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