WA4PTZ Loop Analysis

Using the sketch Tim provided of his loop antenna, I entered the measurements into the 4nec2 program and got the far field patterns shown below. These are calculated with a 50-ohm generator at the feed point -- no feed line included. I expect that the field patterns would be similar with the feed line included.









160 m horizontal gain. Major lobes approximately north-south.










160 m vertical pattern. Mostly straight up, as expected.











75 m horizontal pattern. 4 lobes











75 m vertical pattern. 2 lobes; angle of radiation moving away from vertical.










40 m horizontal pattern. More lobes appearing.











40 m vertical pattern. Lobes continue to move toward lower angle.











20 m horizontal pattern. Lobes increasing rapidly and some nulls are deep.










Here's the 3D color coded version of the 20 m pattern.









20 m vertical pattern. Radiation angles continue to progress lower.











I have not looked at the patterns yet for higher bands. Below are SWR and impedance plots, without feed line included.

Creating a Horizontal Loop Antenna

Tim WA4PTZ wrote a nice paper to inspire other amateurs to discover the excellence of this simple antenna. Thanks, Tim!

W4BXI System Audio

W4BXI has gradually built a versatile interconnection of components that enable him to hear audio from two transceivers and to interface two-way audio between a transceiver and his contacts using Skype for VoIP calls via the Internet. Of course, he also incorporates the 20th century phone patch as another option.

As John's system has grown some issues have emerged about how to best interface various components while avoiding ground loops, feedback and level mismatch. Recent email exchanges with W4UOA and WA5MLF led to their developing some system diagrams to aid in discussion and identification of configuration choices. Below is a diagram from W4UOA that gives a complete picture of the capabilities assembled by W4BXI. (Click on it to enlarge.)

This shows that John can access various audio streams at the mixer as well as participate in a Skype VoIP conference via the Bridge Computer. Carl has included a list of control operator procedures describing how the main components are used. In testing today John determined that he can also initiate a Skype conference on the Rig Computer and connect those participants to his FT-1000 transceiver.

In order to focus more closely on the two-way audio paths, WA5MLF prepared the diagram shown below that does not include all components and audio mixer connections. (Click on it to enlarge.)
The audio mixer is shown with its separate left and right channel inputs from the FT-1000 and outputs to a headset. A new input to the headset's right earphone is the Skype caller's talk path (dashed line in the drawing). Phone patch #4 is used to combine the talk and listen (transceiver transmit and receive) audio while avoiding the creation of a feedback path. Phone patch #2 is used primarily for ground loop isolation, and can be replaced by a single transformer.

Comments and suggestions are welcome.

Smith Chart

Why do we need it? This is answered very well by David J. Jefferies in his article where he says:

"The SMITH chart is a graphical calculator that allows the relatively complicated mathematical calculations, which use complex algebra and numbers, to be replaced with geometrical constructs, and it allows us to see at a glance what the effects of altering the transmission line (feed) geometry will be. If used regularly, it gives the practitioner a really good feel for the behaviour of transmission lines and the wide range of impedance that a transmitter may see for situations of moderately high mismatch (VSWR)."

Many articles about the Smith chart seem to focus first on proving its mathematical validity. It is not necessary to have a rigorous understanding of its mathematical basis to get useful results from it as a tool.

A nice approach to understanding the format and applications of a Smith chart, without getting bogged down in math, is provided in the December 2007 QST, page 48. This is the first of a 3-part series that totals only 6 pages.

Another good introduction is the Smith Chart Tutorial suggested by John W4BXI. This one makes nice use of colors to illustrate the construction of the chart and includes several good simple applications. Don't be put off by the first two pages.

An impressive collection of Smith chart resources was suggested by Phil KB4XX. It includes a list of 20 tutorial articles, including the one referenced in the previous paragraph.

Several web sites provide downloadable copies of the Smith chart that you can print locally for use in working your own transmission line and matching problems. Several are included at the collection link above. My favorite sources are:

• Impedance Smith chart at Print Free Graph Paper. Once there, select Smith Chart Graph Paper on the first pull-down menu.
• Admittance Smith chart
• Combined chart (impedance and admittance scales -- color print is a must!)

Loop Antennas

This morning on 3968 we discussed loop antennas briefly. Phil referenced The Loop Skywire article from QST November 1985. Tim is pleased with the results from his loop antenna and emphasized that it need not be a full wavelength to be usable on 160 m. Larry commented about a recent club presentation about a member's 500+ ft loop antenna fed with high-quality coax and using a remote antenna tuner. The Loop Skywire article seems to emphasize keeping cost low.

N4KC published a detailed article on his loop antenna and its performance relative to other antennas installed at his location.

Another loop design that I found interesting was featured in the March 2008 QST, page 56. The author describes a 187-ft loop of #26 AWG black insulated Copperweld wire. It terminates directly on an SGC SG-237 antenna tuner mounted just below the window sill where the antenna wires enter his antenna-restricted apartment.

160 m Inverted L Antenna

The other antenna that W4BXI likes to use on 160 m is a center-fed inverted L using ladder line. The sketches below provide the configuration details. Click on the image to enlarge it.

I entered this data into the 4nec2 software as a new model for analysis. Since there is not a convenient way to depict the sloped terrain, I used a level ground terrain. The model structure is shown below as it appears in the program. White lines are the antenna elements, blue lines are the ladder line feeder, and green lines are the xyz axes.


The program's main output window is shown below. It indicates the impedance at the end of the feedline to be 131+j691 ohms (703.31 ohms @ 79.27^o) at 1.8 MHz. The SWR for a 50-ohm source is calculated to be 75.7. For comparison, the impedance at the antenna feedpoint, with feedline removed, calculates to be 38.5-j40.5 ohms (55.88 ohms @ -46.45^o).


The variation of SWR, for a 50-ohm source, is shown in the graph below.


The next set of graphs show the variation of impedance, etc. vs frequency at the end of the feedline. The values at 1.8 MHz are highlighted.


The program is able to calculate the component values for several types of networks to match the feedline impedance to a 50-ohm source. The values for each network type are shown below. Component values for a (C-L-C) high-pass T-network are highlighted in green. This gives an indication of the component ranges that may be used in an antenna tuner to achieve a match for this model approximation.


The next plot shows the far field horizontal radiation pattern for the modeled antenna. Major lobes are seen to be perpendicular to the 130 ft leg that is 60 ft above ground. It is not known how this pattern would differ for a sloped ground terrain.

The final figure shows a 2-dimensional slice of the radiation pattern, color-coded for intensity. The elevated 130 ft leg of the antenna appears as a white line emerging from the pattern. The other leg is seen as a white vertical line that connects to another white line extending to the right, close to ground. Blue lines represent the ladder line feeder.

The program enables one to rotate this pattern in all directions for other views.

ICOM IC-7600

Also from Phil:
"New ICOM IC-7600 is now shipping – first reviews are starting to come in. Wide dynamic range and many nice features - looks like another winner for ICOM. Gotta’ like that sexy hi-res display and built-in spectrum scope. Add a USB keyboard and work PSK31/RTTY. "

Solar Powered Station

Phil sends us news about this Australian Solar Powered Station.
"be sure and check out his station and photo gallery web pages – talk about a low noise floor QTH! Note the 670 amp-hour (solar-charged) battery set, his “homemade” antenna rotor/direction indicator and his “neighbors”."

Thanks, Phil.

Plot of Impedance vs Frequency

For a wider view of W4BXI's 160m antenna properties I used the 4nec2 program's feature that plots impedance vs frequency. The above plot (click to magnify) is from 1.0 to 29.7 MHz in steps of 0.2 MHz. (I started at 1 MHz to get whole numbers on the x-axis.)

The color-coded points and scales identify resistance, reactance, impedance, and phase. A nice program feature is the display of numeric values when you click on a specific point. In the above example I clicked on the 7.2 MHz point before I made the screen capture.

This plot shows the repeating peaks and nulls of impedance, etc. as the program sweeps through the chosen frequency range (with a maximum of 256 steps). The corresponding Smith chart (not included here) shows a series of 16 circles passing through the calculated impedance points plotted around the chart.

Smith Chart Views

As mentioned in the March 31 posting, I have used the Smith chart to check the calculation of impedance at the transmitter end of the 120' feedline. The starting point was the antenna's feed point impedance that is marked on the chart shown here (click on image to magnify). This calculated impedance is in the neighborhood of a dipole's impedance near resonance. The SWR circle shown is based on connecting with a feedline with characteristic impedance of 450 ohms, which is the reference point at the chart's center. The same results can be seen on a paper copy of the Smith chart when plotting with coordinates normalized to the 450-ohm characteristic impedance.


This second chart shows the location of impedance coordinates at the end of the 120' feedline, which represents 0.24 wavelength toward the generator (transmitter). The blue radial line (spoke) from the center is aligned with the 0.24 wavelength mark on the outer perimeter scale. The intersection of this line with the constant SWR circle gives the approximate impedance at the end of the feedline, as annotated in red. This value agrees, within the margin of positioning the mouse cursor, with the value calculated previously.





This final chart shows the range of impedance values (black dots along orange curve) that the program calculated when sweeping from 1.6 to 2.0 MHz. The 1.6 MHz point is identified with the green spoke line and its values are shown in the lower right-hand corner of the page.