Category Archives: Antennas

Articles about Antennas, Feedlines, Towers and related equipment. Fixed, Portable and Mobile Antenna Articles are included.

Antennas, Education and Training, Featured, General, Newsletter

Yagi Antenna Teaching Construct Part #3: Directional Gain and Front to Back Ratio

November 8, 2016 Nashua Area Radio Society

In this final article of our series on the “Lego-style” antenna for teaching basic antenna physics and behavior, our focus is a Yagi-Uda 3-element antenna for 2 meters. The Yagi antenna is the most common construction form for a “beam” antenna. The Yagi has high gain in one specific compass (azimuth) direction and very low gain in the opposite direction when compared to the classic 1/2 wave dipole. The Directional Forward Gain and “Front to Back” Ratio (F/B Ratio) features are hallmarks of the Yagi and the reasons for its high popularity. The forward gain dramatically increases “Effective Radiated Power” in a chosen direction; the Front to Back Ratio dramatically reduces interference from signals to the antenna’s backside.

BASIC PRINCIPLE OF THE 3-ELEMENT YAGI ANTENNA

The Yagi antenna shown in the Feature Picture above has 3 radiating elements. The center element (with the coax attached) is the “Driven Element” (DE) receiving RF energy from the transmitter. The other two elements are “parasitic”, meaning they are not connected to the transmitter. These elements absorb energy from the DE RF wave and re-radiate it by Faraday’s Law of Induction. This fact gives both parasitic elements a 180-degree phase shift relative to the DE. In the above picture, the left side element is the Reflector, typically 5% longer than the DE; the right side element is the Director, typically 5% shorter than the DE. The length differences make both elements slightly off resonance to the DE RF wave which adds a second phase shift relative to the DE. Finally, the distances on the boom between the DE and parasitic elements adds a third phase shift since it takes a finite time for the DE wave to reach the Reflector and Director. The distance between the DE and the parasitic elements vary between 0.1 to 0.25 wavelengths depending on materials, construction details and design goals for the forward gain and F/B Ratio.

The Yagi secret is that the sum of these three phase shifts for the 3 elements add in a way that reinforces each other for RF energy moving towards the Director. However, RF waves moving towards the Reflector combine in a way that subtracts and leaves little RF energy.

Figure 2 provides an illustration of this action using only waves interacting between the DE and Director, for simplicity.

Figure 2. Summing RF wave from the Driven Element with the Induced wave from the Director Element (from Wikipedia, Yagi-Uda Antenna)

The DE RF wave (in green) reaches the Director parasitic element and induces a second RF wave (in blue) by Faraday’s Law. The multiple phase shifts that make up the Director RF wave gives a forward moving emission that adds to the DE wave yielding a combined larger RF wave. However, the Director RF wave moving back towards the Reflector is nearly 180 degrees out of phase to the DE wave and the two nearly cancel each other out. A similar analysis at the Reflector element shows that Reflector waves moving towards the DE and Director also adds to their waves to create an even larger wave. However, Reflector waves traveling off the rear of the antenna subtract with waves arriving from the DE and Director yielding very little RF energy leaving the backside direction of the antenna.

THE TOOLBOX FOR OUR EXPERIMENTS

The top Feature Picture shows the “Lego-Style” antenna in a Yagi assembly and the Lamp Bridge Receiver ½ wave dipole antenna is seen in the upper right corner. The Lamp Bridge Receiver lights when resonant RF is sensed and we used this tool in both Parts 1 and 2 of this article series.

New to our toolbox is a breadboard circuit to measure RF power in a semi-quantitative way using an LED Bar Graph display. The breadboard was built on an Analog/Digital Trainer Module and is seen in the lower right corner. The circuit samples the RF signal received by a 1/4 wave antenna (black antenna seen behind and above the breadboard). The induced RF current is converted to a dc voltage that feeds to a 30-stage linear increasing voltage comparator circuit. A comparator turns on an LED when its voltage threshold value is reached. Our circuit is using only 15 LEDs due to breadboard space limits; however, by skipping every other comparator we are still measuring a 30 fold signal range. The picture shows 3 lit LEDs indicating a sensed voltage that is one-fifth the dynamic range of the circuit.

EFFECTIVE RADIATED POWER: COMPARING THE DIPOLE and YAGI-UDA ANTENNAS

A) BASIC DIPOLE PERFORMANCE

We will use the classic 1/2 wave dipole as the benchmark to assess benefits of the 3-element Yagi-Uda antenna. We begin by measuring relative radiated power of the dipole as we increase transmitter power stepwise between 5, 10, 20 and 40 watts. The power detector circuit consists of a standard 1/4 wave stub antenna connected to the LED Bar Graph RF meter. The results will be used to “calibrate” the detector circuit and be our reference point when we calculate Yagi Gain.

The video below explains the use of the LED Bar Graph RF meter; then it shows the actual testing protocol and you can see the LEDs report received RF signal as we increase Tx power to the dipole.

The results of received RF signal versus transmitter power for the dipole antenna are summarized in Table 1. The data show a close to linear correlation (as expected) within the semi-quantitative limits of the Bar Graph Display.

B) 3-ELEMENT YAGI PERFORMANCE

The next video shows the antenna forward radiated power as we stepwise build the Yagi beginning with the basic dipole; the addition of the Reflector element and a third addition of the Director element. Note, the Tx power is constant at 5 watts. Details of the Yagi construction (element lengths, spacing, etc.) are given in the video.

The data for the RF radiation received with the stepwise addition of elements to assemble the YAGI antenna are summarized in Table 2.

There is a significant increase in antenna forward received signal (looking towards the direction of the Director) as we add the two parasitic elements. The signal increases by 3+ fold with the Reflector over the dipole and then by 7+ fold for the combination of Reflector plus Director. However, we want to translate the results to customary power level values in decibels. The combined data of Tables 1 and 2 let us do this as an estimate of Directional Forward Power.

Gain for 2-Element Antenna in dB:
7 Lit LEDs at 20 watts for dipole and 5 watts for Yagi
dB = 10 x Log(20/5) = 6.0 dB (= 1 S meter unit)

Gain for 3-Element Yagi in dB:
15 Lit LEDs at 40 watts for dipole and 5 watts for Yagi
dB = 10 x Log(40/5) = 9.0 dB (= 1.5 S meter units)

THE FRONT TO BACK RATIO OF EFFECTIVE RADIATED POWER FOR THE 3 ELEMENT YAGI ANTENNA.

A) THE 1/2 WAVE DIPOLE

We begin our experiments on Front to Back received power using the basic 1/2 wave dipole as we did above for forward radiated power. However, since the dipole is symmetric side to side we will label our measurements as “left side” and “right side” to the wire axis. Second, our dipole experiment will use the Lamp Bridge Receiver antenna because we will use power levels not requiring amplification (the LED Bar Graph RF meter has a 10x gain built-in). Also, resetting the LED RF meter at the opposite table end is not easy.

The 2-sided picture below shows the responses of the Lamp Bridge Receiver antenna to a 40-watt transmission. Picture 1A shows the received signal on the left side of the dipole; Picture 1B shows the response on the right side of the dipole.

As you would expect, the radiated power of a basic 1/2 wave dipole appears equal broadside to the wire axis, as we learned in the Technician License Class.

B) The 3-Element YAGI-UDA Antenna.

Our last video has a simple experiment showing the Front to Back Ratio effect of the Yagi antenna for radiated power parallel to the boom axis. We return to using the LED Bar Graph RF meter for the experiment. The study is made easy by the simple trick of swapping placement of the Reflector and Director elements, a benefit of the “Lego-Style” construction of our antenna model.

The study results are striking. The data for the Forward RF signal from the Yagi showed an 8x fold increase over the basic dipole with 15 lit LEDs at 5 watts for the Yagi versus the dipole needing 40 watts to elicit the same response. We also saw this result in Video 2 (compare data in Tables 1 and 2). In contrast, the received Back-Side RF signal gave only 1 lit LED. The difference merits visual repetition with a paired picture display.

We can transform this data to estimate dB Power Gain of the Back-Side signal in a similar fashion as the Directional Forward Gain. Again, we use data from Tables 1 and 2. However, we need to interpolate the signal response of the Yagi Back-Side signal of 1 LED with the 2 LED response for a dipole at 5 watts. The factor is 1/3, not 1/2. Why? Remember, the RF detector circuit divides the received signal into 30 equal size buckets, but we only look in every other bucket with an LED. Hence, LED #2 measures the third bucket, not the second bucket. I will repeat the Forward Gain calculation here to make the result comparison easier.

Forward Gain for 3-Element Yagi in dB:
15 Lit LEDs at 40 watts for dipole and 5 watts for Yagi
dB = 10 x Log (40/5) = 9.0 dB (1.5 S meter units)

Backside Gain for 3 Element Yagi in dB:
1 LED (Yagi) = 1/3 (5 watts) =1.7 watts
dB = 10 x Log (1.7/5 ) = – 4.6 dB

We now can easily calculate the Yagi Front to Back Ratio:
F/B Ratio dB = 9.0 dB – (-) 4.6 dB = 13.6 dB
(Remember, dB is a logarithm value, so we subtract the 2 numbers, not divide them)

C) Results Analysis

The result of 9.0 dB for Yagi Forward Gain is likely high considering an expected typical range of 6 to 8 dB (seen in commercial units). The estimate for the F/B Ratio of 13.6 dB seems low, again based on typical commercial units that can have an F/B of about 20 dB. However, the cited values are for “Far Field — Free Space” conditions; conditions not simulated in a 20 x 20 ft. room in my house. Also, the experiments made no effort to optimize Forward Gain or the F/B Ratio by element lengths and spacing.

The list of experimental error sources in our studies are many; a partial list includes detector circuit layout that combined RF, digital and DC signals on one board, the accuracy and precision limits of discrete LEDs, antenna height above ground issues and wall-plus-apparatus surfaces that both absorbed and reflected RF signals.

Still, the “Lego Style” Yagi Antenna Assembly permits an easy way to demonstrate many basic antenna properties while showing performance results that are reasonable. Perhaps, the next ham adventurer to design Version #3 will expand experimental versatility and improve performance areas.

I hope this series of three articles has expanded antenna knowledge to newly minted hams and has re-kindled interest in antenna experimentation for more experienced hams (many more knowledgeable than me). Coming full circle to my thoughts as I began this article series, our antennas are the magic carpet that we ride over the airwaves whether to friends across town or to that rare DX station 10,000 miles away. Enjoy the Ride!

I owe a heartily thank you to Skip Youngberg (K1NKR) for reviewing my draft manuscript for Part #3 and contributing valued suggestions. Also, the complete series of three articles could not have happened without the multi-media assistance, encouragement and full support from a special YL, Teresa Mendoza.

73,

Dave N1RF

Antennas, General, Homebrewing, Newsletter

Hang ‘Em High

October 14, 2016 Nashua Area Radio Society

Dick Powell, WK1J

A Little about me

At 75 yrs old, I have a modest station, consisting of a Mosely TA33 Tri-bander (circa 1986) on a homebrew mount, at the peak on my single-level home (45ft.) and a homebrew 160-80-40M inverted Vee at 65ft. I wound all the loading coils for the 80M and 160M traps and it performs very well and takes only 10 ft. more space than a typical 80M inverted Vee dipole (excellent for 160M on a city sized lot). I plan to write an article on its construction in the future.

I have worked 90 countries on 160M, 96 on 80M and 303 on 40M with only 100 Watts output with this antenna. I am fortunate to be at 840 ft. above Sea Level with a clear shot to Europe, South America, and the Caribbean. Japan is workable but I struggle to get through west coast stations to Asia and the Pacific. Four more confirmed countries on 80M and I will have worked 5Band-DXCC with 100W, proof that at my age, you don’t need a tower, kilowatt and the latest, greatest radio, although they make the challenge easier.

This summer I started in earnest to revamp my “small pistol” station, knowing that the sun spots are declining and that I needed to improve my low band 160m, 80m and 40M transmitting antennas. As well as my ability to hear weak signals better with the increased noise on the bands.

Latest Project: Beverages and 9-Circle Receiving Array (an article coming in the future)

This summer, I worked on improving my ability to receive weak signals by building switchable, (bi-directional) beverage antennas for NE/SW, NW/SE and E/W, switchable from my operating position. You know what they say. “If you can’t hear them, you can’t work them!”

Currently, I am in the final stages of building and deploying a 9 circle receiving array developed by the Yankee Clipper Contest Club of which I am a member. Note: The components are now offered as a kit in partnership with DXEngineering, with DXE supplying all the interconnecting cables and phasing lines. My preliminary tests show a significant improvement (8db better signal RX strength and lower noise floor by 2S units) over the beverages and it is steerable every 45 degrees. The ability to null out interference is unbelievable even when compared to the beverages. Now I need to work them! I am currently building a 160M/80M “double L”(no ground radials needed), separate 40M and 80M Delta Loops, replacing my older dipoles. I hope to have these in place by CQWW SSB later this month.

Now this Article: – Antenna Launcher

How to get these new wire antennas hung from the many tall pine trees on the property? In the past, I have used a sling shot to get dipoles up but usually only 50-60 ft. consistently.

I read with interest the recent article that Brian, AB1ZO wrote: “I can’t believe my antenna’s up!” Seeing the pictures of the method he used to toss a line over the tree got me to thinking about a better approach. I wanted something reliable, easy to use, easy to make (not buy), and portable in the woods, no electricity/batteries, no butane and spark ignitors (I would probably cause a large forest fire, hi!). In other words “a field day” type solution. I also recalled Dennis, K1LGQ’s presentation on the “potato launcher” he demonstrated at project night.

I viewed many YouTube videos and found a lot of excellent approaches. I settled on a pneumatic (compressed air) approach. I designed the antenna launcher to be very compact, most were quite long and not ideal for trudging through dense woods. By no means is this approach unique, but it is proving to be very reliable, cheap (less than $60 in materials, if purchased) and can easily reach heights of over 150ft. I use a simple bicycle tire pump (found for $25 on eBay) to fill the compression chamber. A compressed C02 air refill canister for a Paintball gun or a battery operated car tire pump would also work.

This is a picture of the completed Antenna Launcher. It took 2, 4 hour days to complete as I waited 24 hours to ensure the PVC cement cured in the pressure chamber (important safety step). I chose to spray paint it and added labeling.

I build the launcher in 4 main sections:

Pressure Chamber
A Modified Inline Irrigation Valve
U section (for a compact design)
Launching Tube

Step 1 – Pressure Chamber

I looked at both 3in. and 2in PCV schedule 40 Pipe. The box stores do not carry 3in PVC rated for pressure applications. Some videos on YouTube do use it; I chose the 2in for safety reasons (max. 280psi).

Below is a picture of the pieces needed to assemble the pressure chamber, including the tire (Schrader) valve and pressure gauge (optional). I opted to have the pressure gauge on the chamber when filling it, rather than having to read one located on a tire pump, I blame the bifocals, it couldn’t be old age!

The next 5 pictures show the progression of the assembly. For brevity, (in this article) the pictures will give you a reasonable idea of the construction. I plan to create an accompanying (downloadable) PDF document detailing all the steps with instructions.

Step 2 – In-line sprinkler Valve Modifications

This picture shows the parts needed to modify a common irrigation valve for air pressure vs electrical use. A good YouTube video of modifying the Rain Bird HD 1 in. valve can be found at https://youtu.be/A3EOdNP6Iag

The next 7 pictures show the detailed progression of the modification. They may be a little easier to see than in the video.

Step 3 – The “U” Assembly (or let’s turn the corner!)

I wanted the launcher to be as compact as possible for better portability and chose to assemble some pipe to make a “U” turn, prior to installing the actual launch tube.

The following 2 picture shows the detail of the assembly of the U-turn.

Step 4 – The launcher’s “business end” where all the work gets done…

Conclusion:

I hope I haven’t put you to sleep by now. This was a fun project and it works really well and will last for many trips into the woods in the future. There are very few mechanical parts which could fail (only the inline valve and blow gun) and the selection of higher PSI PVC ensures a good degree of safety, even if over inflated a little. The inline valve is the “weak link” so to speak, rated at 100psi max.

Now to go and “Hang ‘Em High”

73 Dick, WK1J

Questions: [email protected] [email protected]

Antennas, Education and Training, Featured, General, Homebrewing, Newsletter

Yagi Antenna Construct Part #2: Current, Voltage Profiles, and Dipole Pattern

September 23, 2016 Nashua Area Radio Society

In Part 1 of this article series, I presented the “Lego” 2 m 3-element Yagi antenna design that the N1FD ham license teaching team has used over the past year for class demonstrations. The design allows easy assembly of the basic dipole antenna as well as a 3 element Yagi. The configuration of individual elements and spacing between elements can be quickly changed to demonstrate basic physics and behavior of these popular antennas.

The first article described antenna construction details and showed how to demonstrate the criterion for resonance as well as the polarization property of the radio wave. In Part 2 of the series, I will continue a focus on the dipole, specifically the spatial current – voltage profiles on the driven element and the radiation pattern of the antenna. We will use this information in Part 3 of the series next month to demonstrate how a 3-element Yagi works and why it is so popular.

THE CURRENT & VOLTAGE PROFILES ON A HALF WAVELENGTH DIPOLE

Figures 1a and b – Current Profile on a Half Wave Dipole Antenna

Figure 1a reminds us of the basic dipole geometry; and Fig. 1b shows the current and voltage profiles along the driven element. (From http://www.radio-electronics.com/info/antennas/dipole/half-wave-dipole.php)

Note from Fig. 1b that the current profile of a dipole has a maximum current level at the center feedpoint and decreases to zero current at the end of each element arm. Contrasting, the voltage profile has a zero value at the feed point and increases to a maximum level at the ends of the element arms.

Figure 2 – 1/4 Wave Vertical. Note the 7 spaced lamps.

The 1/4 wave vertical antenna seen in Figure 2 can be used to visualize the current profile along the arms of a 1/2 wave dipole. The 1/4 wave antenna is made from a short length of a Christmas tree (incandescent) light string. The string length can be estimated from the standard equation: Length (ft) = 234/Frequency in MHz. Generally, several inches needs to be trimmed off because the lamps add “electrical length”. The shown antenna has the same resonance frequency as the Lego Style dipole we will use later (i.e., 146.550 MHz). The top end of the antenna is marked by the blue tape immediately above the 7^th lamp.

Figure 3 – Transmitting mode. Note pattern of lit and unlit lamps.

The energized antenna with 15 watts RF signal is seen in Figure 3. Compare the pattern of lit and unlit lamps with the current profile sketch shown in Fig. 2b. The three lamps, counting from the picture bottom are brightly lit from an RF current. Lamps 4 and 5 show progressed less light indicating a lower RF current. Lamp number 6 is barely lit and number 7 is dark indicating together very little to no RF current at the element top end. The light pattern is a clear mimic of the diagram in Figure 1b.

Demonstration of the Voltage and RF Radiation Profile on a Half Wave Dipole Antenna

The voltage profile on a center fed 1/2 wavelength dipole is seen in Figure 1b. As mentioned above, the voltage is zero at the dipole center and increases in monotonic fashion to a maximum value at the antenna ends.

Figure 4 – Illustration of Dipole RF Radiation Pattern

The familiar RF radiation pattern of a dipole is shown in Figure 4 (taken from the cited source for Figure 1).

We are all well-schooled on the pattern, so I will just list the three key facts. First, the RF radiation is broadside to the antenna axis. Second, the RF field intensity is equal on the left and right sides of the dipole axis (i.e., there is no discerned “front to back” sidedness. Third, there is (theoretically) no RF radiation off the ends of the dipole wire.

The dipole voltage profile and the RF radiation pattern can be demonstrated using the basic dipole element of our “Lego Style” antenna and two simple tools. The voltage profile, or more correctly, the electric field strength around the dipole is sensed by a small fluorescent light tube. The actual RF radiation from the energized dipole is sensed by the flashlight lamp-bridged receiver antenna introduced last month in Part 1 of this series.

Direct RF Radiation Visual Detection

The video below (double-click in the picture box) demonstrates the use of the lamp-bridged receiver antenna to detect radiated RF power.

The video shows the flashlight bulb bridging the handheld receiver antenna lights up when it detects an RF signal that matches its resonance point at 146.550 MHz The light bulb is dark with no transmitted RF power from the Lego dipole. Keying the radio energizes the Lego dipole and the receiver lights up about equally on the right and left sides of the Lego antenna. This reflects the figure 8 pattern of RF power illustrated in Figure 4.

Voltage Profile witnessed by the Electric Field Strength.

The next video (double-click in the picture box) employs the fluorescent light bulb to map the voltage profile along a dipole arm by sensing its electric field strength. An RF electric field causes a series of chemical reactions within the light bulb that produces a bright fluorescent light.

The light bulb is dark when the Lego dipole is not transmitting an RF signal. Keying the radio generates an RF signal and the associated electric field around the dipole element causes the bulb to light up. Note, the bulb is very bright adjacent with the side end of the dipole arm and extinguishes as it is moved to the dipole centered feedpoint. Also, the light is dimmed at the antenna tip in-line with the dipole axis.

The voltage profile map seen in the fluorescent light bulb video augments the RF signal map seen in the lamp-bridged receiver antenna video. Also, it extends our demonstration to the expected observation that there is (theoretically) no RF radiation off the end tips of dipole elements.

CONCLUSION

In this second installment of our Lego-Style Antenna series, we have shown how this construct together with two simple tools can be used in the classroom to demonstrate basic properties of the ubiquitous dipole antenna; Namely, criterion of resonance, generation of RF radiated waves, the polarization of the RF field (horizontal or vertical) and the general propagation geometry of these waves relative to the antenna orientation.

In Part 3 and last installment of this series we will continue to use the Lego-Style Antenna in its’ Yagi configuration together with the two accessory tools to show how properly designed and placed reflector and director elements on the Yagi antenna can shape and control the dipole rf signal to increase gain via spatial directivity and improve signal selectivity by the “front-to-back” ratio that it creates.

73 & Hope to hear you on the air,

Dave N1RF

Radio Amateurs Developing Skills Worldwide