Tag Archives: Antennas

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

Antennas, Featured, General, Homebrewing, Newsletter

Moxons in the Attic (Part 1)

September 17, 2016 Ed Deichler 1 Comment

A few months ago, I wrote an article on building a stealth antenna farm. Since I live in the land of CC&Rs, antennas must be “dual use” such as a vertical hidden inside a PVC flagpole, or low dipoles and inverted Vees hidden in trees. I spent many years as an avid contester and DX-chaser to appreciate the logic of stacked beams on towers to enhance the thrill of the hunt. Thanks to Layne, AE1N, I checked out the website of Jeff, AC0C (www.acoc.com) for some ideas of how to build a multi-band station in one’s attic without the condo association vigilantes running him out of town. Jeff spent countless hours crawling around his attic to construct multi-element antennas for 160 through 6 meters. Spurred on by Jeff’s success, I decided to explore the attic of my garage to see what I could do. As I described in the MAY Nashua ARC bulletin, I settled on building Moxon antennas for 15 and 17 meters.

An old adage about antenna building states that an antenna must be built-in lousy weather in order to work right. Thirty years in New Hampshire lent credence to this axiom as I spent many a cold, windy day on a tower doing antenna work. In Florida, a similar law applies: build an antenna in the summer months while sweating profusely rather than during the comfortable winter weather. Again this makes sense: DX and contesting fill up the winter months to have time to mess with antennas. It is also important to remember that, during such endeavors, you will become enamored with you antenna as you take breaks to warm your body (in NH) or drink a gallon of water (in FL), all the while cursing this law of antenna building.

But I’m getting ahead of myself. My garage attic is roughly 20 x 20 feet with an apex of about five feet that runs north-south. I had selected the Moxon design because a conventional 2-element beam would not fit in the space available. I elected to build a Moxon for 15 and 17 meters that would fit in the space available without having to encounter obstacles like the ventilation duct work. I was also fortunate in that my home is one of the older models that do not have foil-backed insulation inside the roof that creates a radiation-proof box. The joists junctions are reinforced with metal plates as part of hurricane building codes. My plan was to attach the wires to the roof trusses and stay away from these plates as much as possible to avoid interaction.

The 17-meter Moxon is a little over seven feet between elements while the 15-meter Moxon is about six feet between elements. I used a piece of half-inch PVC pipe as a template to mark the joists for the 17-meter antenna. The antenna is about 2.5 feet above the attic floor for a total height of eleven feet above the ground. I had modeled it at 13 feet so I figured it would be close enough. The 15-meter Moxon is about 15 inches above the 17-meter one. When viewed from the top, the antennas look like concentric rectangular loops.

Over the course of several weeks, I grunted, groaned and sweated my way back and forth measuring and installing the wires. I worked during the morning hours before I was soaked before 10:00 AM. I found myself wishing I could have my five-year-old grandson help me. He can stand upright and is plenty flexible to maneuver around the joists. While I did not have to worry about the obvious safety issue of working on a tower, I did at times feel I was a candidate for the NFL concussion protocol from bumping my head. I tried using my cycling helmet but it interfered with my headlamp. Another similarity to tower work is that I had to make N trips back and forth in the attic for stuff I forgot. This is, however, much more bearable that climbing up and down a tower to get what I forgot.

Figure 1 shows a view toward the south end of the attic. The two pieces of PVC form the element separators for the 15-meter Moxon (top wire) and the 17-meter Moxon (bottom wire).

Figure 1 – Moxon Element Separators, 15m (top) & 17m (bottom)

Figure 2 shows the reflector elements for each antenna as secured to the joists, looking north through the attic. The white standoff fasteners are coax cable tie-downs that I found at the hardware store. Standard house wiring fasteners would have worked but they leave little room for pulling wires if I needed to make adjustments. (My first attempt was to use duct tape to hold the elements up. However, the heat soon made them droop.)

Figure 2 – Moxon Reflector Elements, 15 m (top) & 17m (bottom)

Figure 3 shows one corner of the director of each antenna looking east. The duct work to the left is part of the ventilation system while the open duct vents directly from the garage below. The yellow fence standoff on the upper antenna is the bend point for one end of the 15-meter director. Not visible to the left is a similar bend point for the 17-meter director.

Stealth Antenna - Looking East-from the Reflectors — Figure 3 – Looking East-from the Reflectors

Figure 4 shows the temporary feed points for each antenna.

Stealth Antenna – Feedpoint for 15m (top) & 17m (bottom) — Figure 4 – Feedpoint for 15m (top) & 17m (bottom)

The figures above show the project to date. I installed the 17-meter antenna first and measured its SWR performance with my analyzer. I found that it resonated beautifully at 16.7 MHz with a 1.1:1 SWR while bulging to 3.9:1 at 18.1 MHz. I shortened each element by a foot and ran measurements again, this time the resonant point moved up to 17.3 MHz (1.3:1 SWR) and the SWR at 18.1 MHz dropped to 2.7:1. I folded the elements back another four inches on each end and measured the response. I observed the SWR bottoming out at 1.5:1 at 18.1 MHz where I wanted to be. As a point of interest, I modeled a Moxon designed for 16.7 MHz and noticed the elements were about two feet longer than a Moxon designed for 18.1 MHz, close to the twenty inches I had to shorten the elements. Apparently, there is some interaction with the wiring that runs along the attic floor near the edges.

Armed with the satisfaction I was on the right track, I installed the 15-meter Moxon above the 17-meter antenna. I hooked up the analyzer and fired it up only to find to that its “resonant” point was a dismal 3.0:1 SWR at 22.9 MHz, rising to 3.9:1 at 21.1 MHz. This meant my antenna was too short. I went back to EZNEC, opened the standard dipole model and plugged in 22.9 MHz and found that its length was very close to the overall driven element for a Moxon designed for 21.1 MHz. I lengthened each element by five inches as a starting point to see what would happen. The result was no change in SWR at 22.9 MHz while dropping slightly to 3.6:1at 21.1 MHz. Hmm, looks like I need to get a little smarter about this.

Stay tuned for Part 2 to find out. (Don’t you hate that?)

Ed, K2TE

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