KB6NU finally builds an end-fed, half-wave antenna

I’ve posted about end-fed, half-wave antennas before, but until this weekend, I’d never built one. One of the reasons for this is that most designs are for QRP antennas and not made to handle more than 5 – 10 W of power.

A couple of months ago, I ran across a design rated at 100 W. The design seemed relatively simple to build, requiring only a single toroid and a capacitor made with a short length of RG-174 coax. Well, it just so happens that I bought 100-ft. of RG-174 at Dayton this year, and I found the toroid cores online from the  “Toroid King” for a very reasonable price, so I decided to make my own.

All told, the parts cost about $10, the biggest part of that the plastic junction box I got from Lowe’s for $6.41. Compare that with the $60 that LNR wants for their end-fed antenna.

Sunday was a beautiful day here with temperatures in the 60s, so I decided to put up the antenna. I’d love to report that everything worked perfectly, but this antenna is going to need a little more work. The SWR is 2.6:1 at 14.000 MHz, dropping to about 1.5:1 at 14.900 MHz.

Since the internal tuner on my IC-746PRO is supposed to be good to 3:1, I did use it and made a couple of contacts. The guy in MA even gave me a 599 signal report. I’d be a little more comfortable about using it, however, if I could get the SWR down a bit.

I e-mailed the guy who published the design and asked why he thought the resonant frequency was so high, and he said that all I had to do was add a couple feet of wire to the antenna. He also suggested that adding a turn or two to the coil would bring the SWR down. I did some more reading about end-feds and I’m thinking that perhaps adding a short counterpoise might be something to try, too.

So, while the results so far have been mixed, I’m hopeful that with a little tweaking, I’ll have another antenna to add to my arsenal. It’s been a good learning experience, and I’ve certainly saved a bunch of money over the commercial versions.

UPDATE 11/18/12:
I added 24-in. to the antenna  and it did indeed bring down the SWR of the antenna to below 2:1 in the CW portion of 20m.  I’m happier with this. I still do plan to try a counterpoise. Not so much to improve the SWR, but to see if it makes the antenna a little more efficient.

The box I used for this project is the Carlon E989NNJ, a 4-in. x 4-in. x 2-in. plastic junction box.  This is a very nice box. Not only is it completely enclosed. The screw down cover is gasketed, making it waterproof.

I liked the box so much, I went back to Lowe’s to get a couple more. Not only were they out of stock, when I searched their website for that part number, it came up with no results. It kind of looks to me as though they’re not planning to stack this box anymore.

I Googled the part number and found several places online that had them, but they wanted more money, plus I would have to pay shipping. Fortunately, I was able to find some at a local Home Depot. They wanted $6.88, compared to $6.41 at Lowe’s, but at least I was able to purchase a couple more of them.

Operating Notes – 11/1/12

ICOM/Kenwood Power ConnectorJunk, continued. A couple of months ago, I wrote about the importance of junk in amateur radio. Well, here’s another example.
Last week, my friend, Ralph, AA8RK, acquired a used TS-850. Unfortunately, it didn’t have a power cable, so he sent a message to our club e-mail list asking if anyone had a cable he could buy from them. It struck me that while I didn’t have a cable that I wanted to sell him, I did have all the parts needed to make one. For some reason a year or so ago, I happened to purchase the Molex connector at a hamfest, and at Dayton this year, I purchased more 12-ga., two-conductor wire and a big bag of PowerPole connectors.
Ralph came over Monday night, and we had a fine time making the connector and talking about ham radio in general. Ham radio is more fun if you have the right junk!

Horse-fence antenna revisited. One of the things that Ralph and I talked about was my horse-fence antenna. I was unsuccessful with my first attempt at making a 2m dipole with the horse-fence material and just haven’t had the motivation to get back to working on it. Well, when I showed it to Ralph, he said, “It looks to me like you have a bunch of little capacitors there,” referring to the 14 very fine stainless steel wires running through the plastic ribbon, “maybe that’s detuning it.”
I think he might be right.  I’m going to go get a couple more clamps and clamp the far end of the antenna elements and see what effect that has.

55th JOTAJamboree on the Air (JOTA) 2012. Almost two weeks ago now, we had a great Jamboree on the Air at WA2HOM. On Saturday, we had eight or ten Scouts. Several of them already had their General Class licenses, so all I had to do was sit them down at the radio, show them how to use the controls, and they were off to the races. They even worked a new country for us – Trinidad. The cool thing was that the station they contacted down there was also participating in the JOTA, and they got to talk to some Trinidadian Scouts.

 Sandy quiets the bands. On Tuesday night, the 40m band was oddly quiet as hurricane Sandy took out the power on the East Coast and otherwise occupied hams there. It was quite noticeable that there were a lot fewer stations on the air.

25, 50, and 75 Years Ago in QST

QST publishes a column every month towards the back of the magazine that highlights from issues 25, 50, and 75 years ago. Now that the QST archive is online, it’s really worth taking a look at these articles. Here are a few that were interesting to me this month:

  • October 1937
    • Modernizing the Simple Regenerative Receiver by Vernon Chambers, W1JEQ. This a nicely-designed and built regen using two tubes, a 6K5 pentode and 6C5 triode. I’m going to keep this design in mind if I ever get around to playing with all the tubes I have. As an aside, W1JEQ wrote 87 articles for QST from 1936 through February 1958. This was his third article.
    • Concentrated Directional Antennas for Transmission and Reception by John L. Reinartz, W1QP, and Burton T. Simpson, W8CPC. This article describes two different antennas. The first is a  half-wave loop antenna that the author says works on 2-1/2, 5, 10, and 20m. The second is a square loop antenna called a “signal squirter” for 14 Mc.
  • October 1962
    • In the “New Apparatus” item on page 27, a key made by J. A. Hills, W8FYO, of Dayton, OH is shown under the heading, “New Key Mechanism for Electronic Keyers.” The photo clearly shows a key whose design was adopted by whoever designed the Bencher BY-1 paddle.
    • The Towering Problem by Jay Kay Klein, WA2LII clearly shows that putting up towers have always been a problem for amateur radio operators. This is a humorous take on the problem. What’s notable is that this type of humorous article almost never appears in QST anymore. Amateur radio seems to have lost its sense of humor.
  • October 1987
    • Stalking Those Fugitive Components by Doug DeMaw, W1FB. We often complain about the demise of local parts suppliers, but this article shows that this was a problem 25 years ago as well. W1FB gives some advice that I gave not long ago–stock up on parts, especially when you find a good deal on them, and you won’t have to scrounge around for them when you want them.

Extra Class question of the day: Smith Chart

NOTE: This is the last installment of the Extra Class question of the day. I’m going to be compiling all of these into the No-Nonsense Extra Class Study Guide. Watch for it real soon now.

 

Figure E9-3A Smith chart is shown in Figure E9-3 above. (E9G05) It is a chart designed to solve transmission line problems graphically. While a complete discussion of the theory behind the Smith Chart is outside the scope of this study guide, a good discussion of the Smith Chart can be found on the ARRL website.

The coordinate system is used in a Smith chart is comprised of resistance circles and reactance arcs. (E9G02) Resistance and reactance are the two families of circles and arcs that make up a Smith chart. (E9G04)

The resistance axis is the only straight line shown on the Smith chart shown in Figure E9-3. (E9G07) Points on this axis are pure resistances. In practice, you want to position the chart so that 0 ohms is at the far left, while infinity is at the far right.

The arcs on a Smith chart represent points with constant reactance. (E9G10) On the Smith chart, shown in Figure E9-3, the name for the large outer circle on which the reactance arcs terminate is the reactance axis. (E9G06) Points on the reactance axis have a resistance of 0 ohms. When oriented so that the resistance axis is horizontal, positive reactances are plotted above the resistance axis and negative reactances below.

The process of normalization with regard to a Smith chart refers to reassigning impedance values with regard to the prime center. (E9G08) The prime center is the point marked 1.0 on the resistance axis. If you’re working with a 50 ohm transmission line, you’d normally divide the impedances by 50, meaning that a 50 ohm resistance would then be plotted on the resistance axis at the point marked 1.0. A reactance of 50 + j100 would be plotted on the resistance circle going through the prime center where it intersects the reactance arc marked 2.0.

Impedance along transmission lines can be calculated using a Smith chart. (E9G01) Impedance and SWR values in transmission lines are often determined using a Smith chart. (E9G03) Standing-wave ratio circles are often added to a Smith chart during the process of solving problems. (E9G09)

The wavelength scales on a Smith chart calibrated in fractions of transmission line electrical wavelength. (E9G11) These are useful when trying to determine how long transmission lines must be when used to match a load to a transmitter.

Extra Class question of the day: Direction finding

Direction finding is an activity that’s both fun and useful. One of the ways that it’s useful is to hunt down noise sources. It can also be used to hunt down stations causing harmful interference.

A variety of directional antennas are used in direction finding, including the shielded loop antenna. A receiving loop antenna consists of one or more turns of wire wound in the shape of a large open coil. (E9H09) The output voltage of a multi-turn receiving loop antenna be increased by increasing either the number of wire turns in the loop or the area of the loop structure or both. (E9H10)

An advantage of using a shielded loop antenna for direction finding is that it is electro-statically balanced against ground, giving better nulls. (E9H12) The main drawback of a wire-loop antenna for direction finding is that it has a bidirectional pattern. (E9H05)

Sometimes a sense antenna is used with a direction finding antenna. The function of a sense antenna is that it modifies the pattern of a DF antenna array to provide a null in one direction. (E9H08)

Another way to obtain a null in only one direction is to build an antenna array with a cardioid pattern. One way to do this is to build an array with two dipoles fed in quadrature. A very sharp single null is a  characteristic of a cardioid-pattern antenna is useful for direction finding. (E9H11)

Another accessory that is often used in direction finding is an attenuator. It is advisable to use an RF attenuator on a receiver being used for direction finding because it prevents receiver overload which could make it difficult to determine peaks or nulls. (E9H07)

If more than one operator can be mobilized for a direction-finding operation, they could use the triangulation method for finding a noise source or the source of a radio signal. When using the triangulation method of direction finding, antenna headings from several different receiving locations are used to locate the signal source. (E9H06)

Extra Class question of the day: Effective radiated power

Effective radiated power is a widely misunderstood concept. Effective radiated power is the term that describes station output, including the transmitter, antenna and everything in between, when considering transmitter power and system gains and losses. (E9H04)

The effective radiated power, or ERP, is always given with respect to a certain direction. Let’s think about this for a second. If your transmitter has an output of 100 W, the maximum power that the antenna can radiate is also 100 W. Transmitting antennas are, after all, passive devices. You can’t get more power out of them that you put into them. In reality, the total power output will be even less than 100 W because you will have losses in the feedline.

An antenna can, however, concentrate the power in a certain direction. The power being radiated in that direction will be more than the power radiated in that direction by a reference antenna, usually a dipole or an isotropic antenna, which is an antenna that radiates equally in all directions.

When an antenna concentrates power in a certain direction, we say that it has gain in that direction, and we specify the amount of gain in dB. If the reference antenna is an isotropic antenna, then the unit of gain is dBi. If the reference antenna is a dipole, then the unit of gain is dBd.

With that in mind, let’s take a look at an example. In this example, a repeater station has 150 watts transmitter power output, there is a 2-dB feed line loss, 2.2-dB duplexer loss, and the antenna has 7-dBd gain. To calculate the system gain (or loss), you add the gains and losses, so

Gain = 7 dBd – 2 dB – 2.2 dB = + 2.8 dB

dB Ratio
1 1.26:1
2 1.585:1
3 2:1

Now, if you recall, 3 dB is close to a gain of 2, as shown in the table at right, so in this example, to calculate the effective radiated power, you multiply the transmitter’s output power by a factor slightly less than two. This makes the effective radiated power slightly less than 15o W x 2, or 300 W. The closest answer to 300 W is 286 W. (E9H01)

Let’s look at another example. The effective radiated power relative to a dipole of a repeater station with 200 watts transmitter power output, 4-dB feed line loss, 3.2-dB duplexer loss, 0.8-dB circulator loss and 10-dBd antenna gain is 317 watts. (E9H02) In this example, the gain is equal to 10 dB – 8 dB in lossses or a net gain of 2 dB. That’s equivalent to a ratio of 1.585:1. The ERP is then 200 W x 1.585 = 317 W.

Now, lets look at an example using an isotropic antenna as the reference antenna. The effective isotropic radiated power of a repeater station with 200 watts transmitter power output, 2-dB feed line loss, 2.8-dB duplexer loss, 1.2-dB circulator loss and 7-dBi antenna gain is 252 watts. (E9H03) In this example, the gain is equal to 7 dB – 2 dB – 2.8 dB – 1.2 dB = 1 dB. That’s equivalent to a ratio of 1.26:1, so the ERP is 200 W x 1.26 = 252 W.

Extra Class question of the day: Wire and phased vertical antennas

There are many ways to put up antennas that are directional. Yagis are directional antennas, but they require a structure, such as a tower, to get them high in the air. One way to get directionality without a tower is to use phased vertical arrays.

In general, the phased vertical array consists of two or more quarter-wave vertical antennas. The radiation pattern that the array will have depends on how you feed the vertical antennas.

So, for example, the radiation pattern of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase is a figure-8 oriented along the axis of the array. (E9C01) The radiation pattern of two 1/4-wavelength vertical antennas spaced 1/4-wavelength apart and fed 90 degrees out of phase is a cardioid. (E9C02) The radiation pattern of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed in phase is a Figure-8 broadside to the axis of the array. (E9C03)

A rhombic antenna is often used for receiving on the HF bands. A basic unterminated rhombic antenna is described as bidirectional; four-sides, each side one or more wavelengths long; open at the end opposite the transmission line connection. (E9C04) The disadvantages of a terminated rhombic antenna for the HF bands is that the antenna requires a large physical area and 4 separate supports. (E9C05) Putting a terminating resistor on a rhombic antenna changes the radiation pattern from bidirectional to unidirectional. (E9C06)

Figure E9-2: Elevation Pattern

The type of antenna pattern over real ground that is shown in Figure E9-2 is an elevation pattern. (E9C07) The elevation angle of peak response in the antenna radiation pattern shown in Figure E9-2 is 7.5 degrees. (E9C08) The front-to-back ratio of the radiation pattern shown in Figure E9-2 is 28 dB. (E9C09) 4 elevation lobes appear in the forward direction of the antenna radiation pattern shown in Figure E9-2. (E9C10)

How and where you install an antenna affects its radiation pattern. For example, the far-field elevation pattern of a vertically polarized antenna is affected when it is mounted over seawater versus rocky ground. What happens is that the low-angle radiation increases. (E9C11) The main effect of placing a vertical antenna over an imperfect ground is that it reduces low-angle radiation. (E9C13) When constructing a Beverage antenna, remember that it should be one or more wavelengths long to achieve good performance at the desired frequency. (E9C12)

Extra Class question of the day: Transmission line characteristics

The physical length of a coaxial cable transmission line shorter than its electrical length because electrical signals move more slowly in a coaxial cable than in air. (E9F03) The term we use to quantify the difference in how fast a wave travels in air versus how fast it travels in a feedline is velocity factor.

The velocity factor of a transmission line is the velocity of the wave in the transmission line divided by the velocity of light in a vacuum. (E9F01) Put another way, velocity factor is the term for the ratio of the actual speed at which a signal travels through a transmission line to the speed of light in a vacuum. (E9F08) The dielectric materials used in the line determines the velocity factor of a transmission line. (E9F02)

The typical velocity factor for a coaxial cable with solid polyethylene dielectric is 0.66. (E9F04) That makes the approximate physical length of a solid polyethylene dielectric coaxial transmission line that is electrically one-quarter wavelength long at 14.1 MHz about 3.5 meters. (E9F05)The approximate physical length of a solid polyethylene dielectric coaxial transmission line that is electrically one-quarter wavelength long at 7.2 MHz is 6.9 meters. (E9F09)

The velocity factor of air-insulated, parallel conductor transmission lines is a lot closer to 1 than the velocity factor for coaxial cable. The approximate physical length of an air-insulated, parallel conductor transmission line that is electrically one-half wavelength long at 14.10 MHz is 10 meters. (E9F06)

While having a higher velocity factor is not really such a big advantage, open-wire or ladder line feedlines do have other advantages. For example, ladder line has lower loss than small-diameter coaxial cable such as RG-58 at 50 MHz. (E9F07)

Sometimes we use various lengths of coax to match an antenna system or to filter out frequencies. A 1/8-wavelength transmission line presents an inductive reactance to a generator when the line is shorted at the far end. (E9F10) A 1/8-wavelength transmission line presents a capacitive reactance to a generator when the line is open at the far end.

A 1/4-wavelength transmission line presents a very low impedance to a generator when the line is open at the far end. (E9F12) A 1/4-wavelength transmission line presents a very high impedance to a generator when the line is shorted at the far end. (E9F13)

A 1/2-wavelength transmission line presents a very low impedance to a generator when the line is shorted at the far end. (E9F14) A 1/2-wavelength transmission line presents a very high impedance to a generator when the line is open at the far end. (E9F15)

Extra Class question of the day: matching antennas to feedlines

For many types of antennas, matching the impedance of the antenna to the impedance of the feedline, normally coax, is essential. Mismatched lines create high SWR and, consequently, feedline losses. An SWR greater than 1:1 is characteristic of a mismatched transmission line. (E9E08)

When a feedline and antenna are mismatched, some of the power you are trying to transmit will be reflected back down the feedline. The ration of the amplitude of the reflected wave to the amplitude of the wave you are trying to send is called the reflection ratio, and it is mathematically related to SWR. Reflection coefficient is the term that best describes the interactions at the load end of a mismatched transmission line. (E9E07)

To match the impedance of the feedline to the impedance of the antenna, we use a variety of different techniques. The delta matching system matches a high-impedance transmission line to a lower impedance antenna by connecting the line to the driven element in two places spaced a fraction of a wavelength each side of element center. (E9E01)

The gamma match is the name of an antenna matching system that matches an unbalanced feed line to an antenna by feeding the driven element both at the center of the element and at a fraction of a wavelength to one side of center. (E9E02) The purpose of the series capacitor in a gamma-type antenna matching network is to cancel the inductive reactance of the matching network. (E9E04)The gamma match is an effective method of connecting a 50-ohm coaxial cable feed line to a grounded tower so it can be used as a vertical antenna. (E9E09)

The stub match is the name of the matching system that uses a section of transmission line connected in parallel with the feed line at or near the feed point. (E9E03) What the stub does is to add reactance at the feed point. By varying the length of the stub, you can change the reactance that the stub provides to whatever value is needed. An effective way of matching a feed line to a VHF or UHF antenna when the impedances of both the antenna and feed line are unknown is to use the universal stub matching technique. (E9E11)

Inserting a 1/4-wavelength piece of 75-ohm coaxial cable transmission line in series between the antenna terminals and the 50-ohm feed cable is an effective way to match an antenna with a 100-ohm feed point impedance to a 50-ohm coaxial cable feed line. (E9E10) Note that this will only work on one band as the length of 75-ohm coax you use will only be 1/4 of a wavelength on one band.

Many directly-fed Yagi antennas have feedpoint impedances of approximately 20 to 25 ohms. One technique often use to match these antennas to 50-ohm coaxial cable is the hairpin match. To use a hairpin matching system to tune the driven element of  a 3-element Yagi, the driven element reactance must be capacitive. (E9E05) The equivalent lumped-constant network for a hairpin matching system on a 3-element Yagi is an L network. (E9E06)

Some beam antennas use multiple driven elements in order to make them multi-band antennas. The primary purpose of a phasing line when used with an antenna having multiple driven elements is that it ensures that each driven element operates in concert with the others to create the desired antenna pattern. (E9E12)

I’m not sure that Wilkinson dividers are used much in antenna systems, or why this question is in the section on feedline matching, but here it is. The purpose of a Wilkinson divider is that it divides power equally among multiple loads while preventing changes in one load from disturbing power flow to the others. (E9E13)

Interesting stuff on the Internet – 8/27/12

Here’s some more interesting stuff that I’ve run across on the Internet:

  • HamInfoBar. This toolbar, which is available for Firefox, IE, Safari, and Chrome, give you easy access to a wide variety of net info, including callsign info, APRS station info, eQSL data, and electronic component data.
  • Area51 ham radio forum. Area51 is a set of bulletin boards. Some hams are trying to establish a ham radio forum there, but they need a minimum number of “followers” to do so. Click on the link and follow the forum.
  • Simple Ham Antennas. This ham-radio blog, written by KH6JRM, has some very specific content – all of the posts describe simple antennas that can be home-brewed. For example, the last three are a 40m – 10m loop antenna, a vertical antenna for 40/20/15/10m, and stealth antennas.