VHF/UHF radio propagation in mountainous terrain can be difficult and sometimes may seem impossible. As you know, higher frequencies travel primarily line-of-sight. Unlike HF, they do not reflect off the ionosphere, so their range can be rather limited (although knowledgeable DXer’s know special tricks, like meteor scatter.)
However, all is not lost. It is possible to make sense out of using higher frequencies in mountain communities, and at the November WARS (Willits Amateur Radio Society) seminar Tim Hanna, WB9NJS, described some of his experimentation and research on this topic.
Tim got interested in mountain VHF propagation after being surprised that he could communicate over 2M FM from his home QTH in Willits to Wayne, W6WMV, in Finely (south of Clear Lake.) The signal path is a good distance and there are two significant mountains in the way; regardless, Tim was still able to communicate quite well.
In his presentation Tim reviewed the ways radio signals change direction:
- Reflection: wave changes direction by bouncing off other objects (a metal fence post for example).
- Refraction: wave changes direction due to variations in the air (or other medium). Examples are cold and hot spots where the density of the air differs.
- Diffraction: wave changes direction by passing over sharply defined edges — often called the “knife edge effect”.
Next, Tim introduced a handy mapping tool for analyzing signal paths. The software is from DeLorme and makes it fairly easy to go from a flat map of the path to an elevation profile view that helps you see the obstacles that are in the way.
Once that was done, an elevation profile could be generated:
Looking at this profile, you might begin to wonder how Tim’s signal was able to reach Wayne in Finely. There are a two peaks that should block the signal. Tim speculated that perhaps knife edge diffraction was helping his signal bend over one or more peaks.
Tim also noticed another possibility. It could be that signals were being reflected off Mount Konocti behind Wayne. If so, the signal map becomes:
To plot such an elevation profile, the profile is “unfolded” to show it by distance. It becomes:
To understand this profile, note that that tall peak on the right isn’t blocking the signal, it’s reflecting the signal. That’s how reflection is shown on a profile plot.
So, this signal path seems more likely. Although there may be a knife edge effect at the peak closer to Tim, the reflection of the signal from Mount Konocti behind Wayne makes sense. The middle peak becomes less of a factor because it’s no longer in the way.
Tim went on to show several other signal paths for unusual radio contacts he’s made from his QTH on the west side of Willits. Some are not such direct paths as the one to Wayne in Finely, but looking at the elevation profiles helps you understand how such paths are even possible. For example, he’s able to communicate with K6FTY who lives a considerable distance to the north. The trick again is to use a directional antenna to reflect the signal off various high-elevation peaks.
Thanks again to Tim Hanna for his educational WARS presentation. It’s good information for those of us who live in the mountainous terrain of Mendocino and Lake Counties. If you find you can’t make a VHF/UHF QSO directly, you now know that you can try reflecting or diffracting your signal off various peaks to increase the likelihood of your success.
The Answer to Antenna Puzzle #2 was based on the idea that signal strength is related to power by squares.
If you remember the power formula from ohm’s law, you know that:
power = voltage squared / resistance
and signal strength is most often measured in voltage per unit length (for example micro-volts per meter.)
To show this relationship, here are a few photos of signal strength measured on the 10M band at four different power levels. This was done by connecting an oscilloscope to a nearby monitor antenna, then snapping a photo of the screen.
Each image is twice the power of the previous image.
For 10 watts we see 0.39 volts (peak-to-peak):
For 20 watts we see 0.542 volts. Notice that we don’t see twice the 0.39 volts, what we see is the square root of two times the square of 0.39. So, thats:
0.39 squared = 0.152
0.152 x 2 = 0.304
square root of 0.304 = 0.551 (roughly)
For 40 watts we see about 0.846 volts. (Notice that it’s not precisely what we expected. That’s because I don’t have precise control over the radio’s power output. It’s just approximate.)
For 80 watts we see 1.192 volts:
So, when you go from 10 watts to 80 watts you don’t see your signal strength increase by a factor of 8, you only see it increase by about 3! (Kind of seems like a rip-off, doesn’t it?)
That’s worth keeping in mind if you ever need to measure the signal strength of your antenna or figure out strange antenna puzzles posted to miscellaneous ham radio sites on the web.
It seems like a simple enough question, right?
Three weeks ago at the McARCS meeting in Mendocino, a few of us built cheap Yagi-Uda antennas for 2M. I wrote about the experience in Snipping the Yagi. There’s nothing like building an antenna to make you curious as to how and why they actually work. Yagi-Uda antennas provide excellent gain and directiveness, and the more directors you add, the more they provide!
I remember once reading that the Yagi design was invented back in 1929, but that their theory of operation was not understood until 1975. That being said, good luck finding a decent description of how they work! I’ve searched online and also looked through a number of radio textbooks, but so far I’ve come up empty for an adequate description of the Yagi’s mechanism. There are plenty of descriptions covering its design and properties, but very little about the theory behind it.
While over on the coast, I chatted with Derek, KE6EBZ, who uses a nice 2M Yagi beam that he built from an old TV antenna. We agreed that the function of the reflector seemed like that of a mirror — not difficult to imagine. But, the directors were more of a puzzle. Derek suggested that the shorter lengths of the director elements helped focus the wave in the forward direction.
As Cindy and I drove home later that afternoon… I thought more about Derek’s suggestion. I’d seen that kind of focus pattern before — 30 years ago while building a laser diffraction lab at UC Davis. Essentially, if you paint a big dark circle on a window, light diffracts around it much like a lens. It’s called a zone plate, and… if you continue the pattern of dark and clear in concentric circles around it, it will become even more focused. It becomes a Fresnel lens.
So, is that how Yagi directors works? They form a sort of Fresnel lens? I don’t know. Seems like something to study a bit deeper.
Such a lens might account for the directivity of a Yagi, but where does its high gain come from? Its gain can go beyond 18 dBd. Producing that much gain from parasitic (non-driven) elements seems difficult to comprehend, especially if you think of the directors as focusing only the radiative EM field (the far field) and only in a single dimension.
Recently it dawned on me that many of a Yagi’s directors fall within the near field of the driven element. That means they have mutually inductive and capacitive coupling to the driven element, more than just an effect due to re-radiation of the far field. So, in terms of magnetic induction, the first director works a lot like a transformer. That means that the director can actually pull more energy from the driven element — energy that cannot be tapped from the radiative field alone. Something similar could also be said about its capacitive coupling of the E field to pull energy. So, the directors can pull more power from the feed point… which really helps explain where the high gain comes from. Very cool.
Of course, this also means that the phasing of the whole mechanism must be very precise. The induced and coupled energies must perfectly match the phase of the radiated wave as it passes by. Wow, isn’t that elegant. You could almost think of a Yagi as a medium of propagation… with the wave travelling through it. (Of course, that means the wave velocity is changed while it does so… interesting to consider… hmmm, could be something to this.)
Well, I suppose everyone already knows this, and I’m just the last guy on the block to figure it out. The elmers reading this article are yawning and commenting, “yes of course that’s how it works, dummy.” But, it does get me thinking… what happens in the subsequent directors as the near and far fields pass by… and how far backwards can the induction and coupling have an effect?
Obviously, the mechanism of a Yagi is fairly complex. It sure would be nice to learn more about it. You’ve really got to wonder why such a elegant antenna has so little published about how it actually works.
If you’ve come across in-depth descriptions of Yagi theory, please post a comment here for all to read! Many thanks.
Antenna Puzzle #2 asked why 3 db of gain is obvious for two identical dipoles for reception (3 db): But… it’s not so obvious for transmission!
This is a wonderful question because it shows how power and field strength differ, even though they are directly related.
An insight into the answer comes if you ask the question: if you double the power to your dipole, how much does the field strength increase? The answer is that power is related to E-field strength squared. If your power is 100 watts, and you double it to 200 watts, your field strength does not double. That’s because the square root of 100 is 10, and the square root of 200 is 14.14. So, doubling your power only increased your field strength by about 40%.
With that in mind, you can now analyze the dipoles above for transmission. Here’s a diagram that shows the proportional values (not actual values):
So, here’s what’s happening:
- 100W is divided in half, so 50W for each dipole.
- 50W radiates as a field that is proportional to the square root of 50, thus 7.07.
- At the receive antenna, the two fields combine, 7.07 + 7.07 = 14.14.
- The power received is the square of the field strength: 14.14 squared = 200.
So, you must add the field strength of each dipole, not power. (And of course, the above is written to show the proportions, not actual values. The actual received power is substantially less due to distance and antenna pattern.)
You can now see that the power gain of the system is double (3 db, just like before), even though the power was been split between the two dipoles! Another way to write this is:
pwr-recv = (sqrt(pwr-xmit / 2) * 2) ^ 2
If you enter that into your favorite calculator, slide-rule, or programming language, you’ll see that the pwr-recv result is always double the pwr-xmit input.
So, the principle of antenna reciprocity is safe and sound. The gain is the same for both receive and transmit.
For greater detail see: Signal Strength Relationship to Radiated Power.
Earlier I mentioned the McARCS coastal meeting where we built cheap Yagi-Uda antennas (many thanks to Steve, KJ6EIF.) The design was based on WA5VJB’s cheap yagi, and by “cheap” I mean about $5. The antennas were made from lengths of ordinary house wiring attached to wooden garden stakes. The feed method was a simple half-folded dipole (no gamma match or balun needed.)
Even though we closely followed the instructions, the antennas didn’t tune as well as we expected. They resonated lower than we wanted, and the SWR wasn’t the best. After some experiments we suspected the problem to be the insulation left on the directors (to make the elements stronger in the wind.) That insulation affects the velocity factor, making those elements electrically longer than they should be, and on a Yagi, that creates major problems.
Last weekend I decided it was time to see if the antenna could be fixed, or whether the boom should return to the tomato garden. I discovered that it only took a few clips here and there to make the antenna just about perfect. Here’s what I did:
- Mounted the antenna on a mast with the elements vertical (and mounted it from the end, not the balance point.)
- Connected Steve’s antenna analyzer to the feedpoint. The SWR was about 1.5:1 and resonated at 143.5 MHz.
- Wanting to fix the SWR problem first, I snipped 1 cm from one side of the 1st director. The SWR fell to 1.3!
- Then, trimmed the other side of the 1st director another 1 cm, and the SWR dropped to 1.1:1. Even better!
- Clipped 1 cm from both sides of the 2nd director. No effect (on the analyzer that is, but maybe in the gain and pattern.)
- Thinking I was on a roll, I snipped 1 cm from both sides of the reflector. Whoops… the SWR climbed just a hair.
- Now the big decision was how to deal with the driven element. It’s not easy to cut because half of it is a loop. Throwing caution to the wind, I just snipped 1 cm from the non-loop side and bingo! The resonant frequency jumped to 146.23 with an SWR around 1.1:1.
- Finally, I clipped 0.5 cm from each side of the 1st director. The SWR landed at 1:1!
Hurray! The antenna was right where I wanted it.
I measured the SWR as 1.32 at 145, 1.02 at 146, and 1.3 at 147. That’s just about right for the FM part of the 2M band.
Now, I just need to figure out where to bolt it outside, and then… do I want to point it toward the Mendocino coast, toward the Bay Area, Eureka, or Clear Lake? Decisions, decisions. Hey Steve, could next year’s workshop be about building our own rotators for $5?
Last week’s McARCs meeting/workshop in Mendocino was a lot of fun. The turn-out was good too — better than anticipated. I’d like to thank all of those who made this meeting possible and who helped out with the antenna workshop.
One of the interesting demonstrations was Alan’s (WA6JBK) j-pole jig which he uses to rapidly construct 2 meter J-poles from ordinary copper pipe (same as used for home plumbing). Notice how Alan decouples the coax by passing it within the lower segment of the antenna pipe.
I use a similar J-pole made by Len, WA6KLK, and they’re strong, easy to setup antennas that work quite well. Here’s how to make a J-pole — but note the difference in how the coax is attached. (It would be fun to electrically compare the difference in the two decoupling methods: wire loop vs. Alan’s copper sleeve.)
Another antenna construction project presented by Steve, KJ6EIF was a hands-on build your own super cheap 2M Yagi-Uda antenna (for only $5). The design is based on WA5VJB’s cheap yagi. Steve made one about a year ago and it tested out pretty well.
We made the Yagi antennas from lengths of ordinary #12 house wiring attached to wooden garden stakes. The feed method was a simple half-folded dipole (no gamma match or balun needed.) All-in-all it took about an hour to build each antenna.
Unfortunately… after construction, we discovered that our antenna resonance rang up around 143-144 MHz, quite a lot lower than we had intended. We experimented around a bit with shortening the driven element and shunting its folded-side, but neither changed the frequency by much (although the shunt did give better control over SWR.) We suspect the culprit to be the insulation we left on the directors. We kept it on to make the elements stronger in the wind, but insulation will affect the velocity factor, making those elements electrically longer than they should be. On a Yagi-Uda, you can get by with directors that are a bit too short, but if you make them too long, the phasing of the array is quickly spoiled. Well, not a problem… we will try snipping down the directors to see if we can get the Yagi where we want it. (We’ll let you know how this works out.)
Thanks again to all of those who made this workshop a fun and educational McARCs meeting.
Here’s a simple but interesting antenna puzzle to think about…
Let’s say you use a wire dipole for the 20M band. You decide you want to boost your gain by adding a second dipole (end-to-end, i.e. colinear) and combine their feeds (in phase, of course.)
The two antennas receive the incoming signal (depicted in green below) which gets combined to double its strength at the receiver. Here’s the diagram:
So, this simple array provides 3 dBd of gain. It makes sense because each antenna is receiving an independent field of energy from the distant station.
Now, you use the same setup to transmit 20 watts on CW. Of course, your power gets divided in half between the two dipoles. Each antenna gets 10 watts.
But, wait… if you simply put that 20 watts into a single dipole wouldn’t that be the same thing as two antennas at 10 watts?
The principle of antenna reciprocity dictates that an antenna must behave the same for both receive and transmit. It has the same gain, directivity, and pattern. So where’s your 3 dB of gain on transmit?
Perhaps you’ll answer that the two antenna fields significantly overlap. But, even if they add together perfectly, won’t the distant station still see the same signal as one antenna at 20 watts? Where’s the 3 dB? (The output should be equivalent to 40 watts.)
So, there you go, a puzzle. What’s your solution? Post it here as a comment.
Answer now posted here: Power vs Signal Strength
Don’t forget the special McARCS meeting in Mendocino on April 2. Note the somewhat relaxed schedule of events and pot luck, “bring your own lunch and some to share”.
Date: April 2, 2011
Place: Mendocino High School Multi-purpose room, 10700 Ford Street
Talk-in: 146.820 –103.5
- Getting Acquainted & Introductions
- Tsunami Event and Exercise de-briefing
- School District Exercise Evaluation
- Future Public Service Communications Opportunities
- Lunch – bring your own and something to share
Workshops as requested:
- Getting Started for New Hams
- FLDIGI Communications (bring laptop)
- Copper J-Pole Antenna Construction
- 2 meter Beam Construction
Yesterday, I finally got on 40M PSK31. That happened because on Saturday I finally tuned my 40/80M dipole down to the lower end of those bands (removed the tuner, yeah)… and also because Len, WA6KLK, was so kind to offer me a Rigblaster (for a decent price.) So, I no longer had to fiddle around with 600 ohm transformers and resisters to balance out the AC hum creeping into my various digi modes. Optical isolation is so much cleaner.
Anyway, one of my first contacts was Mike, VE6VIS, in Glenevis, Alberta Canada. We got to talking about antennas, and he told me about his ZZ Wave Net, quite an interesting antenna design. It’s a loop antenna with a cool geometry. It looks like something that might take off and fly. Mike says it puts out a good signal (which I can confirm that from the solid PSK QSO that lasted much longer than I thought in fluttery/fading band conditions.)
As you know (or may not) some of us have been discussing what kind of antenna to build next. I mentioned the TTFD and K9AY in my earlier blogs, and I’ve also been looking closely at the double extended zepp recently proposed by Steve, Ki6EIF. (And if it hadn’t been for the sudden appearance of the rigblaster yesterday and difficulty in measuring the capacitance of a 2 inch piece of coax, I would now have one on the air for 10M.)
But, Mike’s ZZ Wave Net loop definitely looks like a fun antenna to try out. He told me he was originally inspired by those bow-tie UHF TV antennas which are very boad-banded (see further down his page for the ZZ BowTie which is 1:1 across the 20M band.) And, it just so happens I’ve been thinking about building one of those for some of the higher ham bands… mainly due to the ease of construction, low cost, and wide bandpass.
So many antennas… so little time.
I think I’m infatuated with the idea of low noise reception in the HF bands.
My interest started 30 years ago when designing and building an ionosonde for use in Antarctica. I had the opportunity to build an entire receiver from scratch (and more importantly, with someone else’s money.) Now, there are some tricks you can pull to drop the noise of an HF receiver, and there are still a few ideas I’d like to test… but the front end begins at the antenna, and if you can improve the signal-to-noise there, you start out ahead of the game.
I became inspired again in recent months when a vertical antenna I was testing became shorted in an unusual way (involving water) and became low noise but with decent signal reception (and horrible transmission results.)
One antenna that’s popular with SWLers is the TTFD (or T2FD), the tilted terminated folded dipole. It’s essentially a folded dipole, but with greater distance between the folded elements and a termination at the far end from the feed. It’s very broad band and is a standing wave design somewhat like a rhombic where induced opposing currents terminate, rather than reflect. Of course, as a result of the lower feed point currents, the feed has a high resistance, and it’s terminator should be roughly match that. 400-600 ohms is reasonable, and a 6:1 to 9:1 balan comes in handy if you want to use coax. Also, watch out: there’s a lot of misinformation about the TTFD and erroneous construction details. This TTFD document by Rob Wagner seems pretty good, and “Buck” Rogers K4ABT has the best T2FD diagram (he sells the TTFD as a kit.) John Conover’s T2FD page also seems decent.
Tonight, I came across another similar design, the K9AY Terminated Loop by Gary Breed. It’s been getting some rave reviews because it’s much smaller than traditional low band loops, sparing you a lot of real estate. You only need one fairly short support (like a tree), and the entire loop can fit in a 15 foot radius. In addition, when built smartly you can make it four way directionally switchable. A nice feat for a small, ground level, fixed position low band antenna. The front-to-back ratio can be as much as high 40db. Quite remarkable really.
The theory of the K9AY (according to Gary) is that the E and H fields induce voltage and currents that reinforce on the front side, but will cancel when received on the backside. That’s probably due to the oddly placed feed and termination points, which are very near the grounded base and are connected to ground. And, it’s also a standing wave type of antenna, but with an interesting twist in its geometry relative to its feed, termination, and ground.
If what this antenna claims to do is true, it might be superior to the TTFD in several ways.
Somehow I’ve got to find the time one of these weekends to build both of these interesting designs.
Ok, I finally got some time to begin my short series of articles about antenna theory.
So you know… I like the KISS principle. Keep things simple. I’m not going to make this complicated. As an electrical engineer I can tell you that there are hundreds of books on this subject that dive in to a range of details and complexities… often to such an extreme that few readers understand.
I prefer first principles. These are the fundamental ideas upon which a theory is based. For antennas, these principles were studied beginning in the 1880′s by scientists like Heinrich Hertz, and those ideas are still quite relevant today.
A simple experiment provides a great starting point for discussion. You can actually build this and try it out (but that’s not really necessary to understand the theory behind it. )
- A ten inch sphere made of thin copper or other conductive metal.
- Ten feet of copper wire and a few insulators to hang it.
- An AM radio.
- A high voltage DC source, such as a Van de Graaff generator.
Hang the wire and the sphere near each other, using insulators. Here’s a crude diagram (and not drawn to scale):
Using the voltage source, charge the sphere to negative 10,000 volts (or any high voltage, it does not matter.)
Turn on your AM radio, it can be positioned anywhere in the room, and tune it to a clear frequency.
Now, using an insulated rod, touch the wire to the sphere.
You’ll hear a snap or click sound on the radio. Interesting!
Congratulations. You’ve just built the most basic radio transmitter and antenna possible. Experiments of this sort were first performed by Heinrich Hertz (yes, as in megaHertz) in 1887 to prove the existence of electromagnetic waves.
Physics and radio books just love to dive into the details of voltages, currents, fluxes, and fields. However, it’s good to recognize that those are all derivatives of a fundamental concept: charge. If you think in terms of charge, it’s easier to understand how those other terms relate.
- Think of a charge as an electrically unbalanced atom. It has either too many or too few electrons to balance out the protons of its nucleus. When it has too many, it’s negatively charged. When too few, it’s positively charged.
- Different materials can become charged using various methods. What that means is that their atoms have either too few or too many electrons.
- Charge can be distributed evenly or unevenly within a material. Part of a material may be negative while another part is positive.
- Charge can flow from one point to another. In some materials, such as metals, charges flow quite freely (they conduct.) In fact, they act a lot like a fluid or gas, flowing and sloshing around with very little friction. This fact is really important.
- Charges repel and attract with tremendous force. How much force? Physicist Richard Feynman provides the best example: if you stand one arm’s length from another person, and both of you are charged with one percent more electrons than protons, the force of repulsion between you would be enough to lift the entire weight of the earth. That’s correct. Just one percent of your electrons. It gives you a sense of the magnitude of the force we’re dealing with here, doesn’t it?
- This extreme force causes charges to move. For example, if we add a few extra electrons to a thin metal sphere, the repulsive force causes all of the electrons to flow and distribute themselves across the sphere. This is true for all metal objects, not just spheres.
In the experiment above, here’s what happened in terms of charge:
- You charged the sphere by adding extra electrons to it.
- The sphere is made of metal, and its electrons move freely.
- Because of their extreme repulsive forces, the electrons spread out evenly over the surface of the sphere. I’ll call it electron pressure. That’s a good way to think of it: a force or tension between all of the extra electrons.
- The wire has no charge and when touched to the sphere the electron pressure begins to push electrons down the wire.
- Electrons flow down the wire until the electron pressure within the wire becomes equal to that of the sphere.
- As the electrons are accelerated (begin to move in a specific direction, down the wire) they produce electromagnetic radiation: radio waves.
- These waves radiate outward, and the AM radio detects these waves, making the click sound.
Here’s the diagram again, showing the charge beginning to flow down the wire:
The charge moves down the wire very quickly. The extra electrons also push the electrons that are already within the wire, just like a hose that’s full of water when pressured from a facet. Yes, it’s a lot like a wave of water pressure moving through a hose.
After a short period of time the charge has moved halfway down the wire.
Then, after another short period of time the electrons reach the end of the wire:
The electrons no longer have anywhere to go and they distribute themselves evenly across the wire, and radio waves are no longer being produced.
Essentially, this experiment produced a single burst of radio waves which are heard on the AM receiver.
(Ok, those of you who are experts will recognize that I’ve simplified this explanation quite a lot… but I think the first order effects far outweigh the other details for the purposes of this description. I’ll cover some of that in articles to follow.)
This experiment provides a launching point to get you thinking about the process. I think it’s quite interesting how very simple this experiment is, showing how fundamental this process is in nature. It’s all around us and is happening all the time.
Notice that I’ve not said much about voltage, current, fluxes, fields, frequencies, or phases. But then, the process shown here isn’t really that useful, it just sends a single click.
In the next article, I’ll cover what happens if we charge and discharge this wire at regular intervals… that is, at a specific frequency. In addition, there’s a deeper paradox lurking here: no matter where you tune your AM radio, you’ll hear the click sound. The wave emitted by this process is extremely broad in frequency. You might say it has no frequency, or more accurately, it has all the frequencies.
You don’t need to be an electrical engineer or ham radio extra class to understand the basic electro-magnetic (EM) fields of an antenna. Let’s see if I can explain them in one page.
There are four EM fields related to an antenna:
|electric||An antenna has electric charge; therefore, it projects an electric field outward.
What’s a charge? It’s any imbalance in electrons, either extra electrons for negative charge, or shortage for positive charge. For example the charge on the ends of a battery or capacitor, the static electricity when you stand-up from a chair on a dry winter day, or the voltage on the output of your ham radio transmitter.
This field decreases rapidly with distance, and it relates directly to voltage, the electric potential (force).
|magnetic||An antenna has current (a moving electric charge); therefore, it creates a magnetic field around it.
The field is described in terms of a cylinder that curls around the direction of movement. For example, the current (the movement of charges) in a wire forms a magnetic field around the wire. Even if you take a charged ball, and move it in the air, you’re creating a magnetic field as it moves. Wonderful, isn’t it?
Like the electric field, this field also decreases rapidly with distance. It relates directly to current flow, amperes (flux).
I probably should note: when you say that a charge “moves” you must ask “relative to what?” In other words, the magnetic field is relativistic. This makes it even more interesting, but let’s save that for dessert later.
|radio waves / photons||An antenna current changes (electric charges accelerate); therefore, it produces EM radiation that propagates away from it.
We know of it as radio waves or photons (aka, “light”, every physicist will tell you it’s all the same.) It is described in terms of a tiny pane that detaches from the charge and travels outward. It is composed of both electric and magnetic fields that oscillate back and forth, always at a specific frequency, which also means a specific energy.
These waves can travel great distances — across the room, the country, or the universe. That’s why this field is called the far field whereas the electric and magnetic fields mentioned above are called the near field. Their effects over distance are dramatically different.
When you measure the signal strength of your ham radio, you are actually measuring the quantity of these waves/photons passing by your location. Also, by the way, as you whip around the above mentioned charged ball, you’re also creating EM radiation. Interesting, eh?
|heat waves / photons||An antenna has resistance, which produces heat; therefore, it also produces a different EM radiation that propagates away from it.
When you apply a current to a wire, resistance within the wire will generate heat — meaning atoms “bumping around” into each other. When they do, they accelerate electrons (changing “quantum energy levels”) which as you know from above, produces EM radiation.
This field also travels outward and is composed of photons of many frequencies — most notably infrared radiation: heat waves. In general you don’t want your antenna to generate this kind of field, because it is wasted energy that’s not going into your radio signal.
Ok, did you get all that? It’s the basic theory in a nutshell… really quite simple, which is kind of cool.
So, perhaps now you’ll look at your antenna a bit differently, imagining the forces, flows, and energies that are making your ham radio transmission possible.
PS: As engineers, we express the theory in terms of mathematical equations describing potentials (forces) and fluxes within a volume of space or through surfaces such as a sphere. They were first summarized by James Clerk Maxwell about 150 years ago and reformulated by Oliver Heaviside, the inventor of coaxial cable, about 20 years later into what we call Maxwell’s Equations. (Although many folks prefer they be called Heaviside’s Equations, giving credit to his remarkable simplification through the use of vector notation and operator-based mathematics, the form we still use today.)
Len, WA6KLK, dropped by this weekend with the elmer spirit… and a pile of aluminum tubing, plus a base bracket with a built-in SO-239 socket.
It took only a few minutes to piece together a few short segments and cut off the bent tip for a quarter wave 10M vertical. A brandy cork fit well in the tip to keep the water out, and it was ready to bolt to a fence post.
We picked a post that had a unique location: it’s 300 feet above the valley with a steep 45 degree slope on its southern side. Then as part of the experiment, we used the field fencing itself as the counterpoise. We weren’t entirely sure how it would tune.
After tweaking the length of the main element an inch or so, the antenna analyzer gave us a 1.7:1 SWR at around our target frequency, 28.4 MHz. And, because of the large diameter tubes, the bandwidth was reasonably flat over the entire 10M band.
Len suggested we add just a single radial, cut approximately to the band and hung out over a sawhorse. With that single change, the SWR dropped below 1.3:1!
When you think about it, it’s really quite remarkable. The antenna had the entire field fence to use as a counterpoise, terminated in the ground itself, but a roughly tuned wire tied in-parallel with the fence gave us a better SWR. I think it’s just a bit like those fan antennas where current flows into whatever element resonates best.
Encouraged by that change, we cut a second radial and dropped the SWR below 1.2:1.
Of course, this was all an experiment. We know this isn’t an ideal setup, because:
- The antenna is only a quarter wave, so no gain.
- It’s close to the ground (although on southern side, it drops of quickly.)
- The coax is 110 feet long to reach it.
- It could use many more radials.
However, the project was a learning experience, and I appreciate Len taking the time out to share some of his knowledge of antennas. I’m still working on my understanding of the currents generated inside the coax shield, and how to best radiate or terminate them, as well as how to choke return currents that can pass down the outside of the coax and re-radiate unwanted signals. This project helped make the theory real for me.
Sure, the jury is still out on whether this will be an effective antenna for 10M or not. This Wednesday, we’ll give it a trial on the 10M local net (28.405) at 8:20 PM. Please tune in.
It was difficult to climb out of bed at 3:45 AM to head down to Pleasant Hill for the 2010 Pacificon Antenna Forum on Friday, and I appreciate Steve, KJ6EIF, piloting us safely down and back.
I must admit that in my stupor at 4 AM, I was doubting this trip would be worthwhile, but now I’m so glad I went. The forum was a truly amazing, educational, and entertaining event.
Here’s a very quick summary…
- Rich Holoch, KY6R, talked about his experiments with a variety of wire arrays for DX-ing, even when operated from from a fairly restrictive QTH. He showed many photos of his antenna configurations, including an interesting variation on a Bruce Array which performs quite well for him on 40 M and a modified W8JK beam for 20 M.
- Dean Straw, N6BV – Gave a very detailed description of why the 2010 WRTC (“The Olymics of Ham Radio” held every four years) became an amazing event due to the sporadic E layer that formed near Moscow during the contest. Team USA, R33M (N6MJ and KL6A) placed 3rd with 3,603 contacts during the 24 hour contest period. (BTW, that’s 2.5 contacts per minute on average.)
- Steve Sterns, K6OIK, discussed the detailed science, math, and engineering related to SWR. This was the deepest analysis I’ve ever seen or read on the topic (serious electrical engineering level), and included a lot of insights regarding differences between SWR at your rig vs. the antenna, as well as many issues related to coax and mismatches. I don’t think most of us understood every detail, but we have a copy of the notes if we want to dive deeper.
- Jim Brown, K9YC, provided a entertaining and educational in-depth look of coaxial ferrite chokes. The conclusions where quite surprising and interesting, and I’ll be writing more about this subject in MendoRadio in the future. He provided the results of many of the tests that he’s done on different ferrite materials, and it’s not what you would expect! BTW, one conclusion was that if you’re using a coax loop for your chokes, you need to reconsider that!
- Edison Fong, WB6IQN, gave one of the most exciting and interesting lectures… I think we were all on the edge of our seats. As a warm up, he showed a 3 gig sampler board that they built recently that he thinks someday could be made for about $100! This is essentially the receiver side of a SDR on steriods, and also makes a terrific low cost spectrum analyzer. The rest of his talk was the challenge to invent a new type of vertical antenna that was low cost, had no radials, all weather, with 5 db gain (over a standard 1/4 wave whip.) Side note: It is non-trivial in the year 2010 to invent a new type of vertical antenna, but by-golly he did! It’s a collinear J-pole stack — a wonderfully elegant and simple design. And he took all of the measurements to prove that it did in fact have the required gain. This antenna is easy to construct, and I think we’re going to be seeing a lot more of them in the future.
- Tom Schiller, N6BT – made a witty and funny presentation of UAD, unknown antenna disorder, that covered many of his adventures and misadventures of why antenna systems often fail. Think you’ve got good barrel connectors on that lead-in? Well, think again!
Afterwards Steve and I checked out a few of the exhibits in the hall. Steve bought the dual band version of WB6IQN’s new antenna, and we drooled a bit on some of the portable QRP HF rigs being shown. On the way back to the truck, we ran into a demo of hand-held satellite QSO using a dual band yagi with a gun grip for easy pointing.
All in all, it was a great forum this year. My only regret was I didn’t make plans to stick around for the weekend and see the other lectures and exhibits. Maybe next year! -Carl KB6ZST
While not a truly scientific test, I did a short test yesterday with two different ten meter antennas. I have a 10/15/20 meter trapped antenna horizontally polarized that was originally used as the driven element of a three element antenna. Now just a dipole. The other antenna is a homebrew dipole that is hanging vertical from my tower. The bottom is just a few feet off the ground. I am on top of the hill so this will make some difference but not much.
Monday afternoon after repositioning the horizontal antenna so it now favors the northeast–Ukiah is off the end of it–and hanging the vertical, I got on the air.
Listening on ten meters with the horizontal antenna I could hear some southern Mexico stations, Centeral American, and some Brazilian stations on it. Mind you, they were off the end of it so that was the LEAST favorable direction of the antenna. Switching to the vertical made no or very little difference in signal strength. Thus the received signal had to be coming in from a higher angle of radiation so that it got both antennas about the same. A bit later while on the vertical I heard a weak but readable signal from San Francisco. A contact was made and when I switched to the horizontal antenna, all was lost.
As this signal was coming on the horizon, and was vertical polarized, it shows that for local ground wave the polarizations should be the same for maximum signal transfer. The cross polarization from horizontal to vertical will lose allot–typically 20 to 30 db–over the short distrance while not losing so much over the long skip distance.
Just a note, nothing scientific or so. It does show that the need for some of us to have both polarizations is indeed a fact. My problem is that my vertical antenna is causing me audio rectification in the computer speakers. Need some chokes.
Just an observation. Ten has been pretty good about noon on here as has fifteen meters.
From the 10M Mendocino/Lake/Northern Cal net last night I got a better idea of how a 10M vertical performed for nearly-local traffic (see prior blog.) Generally, it confirmed that the more direct (line of sight) the signal, the more important the polarization. In other words, there are fewer obstacles to skew the polarization.
For example, experimenting with polarization in QSO’s with Steve, KJ6EIF, who was also running vertical and line-of-sight (6 mi), the difference in signal strength for vertical vs. horizontal was 12-15 db. So, that’s closer to the 20 db cross-polarization figures I mentioned in the prior blog.
For more distant signals, like Wayne, W6WMV, who was over the ridge and down in the valley just south of Clear Lake (23 mi), and also on a vertical, the polarization difference was only 3-5 db. That’s still significant because it brought his signal up to being readable.
Oddly, I noticed no difference in signal strength of polarization with Dave, N0EDS, who is on the north side of Clear Lake (20 mi) and puts out a great signal here. One possible reason is that my antenna field was being distorted in that direction due to the metal roof of my house. (As mentioned earlier, the test antenna was not that far off the ground.)
One other thing to mention from last night’s net, Lee, N6MIV, over on the coast in Gualala had a terrific signal (40 mi and multiple ridges). He was using a 200 ft horizontal wire (G5RV), and he could hear my horizontal signal, but not so much the vertical. We didn’t get a chance to determine the signal difference. Maybe next time we can experiment more with that.
Well, hopefully sometime in the next month we can repeat the experiment with the test antenna a little higher off the ground. Where it is now is quite likely causing a distorted field, which will throw off the results.
I’d like to invite all Mendocino and Lake county hams (and anyone else who can hear us) to participate in our 10M HF net on 28.405 MHz at about 20:15 PT Wednesday nights.