Direction Finding
Sometimes you need to locate the source of a signal. It could be because you’re doing something fun like a “fox hunt.” Or you may be trying to find where harmful interference is coming from. Either way you would use direction finding to search for the source of a signal. An effective method of direction finding is triangulation. To triangulate you use antenna headings from several different receiving locations. Map their intersection to locate the signal source.
An illustration showing how triangulation is used to track down a transmitter
Direction finders typically use specialized receiving antennas. A popular type is a small wire-loop. This is similar to a pennant antenna. The small loops can be twisted and rotated to null out a signal in a particular direction. Their heart-shaped cardioid pattern creates a single null. That’s a point of zero reception. Nulls are important for direction finding. Signal hunters change their antenna bearing until the signal disappears. This aids in determining direction information for the signal.
Let’s look at the design of some small loop direction-finding antennas. Many include an electrostatic shield. That shield eliminates unbalanced capacitive coupling to the antenna’s surroundings, improving the depth of its nulls. The keyword here is nulls. One thing to watch for on this small wire-loop antenna is that it has a bidirectional null pattern. That could mean false null readings when direction finding.
How about a direction finding antenna with two elements? That’s called a sense antenna. The sense antenna modifies the pattern of a DF antenna to provide a null in only one direction. Sense antennas have been in use for ages. Airplanes were a big adopter of these antennas. It was part of the system they used to track Non Directional Beacons during flight.
It can be helpful to measure the performance of receiving antennas. It’s a big benefit for these directional ones. That’s done via the Receiving Directivity Factor or RDF. The RDF is peak antenna gain compared to average gain over the hemisphere around and above the antenna. The key word here is hemisphere.
An antenna with high RDF helps in two ways. It’s efficient, which is always good. In this case efficiency provides quality reception of the intended signal. Its efficiency is also in its ability to ignore other signals that could create noise.
Loading Coils
Let’s say you want a vertical antenna on your truck so you can operate on 40 meters. A quarter-wavelength antenna would be about 30 feet high. That’s not something most people want to drive around with. So, many antennas with physical space limitations make use of a loading coil.
What does a loading coil in an antenna do? It helps make an electrically short antenna resonant at a desired frequency. In this case “electrically short” means that it’s not a full half-wavelength or quarter-wavelength element. The function of a loading coil as part of an electrically short antenna is to resonate the antenna by canceling the capacitive reactance.
Let’s focus on the coil in an HF mobile antenna. It should have a high ratio of reactance to resistance to maximize efficiency. Part of the coil’s job is to cancel capacitive reactance, not add extra resistance to the antenna!
Loading coil on a mobile antenna
Here’s a drawback to using one or more loading coils to resonate an electrically short antenna. With the coil, the SWR bandwidth is decreased. So, if you want your antenna to cover more of a particular band, you should limit your use of loading coils. This is part of a general antenna theory. It says as the Q of an antenna increases, SWR bandwidth decreases. This is easy enough to remember – as the Q goes up, the SWR bandwidth goes down.
Placement of the loading coil matters. Where is the most efficient place to put a loading coil? Place it near the center of the vertical radiator to minimize losses in a shortened vertical antenna. Of course, experimenting is welcome too. What about top loading an electrically short HF vertical antenna? In that scenario you gain improved radiation efficiency.
Antenna Placements
Where you place your antenna will impact its performance. The surrounding terrain will have an effect on its radiation pattern. That can be from hills, rocks, water, or other features we’ll explore. One term for this is “ground gain” which is an increase in signal strength from ground reflections in the environment of the antenna.
Let’s be clear on that term for this section. In this discussion “ground” is referring to the earth or terrain. We’re not talking about an electrical connection for safety.
How does the ground impact the performance of a horizontally polarized antenna? Height makes a difference. The higher above the ground the antenna is, the less the ground will affect the radiation pattern. The higher it is, the more that pattern becomes horizontal. How does that work? As you increase the height of a horizontally polarized antenna the takeoff angle of the lowest elevation lobe decreases.
So, what if the terrain is uneven? There is a difference in performance if that antenna is mounted over a long slope instead of flat ground. What happens with a horizontally polarized antenna over a long slope? The main lobe takeoff angle decreases in the downhill direction. This is because the ground is not pushing the signal radiation pattern upwards.
Feed point impedance matching comes into play with height as well. Antenna height is an important factor when matching the impedance for antenna tuning. This is because height can make the antenna appear electrically longer.
For your base-fed whip antenna, impedance matching can be related to antenna frequency. What happens when you operate that below its resonant frequency? Radiation resistance decreases in a base-fed whip antenna.
Let’s get back to the kind of terrain you have to work with. Different types of soil and rocks have different reflectivity. You can lose energy and efficiency with a signal that ends up transmitted into the soil. The soil conductivity determines ground losses. That’s for a ground-mounted vertical antenna operating on HF.
If soil is impacting your performance, you have options. Installing a radial system improves the efficiency of a ground-mounted quarter-wave vertical antenna.
Contrast that with mounting your antenna near seawater. Seawater is more conductive to radio waves than the earth. It attenuates your signal. How does the performance change for a vertically polarized antenna mounted over seawater instead of soil? The radiation at low angles increases in the far-field elevation pattern. Maybe this is a good excuse for creating your station near a beach!
