Low Power AM BCB Antenna
The initial motivation for building an AM signal source was so I could characterize the performance of a 2X2 foot box loop antenna I built a few years ago. You can read about the transmitter design here. and initial tests my father and I did during our October, 2020 R&D Expedition.
My father and I are smarter now than we were in October, 2020. We decided we needed to build a better antenna so we could characterize the performance of the box loop antenna in the far field of the signal source.
So, off to the internet I went, looking for information. I got lucky and found the Low Power AM Broadcasters Handbook by Kyle Drake. This is an excellent source. My antenna design is strongly based off his.
The primary design constraint for me was transportability. I live in White Plains, New York, and my father, the electronics expert, lives in Columbus, Ohio. I designed the antenna so it breaks down in to sections that fit in my Mazda CX5 and can be transported on an airplane.
For our first set of tests, V1 of the antenna, I drove to Columbus. For the second set of test, V2 of the antenna, I flew. I purchased a Thule Round Trip 175 cm ski roller bag and transported the antenna parts in it. The radiating element, a 1/2 inch, 10 foot copper pipe, of course was not transportable by either method. I purchased a 10 foot, 1/2 inch copper pipe at Home Depot near my dad's house.
How It Goes Together - V1 - JUNE, 2021
Basic Assembly
I bought the Simpson Fence Post Spike from Home Depot. It is nice and sturdy and did the trick. When removing from the ground it was very tolerant of the rough handling needed to loosen it and pull it out of the ground. For the V2 tests in Ohio, it added too much weight to my luggage so I left it at home. In Ohio, I bought a similar product from Home Depot. It was much lighter weight. It was not satisfactory. It broke the first time I was loosening it to pull it out of the ground. Be advised, spend the money to get the heavier one.
The 4X4 slides in to the fence post spike. I used some wood shims to tighten it up.
The coil element sets on top of the 4X4 and is secured with dowel rods.
Ground Plane and Radials
For the ground plane I ordered a 12X12X1/4 inch sheet of aluminum from Amazon. It cost about $34. I used a jigsaw to cut the circular hole in the plate. Chewed up a couple of jigsaw blades doing that. I also added in 16 bolts for the 15 foot radial wires.
The ground plane rests on two dowel rods and is secured with zip ties. The design worked out pretty well.
In the sources I consulted, there seems to be general agreement that you should use at least 16 radials. There also seems to be general consensus that after 16 radials, you get get less bang for your buck. So, I went with sixteen radials, each fifteen feet long.
Most of the sources I consulted said "Radials should be 1/4 wavelength long." At one megahertz, a 1/4 wavelength is around 250 feet. That's 4000 feet of wire for 16 1/4 wavelength radials.
My radiating element is 10 feet long. 250 foot radials seemed excessive. I came across a source that said they did experiments and found that radials around the length of the radiating element performed well. So, I went with 15 foot radials.
Radiating Element
The radiating element is a a 10 foot 1/2 copper pipe. The antenna is experimental, an opportunity to learn, not a permanent installation. I assemble it, run some tests, disassemble, and improve. So, I did not use any guy wires. My gut feel is you would want guy wires if you were doing a permanent installation with a 10 foot copper pipe as a radiating element.
The Tuning Network
The Coil
Chapter 5 of Kyle Drake's Low Power AM Broadcasters Handbook was my go to resource when building this antenna. It has good practical advice on constructing the tuning coil.
The table in Appendix D recommends 305 uH of inductance for a 10 foot, 1/2 in radiating element at one megahartz. The text after the table recommends adding more inductance than called for in the table. My coil had 144 turns on a 4.5 inch PVC pipe. Total coil length was 24 inches. The calculated value for the coil was 403 uH.
- Formula from chapter 5 of Low Power AM Broadcasters Handbook for number of turns as a function of coil height and diameter
- \(N=\frac{\sqrt{L (4.5 D + 10 H)}}{0.5 D}\)
- N = Number of turns.
- L = Inductance in uH
- D = Diameter of coil in inches
- H = Height of coil in inches
- Solve for L
- \(L=\frac{0.5^2 N^2 4.5^2}{4.5^2 + 110 H}\)
- Plug in my values
- \(L=\frac{0.5^2 144^2 4.5^2}{4.5^2 + 10*24}\)
\(L = 403 uH\)
We measured the coil with my RigExpert AA-600.
Because of measurement range limitations of the RigExpert AA-600 we had to do a piecewise measurement. The measurement came out to around 450 uH. Pretty close for RF work!
It turned out I still needed more inductance for the V2 design described below. I added an extra 80 uH to the coil.
The Variable Capacitors
When tuning the antenna we observed the radiated signal on a laptop running SDRUno hooked to an SDRPlay SDR. The Low Power AM Broadcaster's Handbook recommends connecting to each tap to find the one that results in the strongest signal. Then adjust the tuning capacitor to peak the signal. In our V2 version we had added the extra inductance. We lucked out and we were able to peak the output with the variable capacitor when hooked to the very last tap on the coil.
When we peaked the tuning, one capacitor was at the minimum value. I used the other one to peak the tuning. I ended up not needing two variable capacitors. When building it I figured having the ability to get more capacitance would compensate for any design peculiarities necessitating a larger capacitance. Also, one of the capactors was 1:1 and the other was 8:1. I thought I ccould use the 8:1 for fine tuning.Turns out I did not need the 8:1 variable capacitor.
Hooking it Up
One of the references I consulted used wire alligator clip jumpers to connect the tuning capacitors to the coil and the coil to the radiating element. Our version 1 design used this strategy.
This turned out to not be a good idea. We tuned up the antenna at my dad's house. Then we packed up and went to a large open field by the school my sister worked at. When we reassembled the antenna we could not get the antenna tuned up. I, in a classic "shooting wildly in the dark" moment, wiggled the feed wire and saw the tuning fluctuate wildly. It turns out that the position of the jumper from the coil to the radiating element was super critical. Moving it a little bit, drastically changed the tuning of the antenna.
The feed point of the antenna is at a high voltage, around 160 volts. We theorized that there was capacitive coupling between the jumper and the ground plane. When we noticed this we had run out of time so we could not test if the position of the jumper that connected the tuning capacitors to the bottom of coil was critical too.
We could not get rid of the capacitive coupling between the feed point connection and the ground plain, physics is physics, but we could control the positioning so the coupling is the same each time. The goal for the V2 design was a repeatable design. We wanted to be able to assemble the antenna and get predictable behavior every time we took it to a new location.
Action Shot
Building it Better - V2 - OCTOBER, 2021
Separate Panel for Transmitter and Tuning Capacitors
For version two, I separated the transmitter and tuning capacitors to two different panels. I used a 1 foot coax jumper to connect them. I did this so I could hook my antenna analyzer to the tuning capacitor panel, essentially replacing the transmitter with my antenna analyzer. By doing this I was able to measure what my transmitter "saw" when hooked to the antenna.
Feed Point Connection
I used a ground clamp and a thick, thick, AWG 6, stiff wire to connect the tuning coil to the base of the radiating element. Remember, the design goal was to be able to assemble the antenna and always get repeatable performance. We moved the antenna one time and we met that goal.
Tuning Capacitors to Coil Connection
We did not do any measurements where we moved the position of the alligator jumper wire between the tuning capacitors and the base of the coil. But, I decided that making that part of the antenna rigid and repeatable was probably a good idea. Pictures to the right show how this was accomplished.
More Inductance Please
When I built the antenna I followed the recommendation in the Low Power AM Broadcasters Handbook and added extra inductance in case I needed it. It turned out we needed even more inductance. At this time we are not sure why, maybe because of the 1 foot square ground plane?
I did not have time to do a pretty job so I just wound some wire around the base of the coil. It did the trick and we could easily peak the antenna tuning.
Range Measurements - OCTOBER, 2021
How far can we go?
Once we added the extra inductance and could peak the tuning, it was time to do a range measurement. My brother has a neighbor who is a farmer. He gave us a permission to use one of his corn fields as our test range.
We could not have asked for a more pleasant day.
Range Measurement Station
The goal for the October 2021 iteration was to do a range measurement on our new and improved antenna design. We were concentrating so hard on getting the antenna tuned correctly, I had not given much thought to how we were going to measure the range. On the morning of the test, I grabbed a TV stand my dad had and got to work. I velcroed the laptop, SDRPLAY RSPPro2, and a junction block to the TV stand. A dowel rod and some 2X4 scraps were used to mount a Kaito AN-100 Tunable Passive AM Radio Loop Antenna to the TV stand.
How were the measurements taken?
- Install the antenna and tune it to optimal setting
- Take measurement station to a spot around 250 feet from antenna
- Record GPS position. I used an app named GPS Test on my Samsung S20
- Position sense antenna so signal is peaked
- Record signal amplitude from SDRuno display
- Move measurement station around 250 feet further
- Repeat steps 3 through 6 until signal is lost in noise.
The Data - But First, Some Theory
Remember, our goal was to build an antenna that we could use to characterize performance of an AM BCB receive antenna I built a few years ago. Also, we wanted to observe expected R-Squared of signal strength. R-Squared performance is expected in the far field of the antenna and it seems reasonable that it is better to be in the far field when characterizing performance of the AM BCB receive antenna.
So, an important question to answer is “Where, how far from my low power AM BCB antenna, does the far field start?
Initially that seemed like a relatively simple question. WRONG! As I surfed the internet looking for information, I encountered a bewildering array of eye glazing, mind numbing mathematical treatises on near and far field theory. And, a lot of the sources seemed to be assuming reasonable sized antennas. But, one thing became clear. The far field starts at least 1 wavelength from the antenna but probably more.
So, before I discuss my results I would like to present a little bit of the theory I learned while researching this section.
- The concept of an electrically short/small antenna. Is mine short/small?
- What I learned about where the far field starts
Size Matters – Is My Antenna Electrically Short/Small?
One important characteristic for determining near and far field distances is whether the antenna is “electrically small/short” compared to the wavelength of the frequency being used.
The radiating element is 10 feet (3 meters). Transmitter frequency is one megahertz. Wavelength at one megahertz is 983.6 feet (299.8 meters).
The radiating element is 1% of the wavelength. Seems short to me. Various sources on the web seem to agree.
| These sources say that an antenna length less than 10% qualifies as an electrically short/small antenna. | ||
|---|---|---|
| https://www.highfrequencyelectronics.com/Feb07/HFE0207_tutorial.pdf | "The most common definition is that the largest dimension of the antenna is no more than one-tenth of a wavelength." | |
| https://www.antenna-theory.com/antennas/shortdipole.php | "Typically, a dipole is short if its length is less than a tenth of a wavelength." | |
| http://www.arrl.org/news/tuning-electrically-short-antennas-for-field-operation-is-research-topic | "As the article explains, electrically short antennas — typically 0.1 λ or shorter — ..." | |
My "ka" is 0.063, way less than 1. By this author's criteria, the antenna qualifies as electrically short/small.
Where Does the Far Field Start for my Antenna?
Holy cow, what a journey! I still can't say for sure where far field starts but I suspect there are more "It depends" factors than I currently understand. There is a lot of stuff I do not know.
I started my journey with this Wikipedia article https://en.wikipedia.org/wiki/Near_and_far_field. It had a drawing and text, see below, that seemed like a pretty straight forward definition of where the far field starts for an electrically short/small antenna.
For antennas shorter than half of the wavelength of the radiation they emit (i.e., electromagnetically "short" antennas), the far and near regional boundaries are measured in terms of a simple ratio of the distance r from the radiating source to the wavelength λ of the radiation. For such an antenna, the near field is the region within a radius r ≪ λ, while the far-field is the region for which r ≫ 2 λ. The transition zone is the region between r = λ and r = 2 λ.
Note that D, the length of the antenna is not important, and the approximation is the same for all shorter antennas (sometimes idealized as so-called point antennas). In all such antennas, the short length means that charges and currents in each sub-section of the antenna are the same at any given time, since the antenna is too short for the RF transmitter voltage to reverse before its effects on charges and currents are felt over the entire antenna length.
Our Measurements - Did We See R-Squared?
Measured data
I read the signal strengths from the SDRuno spectrum display. The values are in the below table in the "Measured Signal Strength column." They are also plotted in the graph below the table. The measured data is the blue line with blue circles. Except for measurement 6, which I will talk about later, the measurements seem to fall on a nice curve.
Predicted data
There are four predicted data curves on the below graph. For the gray line with the triangle markers, I started with the measured value at 0.24 wavelengths and predicted the subsequent R-Squared values. I did the same for the other three predicted curves that start at 0.53, 0.79, and 1.04 wavelengths.
The predicted data curves that start at 0.24 and 0.53 wavelengths have the same general shape as the measured data but have higher than expected values. However, the predicted data curves that start at 0.79 and 1.04 wavelengths match the measured data pretty well. According to the above definition stated above for where the far field starts, I definitely was not in the far field. But, I can definitely talk myself in to concluding that we saw R-Squared behavior starting around 0.79 wavelengths.
What's up with measurement position 6?
Notice how measurement 6 is above the curve. When I was taking the measurements I wrote "Local altitude maxima" in my notebook. Don't read too much in to that though. The measurements were taken in a corn field, it was relatively flat.
I plotted the signal levels with respect to altitude. My initial reaction was "The signal is higher because the sense antenna was higher at that location." But if that was the case I would expect measurement 5 to be lower, measurement 7 too. But, there is another but. By measurement 7 the signal was almost in the noise but I could still see it. I don't think there are any conclusions I can make about any relationship between altitudes that vary by a maximum of 6 feet.
What's Next?
Better Radials - Will It Me Get More Range?
Kyle Drake, in the Low Power AM Broadcasters Handbook says "I have used a 5x5' chicken wire screen that was soldered together for the center of an antenna, and it improved range considerably." This leads me to believe that I may be able to get more range if I improve the ground plane/radial system.
Capacitive Top Hat - Will It Get Me More Range?
Kyle Drake, in the Low Power AM Broadcasters Handbook says "A capacitance hat is a component that helps to even the current flow through an antenna. Without one, more of the signal is propagated at the bottom of the vertical radiator than at the top. If more of the signal is propagating at the top of the antenna, the signal does a better job of getting above trees and nearby objects.." This leads me to believe that I may be able to get more range if I add a top hat.
Switch Transmitter From Square Wave To Sine Wave
My dad and I strongly believe we are 100% legal with respect to power input to the antenna. However, since our signal source is a square wave, it is difficult for us to directly measure power to the antenna. We also want to calculate the radiation resistance of the antenna and how well it is matched to our transmitter. I plan on building or buying a sine wave source so we can make these measurements.