Don’s Antenna talks by N4DJ

I have said many times that horizontal antennas give better performance (stronger signals) than vertical antennas at 20 meters and higher frequencies. While that is definitely true, there are times that the strongest possible signal is not the most important thing. Once a signal is readable or of adequate strength to meet the communications requirement it is frequently of no use to make it stronger.  Many times a very simple, small antenna with zero dBi gain ( such as a vertical)  or even negative dBi gain ( such as a mobile antenna) will work and satisfy our needs quite nicely. For those times I will use a top loaded mobile antenna mounted on my truck. Under the right conditions it does not take much radiated power to communicate long distances. While with a good horizontal dipole I might be S-9 plus 10 dB on an S-meter in California, I may still be S-8 with a mobile vertical antenna. The difference is this case is maybe 16 dB. For all practical purposes to make the communica...

You can not simply connect a piece of coax to a wire and expect it to work. We can simplify it somewhat if we think of the coax as simply two wires. We have a center conductor and a braid, also called a shield. If we take a wire, let’s say it’s 66 feet long, and connect the coax right at the center. We do that by cutting the wire in the center and connecting the center conductor to one side and the braid to the other. We have just made a dipole fed with coax. Will it present a 50 ohm pure resistive load to the coax that we want? The answer is maybe.  A 66 foot of wire is resonant at about 7.1 MHz on the 40 meter amateur band. Resonant only means that there is no reactance. The wire will appear as a pure resistance anywhere we break the wire and connect the coax. If we connect the coax at the center we have a point where the resistance will be low. If we move off center we have a point where the resistance will be higher. Near either end the resistance will be maybe 2000 ohms. We wi...

  I am always thinking about how I can better explain antennas. One thing I have always suggested and try to always do my self is go back to the basics. Until you have a good grasp of the fundamentals it’s hard to progress. That applies to probably everything. Two major things   come immediately to my mind. Softball and antennas! I can’t stress enough how important the fundamentals are in softball and in antennas. Most of us use coax. Most coax is called 50 ohm coax. The reason is simple. If you terminate any length of coax with a 50 ohm resistor, all the power that reaches the end of the coax will end up in the resistor and turn to heat in the resistor. I say all the power that reaches the end of the coax because there will be some power lost in any transmission line. That’s inevitable. The amount of power lost when the coax is matched to a 50 ohm pure resistive load is usually small. However the load needs to be either 50 ohms with minimal reactance or it needs to be someh...

We have two distinct types of High Frequency bands. Daytime and nighttime. 20 meters is considered to be a daytime band although as the sunspot numbers rise it will stay open later and later.  Higher bands (also considered to be daytime bands) tend to open later in the morning and close earlier in the evening than 20 meters. The lower bands 40, 80 and 160 are considered to be nighttime long distance bands.  Usually the 40 meter band is “short” during the day and will go “long” at some point in the afternoon. A similar thing happens with 80 meters only later in the evening. 160 meters is frequently below the critical frequency and may not have any skip zone. The critical frequency may go as high or higher than 10 MHz during the daylight hours. Very close in sky wave propagation is possible just above, at and below the critical frequency.   D layer absorption affects 160, 80 and 40 meters drastically during the day limiting the distance. At sunrise and sunset as the D layer...

Another way to look at Antennas I am going to approach antennas from a different direction in this post. At least I think I am!  I want to start with the assumption that I want as strong a transmitted signal that I can possibly get at one single place in the world.  Where?  Well let’s start with some place like Utah.  That would be a distance of approximately 1900 miles away. Real close to 3000 KM.  This thought process could be for anywhere.  Next would be to decide which amateur band I want to use. Let’s start with 20 meters.  Obviously power has a lot to do with signal strength. However the same antenna setup will work for 5 watts as well as 1500 watts. So power really does not need to be mentioned again as we want to concentrate on the antenna. Suffice it to say 1500 watts is 10 dB stronger than 150 watts. Whatever antenna we use needs to be pointed in the right direction and in this case that is a heading of about 290 degrees. That’s the great cir...

My first antenna was an end fed wire with a galvanized pipe for the ground rod. 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 tuner 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 end fed 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 ga...

 If you read part 1 you saw that the reflected power from the antenna was attenuated by the inherent loss of the coax. This loss reduces the reflected power seen by any SWR meter at the transmitter. At the antenna in our previous example only 50 watts arrived but we started with 100 watts. The SWR meter thinks the forward power is 100 watts. It does not know that only 50 get to the antenna. If the true SWR at the antenna is 3:1 we will see 12.5 watts reflected and of that only 6.25 arrives back at the SWR meter. So the SWR meter thinks the forward power is 100 and the reflected power is 6.25. That is a SWR of 1.67. In reality the SWR is 3:1. The coax loss causes us to see a better SWR than we really have.  When checking SWR at the transmitter we need to be careful and not accept that 1.67 SWR until we look further. For example we need to check our antennas when we first put them up to see what the SWR curve looks like. On my 80 meter dipole I get a good SWR on the low end but ...

Is that Good SWR really Good?  Lots of times I hear guys talking about their good SWR and I think to myself “that sounds too good to be true”.  I want to caution anyone who has a low and especially a low flat SWR curve about a couple things. The antenna has an impedance that changes with frequency.  If that impedance is a perfect match to your coax you will get a 1:1 SWR and that is good. However if there is any loss in the coax or in the balun or matching device between the coax and the antenna terminals you will not get the correct SWR reading at the transmitter end of your coax. The SWR meter at the input or transmitter end of the coax can measure the forward voltage or power at that point. Let’s call that 100 watts of forward power. If you have only 3 dB of coax loss only 50 watts will make it to the antenna terminals.  In all cases there will be an amount of power lost simply due to the inherent loss in the coax. In this example we will use 3 dB as the coax loss...

 I use both a halfway dipole cut for 15 meters and a dipole cut for the CW end of 40 meters for 15 meters.  The dipole gives about 7 or 8 dBi gain broadside.The 40 meter dipole gives maybe  a bit more gain but in four narrow directions. If I did not need an antenna for 40 meters I would just as soon have two 15 meter dipoles at right angles.  I did something similar this year on 10 meters. I ended up with three dipoles, one NE/SW, one E/W and one N/S on a three position remote switch.  Sometimes switching gave me one or two S-unit increase with the right antenna. Peak gain was from EZNEC was almost 8 dBi and the minimum gain at any point was never more than 3 dB down from that with one of the three antennas and that was only at precisely two compass points. I have had large Yagis on ten metersand switching between these three dipoles felt like instantly swinging a Yagi! A dipole is a really good antenna if high and broadside to the desired direction. It is the n...

This chart gives the additional  gain that can be expected from combining two identical antennas into a broadside array such as two dipoles or two Yagis or two verticals. For two elements  1/4 wave spacing only gives 1 dB gain but 1/2 wave spacing gives almost 4 dB with a maximum of almost 5 dB possible with 0.6 wave spacing. This chart and similar charts for broadside and collinear phased elements have appeared in most of the good antenna manuals for at least over 70 years. One of my oldest antenna books is dated 1949 and has the same information pertaining only to two elements whereas this chart goes from 2 to 16. Note however that significant gain increases are only shown for 2, 3, 4, 6 8, 12 and 16 elements.The largest single element increase (5 dB)  being from 1 to 2 elements. To get another 5 dB increase you must go from 2 to 6 elements. From there you must go for 6 to 16 for that same 5 dB increase.  Such is typical of increasing antenna gain.  With a Yag...

It is difficult to talk about gain of antennas without pictures or plots. The only antenna that radiates equal power in all directions does not exist but its is used as a reference or baseline with which to compare real antennas. Every antenna radiates its power in some pattern. More power in one or more directions and less power in other directions. Gain should be referenced in dBi. So one antenna may have a peak gain of 10 dBi and another may have a peak gain of 5 dBi. Which one is better? That will all depend on where you want your power to go. The one with the 10 dBi gain may radiate mostly straight up and be good for local communication on 40 meters during the day but have a low angle gain of negative 3 dBi.  The one with 5 dBi gain may radiate at a low angle and be great for long distance communication but very poor for local communication. Its gain of 5 dBi at a low angle means it will be 8 dB better than the antenna advertised with 10 dBi gain as the low angle gain is what ...

This is the link to my Half Square video presentation:             https://youtu.be/2SBGdPMnH-E

A knowledge of dB is, in my opinion, necessary when discussing antennas. dB is a power ratio. It can, however, be calculated by using either power or voltage. When comparing two powers the definition of a dB is:   dB = 10 Log P1/ P2 P1/P2 is the power ratio. If P1 is 2 times P2, than P1 is 3 dB more than P2. When comparing two voltage ratios the definition of dB is: dB = 20 Log V1/V2   Where V1/ V2 is the voltage ratio. When working with radio signals we can measure the field strength of a signal in volts or more usually in microvolts.   A strong signal may be 50 microvolts. This is taken as 50 microvolts across a 50 ohm load. We almost always use 50 ohms as our reference impedance. With our radio receivers we typically have S-meters that are some what calibrated to read S-9 when there is a 50 microvolt signal present at the antenna terminals.  If that signal was decreased to 25 microvolts we would see the S-meter go down. How much would it go down?  Well we dec...

 One almost foolproof receiving antenna for 160 through 40 is a basic Beverage on the Ground.  This antenna should work over a wide variety of locations. The terminating resistor value can be changed to move the null off the back. The wire length can be increased, but the antenna described here will work and probably pretty good! Materials needed are:  Two ground rods. I use one 8 foot rod cut in half. 220 to 300 feet of wire. Even less will work. The wire must be insulated.  Box or plastic jar to house terminating resistor.  Box for transformer. Terminating resistor 200 ohms Plus or minus 25%. Not critical. 4:1 impedance transformer Turns ratio 2:1.   Made by winding a 8 turn secondary and a 4 turn primary using a binocular core BN-73-202.  I used #22 enameled wire.   The coax is connected to the 4 turn primary. The wire and the ground rod is connected to the 8 turn secondary. It is important not to use any kind of transformer where there is a DC...

 This is a follow up on the last post. I mentioned in the last post that current I and the length of wire segment over which that current was flowing was all that determined how strong the field strength would be from an antenna. The actual term that was used by Edmund A. Laporte in his chapter on ANTENNAS in the Radio Engineering Handbook (edited by Henney) was Meter-Amperes. I tend to think in Feet-Amperes but both terms are good. He defines this to be the integral of i dl where i is the rms current in a length of the antenna dl. This means if you take the integral of the current over a length of a halfwave dipole you get the result in Meter- Amperes. The area under the curve represents the summation of the total current over that halfwave length of howevermany meters the antenna is long. Of the current sum was say 10 amps and the length was 20 meters then the result would be 200 meter- Amperes.  In the Radio Engineers Handbook there are charts that shows what the field stre...

Almost anything can work for an antenna Considering just wires. RF current flowing in a wire will radiate a signal. There are well known basic relationships for current and the wire length. I like to go back to the basics. I have a Radio Engineering Handbook by Frederick E. Terman.    I consider it a great reference. Terman was a Professor of EE and Dean of the School of Engineering at Stanford University.  In his book Terman includes a formula for the strength of a radiated field. Copies out of the book are below. In an effort to not make this too complicated there are only two variables in the formula that directly relate to the antenna and that is the magnitude of the current I and the length of the antenna or antenna segment l. Other items are the phase of the current, distance from the antenna, elevation angle, the constant pi (3.14159), frequency, time, c the velocity of light, and wavelength. So we see that simply increasing current and length over which current fl...

I have built a couple of two element wire Yagis. They worked pretty good. I got a certain 3 dB improvement possibly 5 at the most. The two element Yagi antennas are almost fool proof. They can work as unidirectional with pretty good front to back or as bi directional with pretty good gain in both directions. I had one that had the bi directional feature. It may sound strange at first but you can build a two element Yagi and depending on the spacing and length of the parasitic element it can be either a director or reflector and somewhere in the middle it acts as both!  I attached a chart below from an antenna book. The driven element and parasite in this special case are the same length.  As the spacing is increased to 0.1 wavelengths it develops good gain, a little over 5 dB,  as a director in the A direction. As the spacing is increased more it loses some gain in the A direction and develops gain in the B direction. At 0.15 wavelength spacing it has about  equal ga...