Peeking Over the Hill Using Passive Antennas
A Few Passive Thoughts
With the rapid expansion of VHF, UHF and yet higher frequency technology being widely used in Amateur Radio today, most amateur operators have at least had experience with two meter operation or higher. Or, have a good feeling for the propagation of these higher frequency signals in comparison to their HF counterparts. Even the most ardent MF-HF operator will frequently use 2-meters or 440 MHz as a local communications channel to learn where the HF band “HotSpots” are. However, the higher your frequency of operation the more difficult it is to get your transmission to “Bend” around corners and “Climb” over hilltops! Even at 146 MHz you are dealing with a signal that wants to travel a straight path in the direction your antenna is pointing. At the low end of the microwave frequency spectrum your antenna is akin to a spotlight with its beam pointing in a perfectly straight line. Getting your VHF-UHF signal to go where you want it to go “out of the canyon” or “over and down the hill” seems like the impossible dream—or is it?
How to “Bend” your radio waves
Many operators have run into the problem of living in a valley, behind a hilltop or behind a tall building that may just be high enough to block your signal path in the direction you are interested in communicating. Even a few hundred-foot tall hill makes a great dummy load when your antenna is pointing directly at it! There is however a totally passive, almost maintenance free method to correct this problem without going through the expense of constructing, licensing and maintaining a remote base or active repeater site. I am referring to the use of a Passive Antenna System also referred to as a passive reflector system at microwave frequencies. Simply stated, we are going to use common Yagi antennas, or a Yagi in conjunction with an omni directional antenna, to create a pathway for your signals to go up and over, or around and down or both!
A Passive Antenna System
In its basic form a passive reflector is as simple as reflecting your VHF-UHF signal off the side of a cliff or conveniently placed mountain which unfortunately are usually “in the way of rather than on the way to” your signal’s destination. Almost every UHF operator has experienced this from time to time such as pointing a 440 MHz antenna away from the station you are working to reflect the signal off a suitable surface and provide a great improvement in signal quality. This is very similar to pointing a flashlight beam at a mirror and seeing the light beam reflect in different direction.
If we look at the sketch shown in Figure-1 you will see a typical problem illustrated that may well be solved using this passive (no active amplifiers, preamplifiers power supplies or batteries required) antenna technique to point that signal where you want it to go. As shown in this sketch, our VHF-UHF Ham operator is transmitting from his home located on the “wrong side” of a 400-500 foot mountain towering above his QTH. He is attempting to establish contact with a 2-meter repeater system on the opposite side of the hill. He has increased the gain of his antenna, increased his power from 25 to 100 watts, not to mention increasing his tower height all to no avail! His operation on 445 MHz, as you may have supposed, is also not doing well.
Taking a look at Figure-2 we see a basic but effective solution to our fellow Ham’s problem. At the top of this 500-foot mound of dirt and rock cutting him off from the world (on VHF-UHF frequencies) he has erected a short mast or tower, let’s say 20 to 30 feet (28 to 27 m) in height. On this he has mounted two additional 2-meter Yagi antennas. The antennas are vertically separated at approximately one wavelength from each other (about 6.7 feet, 2 m). The lower antenna on the mast is mounted Horizontally and pointed down the hill directly at his home QTH also using horizontal polarization. The second, upper antenna is vertically polarized and pointed at the repeater station he wishes to contact. The two antennas are interconnected using a short length of high quality 50-ohm coaxial cable, preferably terminated in weather tight “N” connectors. The cable selected should be the lowest loss cable you can afford. Fortunately, you only need about an 8-foot (2.4 m) length of cable plus connectors in this example. That’s it, our passive radiator is assembled, with all connections (2) made and two antennas pointing in the right direction. Let’s prepare to test this new “Aluminum Pine Tree” and understand how it works!
Understanding a Passive Antenna System
In this example, the passive antenna system is constructed by connecting two high-gain VHF beam antennas back to back using a short length of very low-loss, high-quality cable and connectors. We then point each of the antennas in the direction desired to transfer the maximum transmit and receive signal strength between the two points that are “blind” to each other without this antenna array.
Obtaining maximum antenna gain for both antennas and minimizing every tenth of a dB of signal loss are key factors along with antenna selection, location, directivity and quality. Thus, the reason for emphasis on low-loss cable and good workmanship atop the hill. If you will permit a few assumptions to be added at this juncture, allow me to assume the directive Yagi antennas used on the hilltop have a gain of 6 dB each. Let us further assume that the cable loss for this short length at 146 MHz is insignificant for this example. And assume other factors such as signal reduction caused by incident and reflected interaction of the propagated signals from the source (our illustrated QTH shown in Figure-3) to the receiving and the repeating antennas is nil. (A bit more on this later.)
Let’s take a look at the signal path between the station QTH and the repeater site on the other side of the mountain. We shall make the assumption the signal strength being received at the top of the hill by the antenna pointed at the station QTH is 50 u volts. This signal strength is of course arbitrary and will vary greatly with station power, antenna gain (ERP), the distance between the station antenna and the receiving antenna on the hilltop. If in fact the received signal is 50 u volts, the 6 dB power gain (20-db log voltage gain) of the receiving antenna will boost this received signal to approximately 800 u volts! This received signal is now passed forward to the second antenna in this passive system also adding its (20-db log voltage) gain to the signal applied producing an apparent signal strength of 3200 u volts (20 micro watts across 50 ohms) of power directed at the repeating station. This may not appear to be a major signal gain but it will make the difference between not being heard by the repeater system “at all” to having your transmitted signal repeated for all working stations in the area to hear clearly!
In receive mode, the exact reverse pathway is present. The signals received on top of the hill will be relayed in reverse order through the two antennas using their respective gains to direct the received signal back to the stations home QTH! This same passive system will also work well between two or more simplex stations working through it. There will have to be a reasonably clear path for the stations being worked and there must be enough signal transferred in either direction to communicate effectively. When a typical two-meter Yagi antenna is used in a vertical position, the beam width will be rather broad at or near 90 to 110 degrees wide. In this configuration this may work out to be a benefit giving you a rather wide range in azimuth without need for any mechanical rotation. A typical 2-meter Yagi antenna will, in general, not provide the same gain in the vertical mode as it will when mounted horizontally. So always purchase a bit more antenna gain “up front” to provide that extra margin of signal strength.
Other Passive Antenna Options
Described above is a basic passive antenna system to illustrate the concept. If for example, the coverage over the mountain top was not to a single point or general area, the second antenna of the pair could be an omni directional, dual beam or cardioid pattern vertical antenna. It is still deriving its input signal from the Yagi receive antenna as in the first example. Another point to remember is a dedicated Yagi at the home station with a fixed mount will be stronger than a rotating antenna assembly. This allows for a bigger (higher gain) antenna to be used without the cost of a rotor, its cable and control, and most importantly, the space required to turn it. This will provide an even greater signal level to be re radiated from the top of the hill.
For vertical orientation, as with repeater and most mobile coverage, the main lobe of the Yagis involved will be broader and, depending upon height—lower in gain than a horizontally oriented Yagi. Hence, aiming is less a problem than system gain estimation (which should be conservative).
Refering back to Figure-3 you will note the Yagi at the home station and the first Yagi on the hill receiving this antenna’s transmission are horizontally polarised. Only the second or re-radiating antenna is vertically polarised because we are discussing a FM communications system in this example. By reversing the polarity of the transmitting (home station) antenna from the re-radiating antenna, you will gain about 20 dB of isolation between the first and third antennas and help prevent reflections and cancellations between the incident wave and the re-radiated wave which will have phase differences.
A frequently asked question by operators using both 2 meter and 440 operation is: “Can dual band directional antennas be used to operate on both bands?” The answer here is yes they can, but give it some thought first. Except for the more expensive dual band 2-meter and .75-meter antennas, most often you will find that some compromise has been made by the antenna manufacture in their dual band Yagis. Perhaps a lower gain on 440 MHz or a reduction in ideal back to front ratio, etc. Also, compare the costs of a dual band antenna with that of a separate antenna for each band keeping the forward gain, beam width and back to front ratio as close as possible for the comparison. Often, depending on sale prices and so forth, the difference in cost between the two methods is minimal. If you are not space limited and there is only a minor cost difference, I would suggest staying with two independent antenna pairs. You might consider dual antennas on the hill and using a single dual bander at the home QTH as one practical compromise.
After you have purchased your antennas, assembled them and your cable assemblies, be certain to test them at home prior to moving them to their new remote home. Don’t wait until then to find out you have some form of a problem in some part of the assembly. Assemble your antennas in the radiating position to be used and mount them on a temporary mast about 20 feet (6.1 m) above the ground and clear of surrounding objects. Test and adjust each for minimum VSWR. Also test your cable assemblies into a known good dummy load to be sure they show no reflected power on your meter. Be certain your mechanical mountings and connections are tight and secure to minimize those winter treks up the hillside to make repairs with half frozen fingers that may have been prevented.
If your remote antenna site has been erected in a wooded area, try to keep away from the surrounding trees particularly to the front and rear of the antennas. Remember, every u volt of input signal saved will be amplified by the combined gains of these passive, parasitic antennas.
A Final Note
I have also used this technique several times to bring TV reception down into hidden valleys or over hilltops with excellent results. In so doing I have had the interesting experience learning the Passive TV Antenna System did a very good job in boosting the performance of my 2-meter equipment even though the antennas in the system were set up for horizontal commercial TV reception.
Give this passive antenna system a test. I am sure you will be surprised by its performance. There are various other antenna combinations that can be experimented with using this basic concept. We would be glad to hear your feedback after you get your passive system working. Please let us know of any problems you had to overcome in your situation. Feel free to send pictures of a system constructed from this article or one that may already be in place at your station.
Originally posted on the AntennaX Online Magazine by August Hoecker, W8MIA
Last Updated : 14th May 2024