 3, 2018

    As both an engineer and a cord cutter, I found this a neat and interesting read.  It is something nice and simple to demonstrate to young folks to help them understand signal progagation visually.  I would have loved to do a bit more demonstrating when installing my antenna with something like this, but I could not with my particular installation.  I had to get a dB8e antenna with two passively combined elements so I could point each in a different direction to pull in TV stations from >30-40 miles.  This meant the antenna must be mounted on the roof.  The wife was nervous enough about me being on the roof as high as it is..the drop from one side is two stories plus basement (three stories basically) onto a concrete pad.  Needless to say she nor I would want our younger kids up there for a direct experiencing! 🙂

    On a related note, there is a great website for understanding TV stations and signal strengths in your aread.  It is over at http://www.tvfool.com.  If anyone is interested in cutting the cord I'd encourage them to check out the site first to see what they can receive and how far away the stations are so they can pick out the right antenna.

    Thanks again for the great read!

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