Radio Communications Systems During Crisis Situations (Antennas)

 This is part 5 in an ongoing series on radio communications during a crisis event.

What type(s) of antenna should I have?

• Antennas are the best and cheapest way to increase RF signal strength and they don’t require any more resource power to provide the amplification.  One can easily double, triple, and even quadruple the effective radiated power out of the antenna. So quite easily you can amplify those 5 watts of transmitter power into 20 watts at the antenna.  Remember the inverse square law where doubling the output power only increased received signal strength by 25%? By quadrupling the power we have increase receive signal strength by 50%. So the receiving end can hear us 50% better! 
•  How is this possible with no extra resource power consumption?  Quite simply by the way the antenna materials act with the RF energy.  A typical dipole antenna in free space radiates the energy equally in all directions.  Since the receiver’s short path is in only one direction that’s a lot of wasted energy going in all other directions where it’s not needed.  What if we could turn all that wasted energy in the direction of the receiver so they get ALL the radiated power from the antenna?  Well we sure can. 
• RF energy reacts with metal objects that reside in an area a certain distance (based on the frequency) from the main radiating element. If a longer (about 5%) element is placed to one side of the radiating element it tends to reflect the energy in the opposite direction. This element is called a reflector. If another element (5% shorter) is placed on the opposite side of the reflector it will tend to pull the RF energy in the same direction. More than one (incrementally) shorter element can be placed in the directed path further refining the RF energy into a well defined beam of energy. These elements are called directors.  The reflector and director elements are passive parasitic elements that are not directly connected to the radio coax antenna cable.  They do not require any power resource to accomplish the reflecting and directing of the RF energy.  They accomplish this task by their parasitic nature by robbing the RF energy pulling it toward them and thus in a specific direction.  If the reflector and director elements are all in the same plane then all the energy is concentrated into a beam.  The more directors the more concentrated the beam becomes though with diminishing returns. Because the RF energy is formed into a beam, these antennas are called beam antennas or Yagi antennas after the inventors Uda and Yagi.  The old TV antennas are also beam antennas but are of a different design. They are actually log periodic and have a wide frequency range and mainly used for receiving only. Amateur band antennas are not as wide a frequency range and usually limited to a large portion of the band they were designed for.
• The lower the radio frequency the longer the antenna elements and the higher the frequency of the radio wave the smaller the element lengths.  This makes constructing large element arrays at lower frequencies more difficult and expensive due to material costs and structural framing to hold all the elements in the same plane. However higher frequency antennas can support much larger element arrays again due to material costs and structural requirements.  This makes it easier and less costly to make a higher gain VHF or UHF antenna than to make a high gain HF antenna.  Typical element lengths are approximately half the wavelength. So an 80 meter element will be about 40 meters long, a 40 meter element will be about 20 meters long, a 20 meter element will be about 10 meters long and 10, 6, 2 and 70cm elements will be 5, 3, 1 meters and 35cm long respectively.  These element lengths are approximations and the true element length depends on the actual frequency being transmitted but it does give one an appreciation of how large an antenna can get for the lower HF bands.  Practically speaking, when building your own antenna out of scrounged parts after TSHTF, it would be difficult to build a gain antenna for 80 or 40 meters. For these bands a simple dipole is hard to beat cost and construction wise.  A 20 meter beam antenna might be practical and for 10 meters and higher it’s quite practical and even preferred over a simple dipole since the structure can now be supported by one supporting post rather than two for a dipole.  For the added cost in materials and ease of building it just doesn’t make sense to not use a beam gain antenna at the higher VHF/UHF frequencies.

How high should the antenna be?

• Antenna height is a much debated topic when it really shouldn’t be if you know all the factors about antenna height.  What I think usually happens is someone asks someone how high their antenna should be and the answer given is only one of many possible answers.  This then starts the heated discussions I’ve seen in forums and at Ham meets where each is arguing that their height is the best (and for their application it most probably is) but it won’t help the other guy that is trying to accomplish something different with their radio.
• So let’s start with a little theory and hopefully it will explain that the best antenna height depends on where you are trying to communicate to.
• Remember the question about which antenna should I use there was the isotropic di-pole antenna that radiated that precious RF energy equally in all directions and wasting it? And by putting parasitic elements in an array we could form the energy into a more compact beam and direct it at our receiver? Well we can do the same thing by bringing that isotropic di-pole down to earth and having the earth act as a parasitic element.  What this does (and I’ll try to explain this without a picture) is to squish the round sphere of energy so it no longer sends energy toward the ground or toward the sky but only out to the sides of the wire element. By directing the energy wasted in the air and ground we’ve almost doubled the energy going out the sides.  It’s not quite double as the beam width is still too wide and going in 2 directions.  I’ve never done the calculations myself but it has been calculated about 2dB gain (3dB gain is double power) can be had over an isotropic dipole. 
• If the dipole is set about ½ wavelength above ground the energy is concentrated into a beam that points at about 30 degrees (for simple math) into the air.  This angle is called the takeoff angle.  If we raise the antenna to 1 wavelength the beam splits into 2 separate beams the lower one at 15 degrees. As the antenna is raised the beam splits into more beams with tighter patterns and the angle of the lower beam is lower, about ½ the previous angle per wavelength in height. Now in between each beam is an area called a “null” area that will reduce the signal strength of any RF energy coming in at that angle. 
• These beams at different angles shoot into the sky and the beams that point higher will bounce at a less of an angle angle in the sky and land at a closer distance than the lower angled beams.  The lower angle beam thus can travel further distance across the ground in one bounce (hop).  Each hop chews up some signal strength so to talk long distances it’s best to do it in the fewest possible hops. 
• Now remember the nulls? The angle of these nulls will produce mini skip zones where the signal strength will be less at the distant receiving and could be so much less at a specific angle so the signal may not be heard. For an antenna at ½ wavelength with the one wide beam at 30 degrees there will be a significant initial skip zone then several progressively smaller skip zones because the bouncing beams will scatter a little causing the energy to spread out.  After several hops the skip zones will be minor however if for some reason the beam of energy is only received at a certain angle and the height of the receiving antenna is such that there is a null at that angle the signal will be weak or unreadable. 
• Set the transmit antenna at 1 wavelength and now there are 2 beams one that bounces closer and one that bounces further.  We now have 2 skip zones but after a few hops of beam spread they are greatly reduced because of the overlap.  Raise the antenna further and you have more beams, more nulls, but lower angles for longer hops.  The same happens for a beam antenna only the elevation patterns take on a more complex beam and null pattern with the lowest beam being the strongest and many smaller beams and quite wide nulls in places.
• So armed with this knowledge you can see why there might be some heated discussion going on about antenna height.  For one person talking to Europe his antenna height may be perfect but someone a little further away may not hear Europe very well.  And vice versa the guy that can hear Europe may not hear Japan but the guy that can’t hear Europe may hear Japan just fine.  If either one could change the antenna height then they could hear the other locations just fine also.
• So what’s the correct answer? Antenna height will need to be calculated for your transmitter location and receiver location.  Most people with simple setups put the antenna at ½ wavelength and leave it.  That’s what I did in Florida. Actually the antenna was at 20’, not quite ½ wavelength for 20 meters which is what I usually transmitted on. It was a bit of a “sky warmer”. Outside the initial skip zone I could talk all up the east coast on the first hop and all of Europe and some of Eastern Russia on the second hop.  Even though the wide beam pattern at only one angle may not be as strong as the more concentrated lower beam of a dipole at a higher elevation I had enough power for 2 hops and no receiving nulls. That was during solar minimum too.
• One important area we briefly mentioned, the initial skip zone, is a very important zone is a SHTF situation.  At HF frequencies it’s too wide to cover with line of sight VHF so there is this zone around us that we can’t hear, literally. From my receiver in NW Florida I could not hear Georgia, Alabama, Mississippi, Arkansas, Louisiana, and most of Florida.  Extend that circle around into the Gulf of New Mexico and that’s how much area around a receiver in the Midwest would not be able to hear. That’s almost half the country.  So what can we do to remedy that problem?
• Remember the dipole at ½ wavelength radiated the energy in beams off the side at about 30 degrees and when raised these beams multiplied and pointed lower in the air? If you do the opposite the beams actually point higher into the sky to where they become one beam pointing straight up and less RF energy is radiated at the lower angles.  The antenna at this height is horrible for long distance communications since the energy is pointing straight up and there is no skip.  This antenna is called a “cloud warmer” by some and meant in a bad way when trying to communicate long distance.  The term used for this antenna when used for its practical purpose is Near Vertical Incidence Skywave (NVIS) antenna.  Its practical purpose is for communicating to people located in the skip zone.  The energy is not so much reflected like a beam hopping over the Atlantic, but scattered in all directions (backscatter) filling in the skip zone void. A NVIS antenna is also great when you station is located in mountainous country and the takeoff angle of a higher antenna points right into the side of the mountain.
• So now knowing this much information it seems like you would need a of couple antennas to cover long distance and the skip zone.  That would be the best idea, a beam antenna at a calculated height to cover the long distance areas of interest and an NVIS dipole to cover the skip zone.  If that’s practical for you and you can afford to hang a HF beam higher than ½ wavelength then it may be your answer.  However for OPSEC reasons I’d not like to advertize I had communications to everyone who drove by. A wire strung between two poles is harder to see from a distance than a big HF beam on a tower.
• My choice STSHTF would be a single Off Center Fed (OCF) Di-Pole cut for the 80 meter band stretched between 2 masts 30-60ish feet off the ground and on a pulley system so the height of the dipole can be adjusted for long distance or skip zone communications.  The OCF dipole is a special dipole pole that is not fed in the center but off center a bit. By doing that the impedance match is now off by 4 and the antenna becomes unbalanced so to feed it with coax take a couple of extra components called BALUNS.  I’ll not get to technical on how the antenna works, but this antenna not only radiates well on 80 meters but also quite well on 40, 20 and 10 meters.  The radiation pattern is not quite the same as the standard dipole nor is the takeoff angle the same when transmitting on a frequency off the 80 meter band.  But taking all that into account this makes a wonderful single antenna that covers 4 HF bands.  You can cut the OCF for 160 meters but the wire is so long that it needs a center support as well. Now it becomes more expensive to support, needs more surface area to stretch out and more work involved to raise and lower it.  During SHTF situations my bet is communications will be concentrated on the 3 work horse bands, 80 (NVIS – local night comms), 40 meters (NVIS – local day comms) and 20 meters (30 to 60’ - long distance day comms), since simple antennas for these bands are easily deployed due to their smaller size and height requirements.
• One other antenna that I haven’t mentioned yet is the HF vertical.  If you have the time to prepare beforehand then a tall vertical makes a decent antenna also for long distance work.  They are more complicated, need a foundation and heavier materials to support it and many wires strung out underneath it for a ground counterpoise.  I don’t recommend this antenna if you are going to construct it ATSHTF. Also the vertical has a low take off angle making it a poor choice for local comms.
• So my final recommendations for SHTF antennas are:
• An 80 meter OCF dipole stretched between two poles or trees 33 or 66 feet in the air on a pulley system to raise or lower it. This will cover 80/40/20/10 and even 6 meters.  You will need 140 feet of available ground to stretch it out.
• A 20-15-10 meter triband yagi at 30ish feet.  You can also add  6 meter parasitic elements to add 6 meters to the antenna.
• A copper pipe (or whatever metal you can scrounge ATSHTF) J-Pole cut for each of your VHF bands.  Though one 2 meter J-Pole will work on 70cm quite well so if you only build one antenna build the 2 meter version. I only use the 2 meter version for my 2 meter/70cm dualband handheld. This is a line of sight antenna so the highest practical height you can get it the better. I'm at 20 feet supported by the roof apex on the end of the house. If you house is taller, then any height about 6 to 8 feet above your roof peak would be good. Using the building to support the mast eliminates the need to use guy wires. A cheap mast for light antenna work are 10 foot chain link fence top rails.  

What polarization should my antenna be? 

• Antenna polarization is how the electrical field of the electromagnet wave (RF energy) is in relation to the earth’s surface. If the electrical field is perpendicular to the earth then the polarization is considered vertical.  In demonstration, a person walking upright is vertical and a person lying down is horizontal. 
• In the RF communications world, horizontal polarization is used for long distance and vertical polarization is used for local line of sight communications. Some satellite communications use circular or cross polarization but that’s for a specific use. 
• Vertical antenna for HF bands can also use vertical polarization because once the beam reflects off the ionosphere it becomes more of a random pattern. More hops equals more randomization. 
• The biggest factor when communicating direct (no ionosphere) is that the antenna polarizations match.  If not the received RF energy will be reduced dramatically.  
• The venerable di-pole antenna is a horizontally polarized antenna while the sturdy J-Pole is vertically polarized. 
• To change the polarization of a beam antenna just rotate it so the elements point up/down for vertical or lay flat for horizontal polarization.   
• If you are using a beam antenna to work a distant repeater the antenna will need to be vertically polarized to match the repeater’s antenna. 
• If you are working SSB long distance then the beam antenna should be horizontally polarized and so should the distant end.
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What should I use to connect my radio to the antenna?

• Normally ordinary wire can't be used to connect  the radio to an antenna, unless you are using a long wire antenna that leads directly out of the antenna jack on the radio or an antenna tuner.  A long wire antenna is simply that, a long wire that is several wave lengths long and not an even multiple of any wavelength you want to listen too.  It is a receive antenna but can and has been used successfully as a QRP or low power antenna for CW (or any mode really).
• To connect the radio to any other sort of antenna means using either balanced ladder line or coaxial line.  Balanced ladder line has been used since the dawn of radio use and is well suited for any balanced antenna like the di-pole.  The problem with it is it involves very stringent installation methods and is therefore not easy to work with.  It is very low loss but unless you are very familiar with balanced line installation I will discount it as a viable means to connect to the antenna. Most modern radios will only have coaxial PL259 connectors and attaching ladder line will involve a separate piece of equipment to match the ladder line's 450 ohm impedance to the radio's 50 ohm.
• Coaxial cable is bar far the easiest to install and connect to your modern radio. It has a 50 ohm impedance so it matches the radio with no additional equipment.  If the antenna is also 50 ohms at the desired transmitter frequency then all should work well enough to radiate some signal off the antenna.  Coaxial cable is not without some drawbacks that must be discussed otherwise you could be wasting power.  Antenna impedance at the desired frequency is very important.  The antenna must be cut or trimmed to provide the best possible SWR. In a SHTF situation or a temporary  antenna installation a 3:1 SWR might be all that you can manage and it will work but at greatly reduced power.  First the impedance mismatch between the radio and antenna will lose power and most solid state rigs will automatically reduce output power if the SWR is 3:1 or greater.  Some solid state radios will not even transmit if the SWR is too high.  So it's best to try to get an SWR of less than 2:1 even in a temporary antenna installation.  For permanent installations SWR readings of less than 1.5:1 are good.  Don't spend too much time trying to get the SWR below 1.2:1. In most installations that's about as good as it's going to get.  Anything less than 2:1 is going to be OK.
• The reason the antenna SWR needs to be as low as possible is that any energy not radiated off the antenna due to impedance mismatch is sent back down the coax to the radio.  This is what is called reflected power and that power is sent right back to the transmitter and in severe cases will cause permanent damage to the transmitter.  This is why the transmitters automatically reduce output power in a high SWR situation so they don't blow themselves up.  If you use an antenna tuner between the radio and the antenna and the antenna mismatch is off the reflected power is absorbed by the tuner and not reflected back to the radio.  The antenna tuner does not have delicate solid state final transistor amplifiers to get damaged.  The antenna tuner always show 50 ohm impedance to the radio so the radio will never reduce power in a high SWR condition.  However the antenna mismatch will still cause power to be reflected back to the tuner so that power is lost.  But when using an antenna tuner the antenna reflected loss is not combined with a transmitter reduction loss so the overall loss is less.  Antenna tuners are good if you can not for what ever reason obtain a less than 3:1 SWR on the antenna.  If your antenna is well matched (less than 3:1) to the transmitter frequency then I'd take out the tuner and connect the radio directly to the antenna as the antenna tuner does rob some of the transmit power.
• Another important factor with coax cable is it's efficiency.  Coaxial cable is reactive to RF energy and as the frequency goes up the more resistant the coax is to the RF energy.  Some cheaper coaxial cables can reduce power as much as 6dB (for VHF frequencies, more for UHF) or 1/2 the transmitted power in 100 feet.  So if the coaxial run from the radio to the antenna is about 100ft, not an unrealistic length, then you have lost half your transmitter power.  The better coax cable with less impedance to high frequencies costs more but is an investment if you plan to operate on low power.  A 100ft piece of 3dB/100ft loss coax compared to a same length piece of 6dB/100ft loss means you can transmit at 5 W for 20hrs/100aH battery compared to 10W for 10hrs for the same effective radiated power (ERP).
• So in summary, get the best flexible low loss cable (UV resistant) that you can afford and it will be a good investment toward power savings in the end. Another option would be some super low loss 75ohm TV coax used for digital cable.  Even with the impedance mismatch the line loss is so low it makes up for it and the net gain is in the positive.  Good low loss TV coax should be readily available in SHTF conditions especially in urban settings.  Many Hams use 75 ohm coax connected to a dipole antenna (for HF not VHF+) and it works just fine for what they are using it for.