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Fall Antenna Projects at AB1OC/AB1QB

Anita and I like to take advantage of the mild fall weather to do antenna projects at our QTH. We have completed two such projects this fall – the installation of a Two-Element Phased Receive System and a rebuild of the control cable interconnect system at the base of our tower.

Receive Antenna System Electronics

Antenna Projects - NCC-1 Receive Antenna System Components
NCC-1 Receive Antenna System Components

Our first project was the installation of a DXEngineering NCC-1 Receive Antenna System. This system uses two receive-only active vertical antennas to create a steerable receive antenna system. The combination can work on any band from 160m up to 10m. We set ours up for operation on the 80m and 160m bands.

Antenna Projects - NCC-1 Receive System Antenna Pattern
NCC-1 Receive System Antenna Pattern

The NCC-1 System can be used to peak or null a specific incoming signal. It can also be applied to a noise source to null it out. The direction that it peaks or nulls in is determined by changing the phase relationship between the two Active Antenna Elements via the NCC-1 Controller.

Antenna Project - NCC-1 Filter Installation
NCC-1 Filter Installation

The first step in the project was to open the NCC-1 Control Unit to install a set of 80m and 160m bandpass filter boards. These filters prevent strong out-of-band signals (such as local AM radio stations) from overloading the NCC-1. The internal switches were also set to configure the NCC-1 to provide power from an external source to the receive antenna elements through the connecting coax cables.

Receive Antenna Elements and Coax

 

Antenna Projects - Installed Active Receive Element
Installed Active Receive Antenna Element

The next step in the project was to select a suitable location for installing the Receive Antenna Elements. We choose a spot on a ridge which allowed the two Antenna Elements to be separated by 135 ft (for operation on 160m/80m) and which provided a favorable orientation toward both Europe and Japan. The antenna elements use active circuitry to provide uniform phase performance between each element’s 8 1/2 foot whip antenna and the rest of the system. The antenna elements should be separated by a 1/2 wavelength or more on the lowest band of operation from any towers or transmit antennas to enable the best possible noise rejection performance.

Antenna Project - Received Antenna Element Closeup
Received Antenna Element Closeup

The two Antenna Elements were assembled and installed on 5 ft rods which were driven into the ground. To ensure a good ground for the elements and to improve their sensitivity, we opted to install 4 radials on each antenna (the black wires coming from the bottom of the unit in the picture above). The Antenna Elements are powered through 75-ohm flooded coax cables which connect them to the NCC-1 Control Unit in our shack. The coax cable connections in our setup are quite long –  the length of the pair being approximately 500 ft. The use of flooded coax cable allows the cables to be run underground or buried. Should the outer jacket become nicked, the flooding glue inside the cable will seal the damage and keep water out of the cable.

Antenna Project - RFC-1 Receive Line Choke
Receive RF Choke

It is also important to isolate the connecting coax cables from picking up strong signals from nearby AM Radio stations, etc. To help with this, we installed Receive RF Chokes in each of the two coax cables which connect the Antenna Elements to the NCC-1. These chokes need to be installed on ground rods near the Antenna Elements for best performance.

Antenna Project - Underground Feedline Conduit
Underground Cable Conduit In Our Yard

We ran the coax cables underground inside cable conduits for a good portion of the run between the antenna elements and our shack. The conduits were installed in our yard when we built our tower a few years back so getting the coax cables to our shack was relatively easy.

Antenna Project - Receive Antenna Coax Ground System
Receive Antenna Coax Ground System

The last step in the outdoor part of this project was to install a pair of 75-ohm coax surge protectors near the entry to our shack. An additional ground rod was driven for this purpose and was bonded to the rest of our station’s ground system. We routed both of the 75-ohm coax cables from the two Antenna Elements through surge protectors and into our shack. Alpha-Delta makes the copper ground rod bracket shown in the picture for mounting the surge protectors on the ground rod.

Shack Installation

 

Antenna Project - Equipment Shelf In Our Shack
Antenna Equipment Shelf In Our Shack (The NCC-1 Control Unit Is At The Bottom)

The installation work in our shack began with the construction of a larger shelf to hold all of our antenna control equipment and to make space for the NCC-1. The two incoming coax cables from the Antenna Elements were connected to the NCC-1.

Antenna Project - uHAM Station Master Deluxe Controller
microHAM Station Master Deluxe Antenna Controller

Antenna switching and control in our station is handled by a microHAM System. Each radio has a dedicated microHAM Station Master Deluxe Antenna Controller which can be used to select separate transmit and receive antenna for the associated radio. The microHAM system allows our new Receive Antenna System to be shared between the 5 radios in our station.

Antenna Project - Switching Matrix
Antenna Switching Matrix

The first step in integrating the Receive Antenna System was to connect the output of the NCC-1 to the Antenna Switching Matrix outside our shack. We added a low-noise preamp (shown in the upper left of the picture above) to increase the sensitivity of the Antenna System. The blue device in the picture is a 75 ohm to 50-ohm matching transformer which matches the NCC-1’s 75-ohm output to our 50-ohm radios. The other two preamps and transformers in the picture are part of our previously installed 8-Circle Receive Antenna System.

Protection From Overload

 

Antenna Project - Multi-Radio Sequencer
Multi-Radio Sequencer

The Antenna Elements must be protected from overload and damage from strong nearly RF fields from our transmit antennas. In a single radio station, this can be handled via a simple sequencer unit associated with one’s radio. In a multi-op station such as ours, it is possible for a different radio than the one which is using the Receive Antenna System to be transmitting on a band which would damage the Receive Antenna System.

To solve this problem, we built a multi-radio sequencer using one of the microHAM control boxes in our station. The 062 Relay Unit shown above has one relay associated with each of the five radios in our station. The power to the Receive Antenna System is routed through all 5 of these relays. When any radio transmits on a band that could damage the Antenna Elements, the associated relay is automatically opened 25 mS before the radio is allowed to key up. This ensures that the system’s Antenna Elements are safely powered down and grounded.

On The Air Performance

Antenna Project - NCC-1 Controls
NCC-1 Controls

So how well does the system work? To test it, we adjusted the NCC-1 to peak and then null a weak CW signal on 80m. This is done by first adjusting the Balance and Attenuator controls on the NCC-1 so that the incoming signal is heard at the same level by both Antenna Elements. Next, the B Phase switch is set to Rev to cause the system to operate in a signal null’ing configuration and the Phase control is adjusted to maximize the null’ing effect on the target signal. One can go back and forth a few times between the Balance and Phase controls to get the best possible null. Finally, the incoming signal is peaked by setting the B Phase switch to Norm.

Antenna Project - Peaked And Nulled CW Signal
Peaked And Null’ed CW Signal

The picture above shows the display of the target CW signal on the radio using the NCC-1 Antenna System. If you look closely at the lower display in the figure (null’ed signal) you can still see the faint CW trace on the pan adapter. The difference between the peak and the null is about 3 S-units or 18 dB.

Antenna Project - Peaked And Nulled SSB Signal With NCC-1 Used For Noise Cancellation
NCC-1 Used For Noise Cancellation

The NCC-1 can also be used to reduce (null out) background noise. The picture above shows the result of doing this for an incoming SSB signal on 75m. The system display at the top shows an S5 SSB signal in the presence of S4 – S5 noise. Also, note how clean the noise floor for the received SSB signal becomes when the unit is set to null the noise source from a different direction than the received SSB signal.

We are very pleased with the performance of our new Receive Antenna System. It should make a great tool for DX’ing on the low-bands. It is a good complement to our 8-circle steerable receive system which we use for contesting on 160m and 80m.

Other Antenna System Maintenance

Antenna Project - Tower Control Cable Interconnects
Tower Control Cable Interconnects (Bottom Two Gray Boxes)

Our other antenna project was a maintenance one. We have quite a number of control leads going to our tower. When we built our station, we placed surge protectors at the base of our tower. We then routed all of our control leads through exposed connections on these units. Over time, we found that surge protection was not necessary. Also, we became concerned about the effects that sunlight and weather were having on the exposed connections. To clean all of this up, we installed two DXEngineering Interconnect Enclosures on our tower and moved all the control cable connections inside them.

Antenna Project - Inside View Of Interconnect Enclosures
Inside View Of Interconnect Enclosures

We began with a pair of enclosures from DXEngineering and we mounted screw terminal barrier strips on the aluminum mounting plates in each enclosure. The aluminum plates are grounded via copper strap material to our tower.

Antenna Project - Closer Look At The Interconnects
Closer Look At One Of The Interconnect Enclosures

The picture above shows one of the interconnection boxes. This one is used to connect our two SteppIR DB36 Yagi Antennas and some of the supporting equipment. The barrier strips form a convenient set of test points for troubleshooting any problems with our equipment on the tower. There are almost 100 control leads passing through the two enclosures. This arrangement keeps everything organized and protected from the weather.

With all of our antenna projects complete, we are looking forward to a fun winter of contesting and low-band DX’ing.

Fred, AB1OC

Antennas, Featured, General, Homebrewing, Newsletter

Moxons in the Attic (Part 2)

 It has been a couple of months since I wrote about my project to build a Moxon antenna for 15 and 17 meters in the attic of my garage.  The weather has since cooled down to where I can work in the attic without sweat dripping in my eyes and my hands slipping on everything.  When I wrote about my initial measurements in the last article, I was experiencing erratic SWR readings on the 15-meter beam with the lowest SWR being around 23 MHz.  I did some recomputing and figured I would need to lengthen each element by 24 inches to bring resonance down to 21.1 MHz.  I was not too thrilled with the idea of having to solder wires in the cramped space of the attic so I decided to check the SWR behavior again.

There is an old adage in carpentry that says measure twice, cut once.  The same applies for adjusting antenna elements except “twice” becomes “until consistent”.  When I connected my analyzer to the 15-meter beam, I did the flex test of the connecting cable.  Lo and behold, the SWR started jumping around.  I checked the connector hardware and noticed the PL-259 reducer shell was loose.  Once I tightened it, I found the resonant point to be way down at 19.0 MHz.  My antenna was too long; it would have to be SHORTENED by 24 inches.

Armed with the new readings, I shortened each element accordingly and measured the antenna again.  My efforts paid off with a reading of 1.1:1 at 21.2 MHz and a 2:1 bandwidth from 20.8 to 22.78 MHz.  I was now ready to move on to final installation.

I needed more coax, a couple of baluns, and a remote antenna switch to complete the project.  A hamfest scheduled for the first weekend in October in Melbourne, FL looked like a good prospect for finding what I needed.  Unfortunately, Hurricane Matthew had other plans and forced a postponement of two weeks.  When I attended the hamfest, many vendors were absent due to conflicting plans so pickings were slim.  I did manage to find the baluns and more coax but the switch would have to be ordered.

I found an Ameritron RCS-4 online at a reasonable price and ordered it.  I had used this model for many years when in NH so I was familiar with its reliability.  When the switch arrived, I hooked it up to check it out before installing it.  Murphy said hello to me with a non-functioning control unit.  I called the company I bought it from and they arranged to have it returned for a replacement.  I finally received a working unit two days before Thanksgiving.  With the CQ WW CW contest coming up, it came just in time.

I routed a 50-foot run of coax from my shack around the front of the house and into the garage attic.  Figure 1 shows the coax run.  If you can’t see it, good; I do not want the Village Vigilantes to come knocking on the door to question the aesthetics of the cable.  Figure 2 shows a closer view of the coax entering my attic.  So far, the XYL hasn’t noticed it so I’m safe.

Stealth Antenna - Coax Run Across Front of House
Figure 1. Coax Run Across Front of House
Stealth Antenna - Coax into the Attic (Upper Left)
Figure 2. Coax into the Attic (Upper Left)

The next step involved hooking up the baluns and the remote switch which was a straightforward process.  Once everything was in place, I fired up the rig on 17 meters and found a spot for UA0ZC.  I was happy to hear him and gave Val a call.  A minute later I had a rare one in the log without having to hammer away indefinitely.  I checked 15 meters and did not find much activity.  The operational SWR was a bit higher than my measurements but still under 2:1.

At least, it started out that way.  I got on the air the next day and found the SWR on 15 meters hitting 6:1 and higher.  I made a trip up to the attic to find out what was going on.  At first glance, nothing appeared amiss.  I plugged the analyzer into the 15-meter beam and noticed the SWR jumping around.  Flexing the cable made some difference but not much.  My initial thought had been some incomplete switching in the switch unit but the SWR behavior when directly connected to the analyzer ruled that out.  I tried swapping baluns between the two beams to no avail.  With other pressing holiday matters to attend to, I decided to remove a balun from the 17-meter beam in favor of the 15-meter beam.  (During my troubleshooting efforts, I noticed the 17-meter beam behaved as designed, with or without a balun.)  The 15-meter beam shows no discernible difference in performance with or without the balun.  Figure 3 shows the present feed point installation with the switch on the attic floor.

Figure 3. Feed Points and Switch
Figure 3. Feed Points and Switch

I have to admit that I am stumped at this point.  There is some consolation, however, in that my K3 tuner easily matches up the 6:1 SWR imbalance.  I imagine there is interaction with the other structures in the attic (house wiring, AC ducts, 17-meter beams, etc.) that are making a good match difficult to achieve.  At any rate, I am happy to have a worthwhile antenna for 17 meters vice my low inverted V.  As the sunspots continue to degrade, 17 meters may well end up as the MUF.

Ed, K2TE

 

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

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

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   

Current Profile on a Half Wave Dipole Antenna
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.

1/4 Wave Vertical. Note the 7 spaced lamps.
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 7th lamp.

Transmitting mode. Note pattern of lit and unlit lamps.
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.

Illustration of Dipole RF Radiation Pattern
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.

  1. 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.

  1. 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