Wayne, KB1HYL hid his fox, which gave us all a challenge to find it. Here are Jamey and the Wayne getting ready to hunt the fox.
We had a potluck meal and more good food than we could eat. Thanks to all members who brought food and to Merle for bringing her grill and doing the grilling. John welcomed a special guest who was quite friendly. Now that was quite an eyeball QSO!
Thanks to everyone who came out and made our picnic a lot of fun!
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
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.
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 7th lamp.
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.
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 it’s 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.
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.
In an effort to find speakers for the club meetings, I have been reaching out to other HAMs in the community. One HAM, Mark Richards (K1MGY), serves on the Amateur Radio section of the Boston Athletic Associations (BAA) Communications Committee and is reaching out for help with some events. If you are interested in helping out with any of the below events, please contact Mark at email@example.com.
Jimmy Fund Walk, this Sunday. Opportunities to work medical tents along the Boston Marathon course with a top-notch comms team.
Nashua ARC will be hosting its first Youth Outreach event on Saturday, October 1 from 8am-2pm at Mine Falls Park in Nashua, NH. Mine Falls Park is located on Whipple St.
Mine Falls Park hosts many youth soccer games every Saturday throughout the Fall. We are hoping to take full advantage of that fact in order to provide kids a bit of insight into what comprises amateur radio.
Some of the activities that we will likely host are:
Get-On-The-Air (GOTA) Station
Use some HT’s and get on the Nashua repeaters
This we hope is to generate enough interest so that we can plan a follow-on activity a bit later.
Now comes the part where AB1ZO looks at you warmly, scratches his cheek, and asks you for a favor. It might go down something like this:
And that favor, is that we really do need your help to come out to the Park:
Help us set up beginning around 8am
(Setup is pretty important to have able bodies around for. We’ll shoot to get on the air as quickly as possible after. Perhaps by 9am)
Work the booth
(Most importantly) Interact with the kids.
Help breakdown around 2pm
The more the merrier and the longer you’re able to give a bit of time, the better as well. Additionally, you are completely encouraged to bring family and friends, especially if they are < 18 years old. But in all honesty, I think it would be fantastic if we had a significant contingent of the club at the Park, to welcome our guests. But don’t do it for me, do it for the kids:
If you think you can give some time, please contact myself (firstname.lastname@example.org) or Jamey (KC1ENX) to let us know what you would like to help with. Again, we really could use the volunteers in all the aforementioned areas. We’re hoping this is a facet of the club you will all embrace as it evolves.