Don’s Antenna talks by N4DJ

In order to work DX you need an antenna with low angle radiation. The more power you can focus at the low angles the better your DX results will be.  The problem with horizontal antennas on 80 meters is that you have to have them really high to get low angle radiation. An antenna only a half wavelength high has most of its radiation at 30 degrees. Having the maximum radiation at 15 degrees would be much better. With a 80 Meter dipole to even get the maximum down to 30 degrees you need the dipole up at about 130 feet. Vertical antennas, in general, are not dependent on height above ground to obtain a low angle of radiation. The quality of the ground under and around the vertical is important. Another factor we need to be concerned with in the performance of a vertical is that of a radial system to reduce the ground loss. In most verticals the ground makes up half the antenna and unless you have a good radial system you can lose a lot of your power in the ground resistance. One way...

On any antenna certain things will always be true: Voltage will be high or highest at an open end. Current will be zero or near zero at an open end. Voltage is always high a half wave or any integral multiple of a half wave from an open end. Current will always be zero or near zero at the end and at an integral multiple of a half wave from an open end. Low voltage points coincide with high current and vice versa. The voltage to current ratio is the impedance.  High voltage points are high impedance points. High current points are low impedance points.  Antennas shorter than quarter wave are capacitive.  Antennas between quarter and half wave are inductive. For best results, horizontal antennas should be at least a half wave above ground. Any conductor near an antenna can affect that antenna. Any nearby conductor a half wave long will have a significant effect on the antenna. Worst case is if that conductor is parallel to the antenna. It can act as a parasitic director or ...

End Fed Wire Antenna Another simple wire antenna is a piece of wire that is fed power at one end. Typically it is called an end fed antenna or random wire antenna. Power is supplied to the end of the wire through 50 ohm coax as in the dipole, but one or two other items are necessary.  First of all there must be a ground connection. If the impedance between the end of the antenna wire and ground is low or near 50 ohms, coax may be directly connected. For this to happen the antenna must be 1/4 wavelength long or an odd multiple of 1/4 wavelength. The center conductor of the coax is connected to the end of the wire and the shield of the coax to the ground. This will be the case only if the length if the wire is one quarter wavelength long or an odd multiple of one quarter wavelength. In this special case the reactance will be zero and the resistance will be somewhat greater than about 35 ohms. In most all cases, if the load is resistive and anywhere from 25 to 100 ohms it may be conne...

Wire Antennas by Don Johnson, N4DJ Part 1 Wire Antennas at first glance can appear to be very simple. In some respects they are just that. In other respects they can be quite involved. There is probably an infinite number of possibilities for a wire antenna. For example, there is the feed point location. It can be in the center, at either end or anywhere in between. Next is the length of the wire. It can be anywhere from very short to very long. It can be resonant or non resonant. We usually talk about the length of the wire in terms of wavelength. For example it may be 1/2 wavelength or one wavelength. It may also be measured in degrees, 180 degrees being 1/2 wavelength and 360 degrees being one wavelength.  The length is the major characteristic that determines the radiation pattern or directivity. The length and feed point together with distance above ground are the major items that determine the impedance of the antenna. Almost any piece of wire can be used as an antenna, but t...

My first antenna was an end fed wire with a galvanized pipe in the ground for the ground. My DX-60 had a pi network and was able to match the wire directly on some bands. Soon after I built an L network to match it on all bands. I wound a coil about 2 or 2 1/2 inch diameter and about 6 or 8 inches long. I used an old cigar box as the chassis. The coil was on top of the box and I mounted a 140 pf capacitor inside the box. I had a short piece of coax from the transmitter to the cigar box. Eventually I got a Heathkit SWR meter to make the adjustment easier.  This first wire was about 50 to 70 feet long. It probably worked simply plugged into the DX-60 on 80 meters because it was a quarter wave long.  Later I tried dipoles, but could not get very high. I then lengthened the wire to around 300 feet. It went from about 15 feet on my house to about 20 feet high (on a pole) 45 feet from the house then 250 feet to the top of a pine tree. This antenna started with two galvanized pipes a...

Impedance  Impedance is simply the ratio of Voltage to current. That ratio (impedance ) changes from very high at the open end of any antenna wire to low ( let’s just say around 50 or 75)when you move 1/4 wave back from that end. So take a half wave piece of wire, at the frequency for which it is a half wave, the impedance will be about low at the center. (About 50 ohms) That point is 1/4 wave from both ends. It is a good match to 50 ohm coax.    SWR will be low. In reality the impedance will vary with the height above ground. Never too far from the coax impedance be it 50 or 75 ohm cable. At this point there will be very little reactance. We call that resonant. Some antennas have different impedances at the feed point and then you need to “match” them to the coax. Depending on the situation a 2:1 transformer can match 25 or 100 ohms to 50 ohm coax. 4:1 transformers can match 12.5 ohms or 200 ohms to 50 ohms.  Baluns are really not necessary with dipoles. However the...

 There seems to be a lot of mis information out there on antennas and transmission lines. I will from time to time add a short post of fact that I thing is good to know. Hopefully they can be of use to the average amateur radio operator on anyone who want to learn about antennas and how they work. Mostly we will consider wire antennas and coax transmission lines. There will most likely exceptions to every rule. I will try and avoid the use of always in most cases. If I do use "always" please remember there is almost "always" going to be an exception! SWR is only an indicator of how well the antenna impedance is matched to the feline or coax. SWR is not an indicator that the antenna is radiating any kind of signal. Low SWR can also mean excessive feed line loss and not a real good impedance match For any new antenna and known good feed line you should pay attention to the SWR at the ends of the band or where the SWR is 2:1 and be aware of any changes that could indic...

  Propagation The last post on the 15 meter antenna brought up the radiation angle and the height of the ionosphere. I thought I would switch gears here and discuss some radio wave propagation. To make effective use of any antenna or to choose the right antenna it helps to know something about propagation or how a radio wave gets from here to there. Most of our high frequency radio communication depends on the ionosphere bending the radio signal so it will return to earth. The ionosphere is an area of the upper atmosphere where the air is much less dense than here on the surface of the earth. It is also gets the full force of the radiation from the sun. It is the radiation from the sun that causes the air molecules to form ions. Positive and negative charges are created by the radiation from the sun. The strength of the radiation from the sun varies considerably. To put it simply, the radiation varies somewhat predictably with the number of sunspots following an eleven-year cycle. ...

  The 15 Meter Dipole and elevation angles The higher bands (15 and 10 meters) are now starting to open for DX. If you do not have an antenna for these bands yet, I suggest building a simple dipole fed with coax. For lengths of up to 100 feet, RG 8X is not a bad choice. The loss for 100 feet of RG8 X should be less than 2 dB at 15 meters. Less loss will require larger and heavier coax. I feel it is most important to get the dipole outdoors if possible. It does not have to be real high as long as it is away from any nearby objects, especially metalic objects. On 15 meters I consider 20 to 25 feet high. That height puts the antenna a halfwave above the ground. Any antenna that is a halfwave above the ground is frequently a better general purpose antenna than one twice as high. I will go into more detail about antenna patterns at a later date. For now let's just say that I think the magic number for the height of a horizontal antenna (not a vertical antenna) is one half wavelength! Th...

  Verticals only if you can not get horizontal antennas "high" I have recommended that horizontal antennas be a half wave above ground for best performance. For the lower bands, this states to be a problem for most of us. While a half wave on 20 meters is only 33 feet, on 40 this becomes 66 feet and on 80 meters this is now 120 or 130 feet! Most of us can only have "low" dipoles on 80 or 160 meters. In order to have a good performing antenna on these bands, especially for DX, we have to either put up a very high tower or use some form of vertical. It is amazing how much improvement a simple bent vertical or inverted L antenna can be over a dipole at the same "low" heigth. The low dipole does a great job for distances up to maybe 500 miles on 160 or 80 meters. If you want a reasonably strong signal at greater distances, some form of vertical is porbably your answer.   I was able to hang a half square antenna for 80 meters between two pine trees. I was not a...

  More about the Impedance Concept The concept of impedance is very important in antennas and transmission lines. Remember that impedance is the ratio of voltage to current. Let's look at what happens when we apply a radio frequency voltage to one end. When the voltage is first applied, a certain amount of current will flow. What ever the ratio voltage to current turns out to be is called the characteristic impedance of the transmission line. If we apply 10 volts and find that initially .2 amps flows, then 10/. 2 = 50. The impedance of the line is 50 ohms. If the line was 186,000 miles long we would be able to measure this .2 amp current for about two seconds. After however long it takes the applied voltage and current to reach the end and be reflected this current may change. If the transmission line is connected to an impedance equal to the characteristic impedance, then there will be no reflection and the current remains at its initial value. You will also hear the term surge im...

  Two halves does not always make a whole Antennas do not have to be resonant to work well, but it does make feeding them a little less complicated in general. When learning the basics it may make it easier to study resonant antennas first. It is my intention to stick with resonant antennas for a while longer. There may be exceptions as there is with anything. The shortest resonant antenna is a half wave long. Remember a half wave in free space is determined by the formula 492/frequency in MHz. When confined to a wire, the speed changes and there are other effects that appear to change the speed. The sum total of all these effects is to reduce the length by about 5%. So you change the formula to 468 if you are using most kinds of wire. That means that a half wave of wire is determined by dividing 468 by the frequency in MHz. So for example on 1.850 MHZ (160 meter band) the length of a half wave of wire is 468/1.85 = 252.9 feet or about 253 feet. The formula for a half wave in a wir...

  Sketching Current on the Antenna I may be repeating a few things as we go along, but I think the important principles of radio and antenna theory need to be reinforced. I have found over the years that current, voltage, impedance and standing waves were among the most misunderstood concepts in radio or electronics. I think everyone has heard the term SWR (Standing Wave Ratio). I have been talking about standing waves on antennas. These are both standing waves of voltage and standing waves of current.   The ratio of voltage to current is the impedance at that point. When dealing with transmission likes, the common SWR is not a voltage to current ratio, but a ratio of two voltages measured at two different places on a line (VSWR). It could also be the ratio of two currents measured at different places on the line (ISWR). In fact there are numerous methods that can be used to measure transmission line SWR. I will eventually get around to explaining that. Right now I want t...

  If the antenna is a resonant half wavelength the currents and voltages are easy to figure. The voltage is normally zero every half wave. The current is also zero at every half-wave point. Not necessarily at the same point. So the current is quite happy ☺ if its natural zeros are a half wave apart as at the two open ends of a wire. Likewise the voltage has peaks every half wavelength and is also quite happy with those peaks occurring naturally at the ends of the dipole. Under these ideal conditions all is well in the world. To top it all off the ratio of voltage to current at the center (that’s where you normally connect coax) is low. Remember that the current is as large as it will ever be at the center and the voltage is at its low point. We said zero but anytime we say zero it may actually be really small. So what is the ratio of voltage to current? Hint: V/I Another hint E/I. Remember Ohms law? R=E/I Resistance is voltage divided by current. Resistance is impedance to current ...

  Slight review and trap dipole principle In a resonant half wave dipole, the voltage and current enter the antenna at the center, travel to the ends and get reflected. The current reflection is out of phase so the currents at the ends cancel. There is always zero current at the ends. (There is always zero current at the end of a an open wire. The voltage is also reflected but is not reversed in polarity like the current, so the voltage at the end is pretty much twice the original voltage. Due to the fact that the voltage and current vary with time, the actual currents and voltages that are set up on the antenna (these are called standing waves) varies or oscillates between two values. At the center the current varies at any given instant between a maximum positive value and a maximum negative value. As you move toward either end the current varies between two limits of decreasing value until at the end the two values go to zero. The voltage and current at the antenna terminals wil...

  Voltage on a wire (especially at the end) Voltage on an antenna works in a similar manner to the current. When the voltage wave reaches the end of the antenna wire, it also is reflected. Unlike the current which totally reverses its direction resulting in zero total current, the voltages do not cancel but add together. A voltage wave consisting of say 50 volts will be reflected at the open end of a wire. The resultant voltage will be 50 plus 50 or 100 volts. Remember the current had to be zero by law. There is a law that says the sum of all currents, into and out of a point must be zero. This law is called Kirchhoff current law. That law does not apply to voltage. Kirchhof does have a voltage law that we, however must follow. More on that later. At this point think of the voltage wave similar to an ocean wave hitting a breakwater. Water flows in, abruptly hits the breakwater and is stopped. Piles up and bounces back away from the wall. If you have ever watched two water waves pas...

  Current on a wire obeys the law I hope the last post gave a good idea of why an antenna can act like a capacitor when short and like an inductor (coil) when long. It is all about the voltage waveform and the current waveforms either being in phase (increasing to a maximum, decreasing to zero and to a minimum all at the same time) or being somewhat out of phase (having one or the other reach a maximum, zero or minimum before the other). There are various reasons for this to happen. I think I explained why it happens in a capacitor or inductor. You may wonder why it happens in an antenna or wire when that wire may not look very much like a capacitor or coil. (Note I will use the terms inductor and coil interchangeably. A coil is an inductor and an inductor is usually a coil.) There are certain things that just happen in this world. You just need to recognize it and believe it. We call these things natural laws. You throw a ball up in the air and it will come down due to the Law of ...

  Why an antenna is inductive or capacitive Now we know how to calculate wavelength and half wavelength on free space using the formulas: wavelength in meters = 300/MHz or  wavelength in feet = 982/MHz It is of course more useful to get a half wavelength in feet from: Half wavelength in feet = 491/MHz As an example if we want to know what a half wave is for the middle of the ten meter band we would find it by dividing 491 by 28.5 and the answer is 17.22 feet. This is the free space half wavelength in feet. It is based on the speed of the wave in space. If we are going to use a wire or some other conductor to carry this signal, then the speed will be less by several percent. Lets assume that we pick a wire with a certain type of insulation, such that the speed is reduced to 95% of the speed in free space. We then need to multiply the 17.22 feet by .95. We get 16.36 feet. This tells us that a half wavelength for our radio signal is only 16.36 feet. If we were going to make a hal...

  The Basics of Calculating Wavelength In order to gain an understanding of how antennas work, I think one needs to have some knowledge about capacitors and inductors. If an antenna is short, (by this I mean shorter than required for resonance) it acts a bit like a capacitor. We say the antenna is capacitive. If the antenna is long or longer than it would be at resonance it acts like a coil or inductor. We usually say that an antenna is capacitive in the first case and inductive in the second case.   It naturally follows that it is important to know if the antenna is resonant or how long would it need to be to be resonant before we can decide if it may be short or long. (This is where I would like to have a blackboard) When dealing with radio frequencies the distance the electrical signal (in a conductor) or radio wave (same signal after it leaves the antenna) travels in one cycle is the key to everything.  If we are using a frequency of 10 MHz, that means that the r...

Introduction For the past 50 years I have been studying antennas. I did not realize it had been that long until I just wrote this. It was 1960 when I first needed an antenna. I got my amateur radio license in November 1961. Becoming an amateur radio operator probably,more than anything else, took me down the path to going to Virginia Tech and becoming an electronics engineer. I am not exactly sure how I started teaching amateur radio. I think it all started when I began teaching morse code during my lunch half-hour to people who wanted to become "hams". After teaching them the code I just naturally helped them with the theory. After a while I became known as a teacher. Back in the late 70's I taught the Advanced and Extra Class for the Southern Peninsula Amateur Radio Klub (SPARK). I have always wanted to write a book about amateur radio and antennas in terms the average person can understand. It just struck me that maybe I should do a "blog" about antennas. Thi...