Antenna Comparisons using JT65 & JT9 Signals along with NEC Models
True understanding of a problem is confirmed when one can validate theoretical model predictions with measurements. It is too easy to believe pretty pictures that modeling programs produce, especially when making meaningful measurements is so difficult. In this article I try to take the bull by the horns and see how close I can come to declaring that I understand my antennas.
Last time we discovered that the noise performance of my two old radios was essentially identical to one another. This means that I can use the two receivers to listen to signals from my two antennas and be able to make comparisons between them. When digital mode JT signals are decoded, the software determines a signal-to-noise measurement for the received signal. We can use these measurements which we get for free with the WSJT-X software for every received signal to quantitatively make comparisons between the two antennas. In practice this means leaving the two receivers tuned to the same JT65 and JT9 band, have both receivers running an instance WSJT-X software, and recording all of the transmissions received by both radios.
In my case, I have a 40m loop that is about 30 feet high and an off-center-fed dipole (OCFD) at about 75 feet up in my tall Douglas fir trees. Both of these antennas are horizontally polarized and have distinct directional properties that are unique to each geometry. This means that I would expect signals from stations a various locations to be preferred by one antenna or another.
To begin with we will look at the predicted propagation pattern of the two antennas on the 20m band. Since I am now getting down to making comparisons for real antennas with a model, I spent some time adding in as many parasitic elements into my NEC model as I could, and included both of the antenna geometries in the same model – just substituting the driving point of the unused antenna with the matched transmission line impedance. Besides the two antennas I also included the aluminium rain gutters on the house in the model, and I did my best to correctly locate the antenna elements as they really are laid out. The NEC model and the two radiation patters for the two antennas are shown in the following figures.
The pattern for the 40m loop antenna is less complex at 20m, since there are fewer harmonic resonances on the loop.
Since the OCFD is considerably higher than the 40m loop, it generally has better low elevation gain; but the two antenna patterns are really quite difficult to directly compare to one another. Fortunately, 4NEC2 can give us tabulated data for both patterns. Since we will be looking at differences in signal levels, we can also look at differences in the predicted model radiation patterns. I don’t really have the tools to do this with the full 3D pattern, so instead I just looked at the difference in total far-field radiation at the 1o and 15 degree elevation ranges where long distance propagation is most often successful.
With the model results now in hand, let us look at the results of the JT65 & JT9 signal comparisons. After collecting a few days worth of JT signals and spending some time pouring over the Excel spread sheet I was able to generate the antenna difference radiation pattern. Rather than having a nice uniform distribution of signals, there are large concentrations of data points from US, Japanese and European hams and otherwise a fairly sparse azimuthal distribution.
Is there agreement? Hard to say, but some things are clear. Usually the OCFD does better than the Loop, often by 5 dB or more. We see various “lobes,” although they do not necessarily line up very well with the ones predicted by the NEC code. The lobe aimed at the EU at 30° seems in place, and stations due North are suppressed on the OCFD as we would expect along the wire direction. The polar plotting is best for comparison with the NEC model. Below we show the same data as function of compass bearing angle along with the a measure of the statistical errors present in the data, and some geographical reference points.
The graphs above shows the result after quite a bit of data manipulation in Excel. Remember that peaks in the above chart can come about because of either a strong transmission lobe for the OCFD antenna or because of a null on the Loop antenna. Many factors can give rise to a spread in the gain difference at a particular bearing angle. The actual propagation path for a particular transmission might be at any of a range of propagation elevation angles, all of which are it different for each antenna. The transmitters have unknown polarization and the antennas will respond differently according to the actual polarization vector. The radiation patterns I’ve presented are just the total gain and do not consider this added subtlety. Nevertheless, there are definitely directions that favor one antenna or the other.
The data file is included here as an example and template for others to use. Excel Antenna Comparison Calculation Template The template file has some useful Excel formulas embedded in the worksheets that will calculate Latitude and Longitude from the Grid location code, and will then calculate bearing and distance from your location.
Several steps are required to get the cleanest results.
- Only signals received simultaneously by both receivers are considered.
- JT65 signals when both receivers showed signal levels less than -5 dB s/n are included. Strong JT65 signals are rejected from the data considered.
- Grid data is added to signals where it is possible to unambiguously determine the grid.
- All signals from a particular grid square are averaged to get a single number of each grid square.
Once the averaged signal differences for each grid point are determined, this data is further processed .
- Latitude and longitude are determined from the grid location code.
- Bearing and distance are determined from the latitude and longitude of the transmitter and receiver locations.
- Data for distances less than 500 km is excluded.
- Signal differences are plotted on a polar graph.
The details I presented above for the 20m band show the level of data analysis possible with the kind of information that is relatively easy to collect in just a few hours of listening with a couple of radios. Even a much more cursory look at the data is valuable, however. Merely taking averages for all matching transmissions on a given band can give a single-number figure of merit for comparison of two antennas.
Here is such data for my two antennas for the four bands where the loop works well.
First thing to notice is that although the OCFD up at 75 feet does significantly better on the lower bands, 40, 20 and 15 meters, the Loop wins out on the 10 meter band. This was a bit of a surprise, but goes to show that height is not always your friend.
Also notice the rather large standard deviation. In some sense, this is just a measure of the depths of the lobes of the two antennas.
Direct comparison to the NEC models was marginally satisfactory. However, there were many small uncertainties for both of my antenna geometries which, when all combined, proved to make the direct comparison with the theory less than perfect. The models are only as good as the input data, but the radio signals tell no lies.
In the future, doing a similar experiment with a vertical antenna as a reference might be a good plan. The uniform radiation pattern from the vertical would provide a constant reference that would allow the pattern of the test antenna to be more accurately determined.