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.

Extra Class question of the day: Aurora propagation, selective fading; radio-path horizon; take-off angle over flat or sloping terrain; effects of ground on propagation; less common propagation modes

One of the most interesting propagation phenomena is Aurora propagation. To make use of this phenomenon, radio amateurs actually bounce their signals off of the Aurora Borealis, also known as the “Northern Lights.” All of these choices are correct when talking about effects Aurora activity has on radio communications (E3C01):

  • SSB signals are raspy
  • Signals propagating through the Aurora are fluttery
  • CW signals appear to be modulated by white noise

The cause of Aurora activity is the interaction of charged particles from the Sun with the Earth’s magnetic field and the ionosphere. (E3C02) Aurora activity occurs in the E-region of the ionosphere. (E3C03) CW is the emission mode that is best for Aurora propagation. (E3C04) From the contiguous 48 states, an antenna should be pointed North to take maximum advantage of aurora propagation. (E3C11)

Normally, we think of the ionosphere as a mirror, reflecting HF signals back to Earth at the same angle at which the signal hits the ionosphere. While this is normally the case, sometimes the ionosphere does not get refracted sufficiently to return directly to Earth, but instead travels for some distance in the F2 layer before finally being returned. The name of the high-angle wave in HF propagation that travels for some distance within the F2 region is called the Pedersen ray. (E3C08)

While we say that VHF/UHF communications is “line of sight,” the distance that a VHF/UHF radio wave will travel is slightly longer than the line-of-sight distance. We call this distance the “radio horizon” or “radio-path horizon.” The VHF/UHF radio-path horizon distance exceeds the geometric horizon by approximately 15% of the distance. (E3C06) The radio-path horizon distance exceeds the geometric horizon because of downward bending due to density variations in the atmosphere. (E3C14)

Another phenomenon that sometimes makes VHF signals beyond the line of sight is tropospheric ducting. Tropospheric ducting is usually responsible for causing VHF signals to propagate for hundreds of miles. (E3C09)

One of the most frustrating propagation phenomena is selective fading. Selective fading is partial cancellation of some frequencies within the received pass band. (E3C05) It is frustrating because it sometimes makes portions of an otherwise perfectly readable signal unreadable.

Amateur radio operators may sometimes use ground-wave propagation to communicate. One important thing to know about this type of propagation is that the maximum distance of ground-wave propagation decreases when the signal frequency is increased. (E3C12) Vertical polarization is the best type of polarization for ground-wave propagation. (E3C13) So, if you really want to make a contact via ground wave, use a vertical antenna on the 160m band.

To take advantage of some of these phenomena, or to just make your antenna work better, you should know how antenna’s performance changes with changes in its design or installation. For example, the radiation pattern of a horizontally polarized 3-element beam antenna varies as the height above ground changes. What happens is the the main lobe takeoff angle decreases with increasing height. (E3C07)

The performance of a horizontally polarized antenna mounted on the side of a hill will be different from the performance of same antenna mounted on flat ground. Specifically, the main lobe takeoff angle decreases in the downhill direction. (E3C10)

Interesting stuff on the Internet – 8/21/12

Here are three more interesting links that I’ve gleaned from the mailing lists and ham radio groups that I belong to:

  • G8MNY Technical Bulletins. G8MNY has been providing these technical bulletins over the packet network in Europe for many years. They were written by many different authors and cover many different topics including aerials, baluns, filters, microphones, and more.

These last two are from the LinkedIn amateur radio groups. If you’re on LinkedIn, send me a connect request and I’ll add you.

  •  Tubebooks.org. This site has PDFs of many older electronics books. Among them:Audels Radiomans Guide, Edwin P. Anderson, 1945, 880 pages
    An odd book, about 4-1/2″ x 6-1/2″ and a whopping 880 pages, “covering theory, construction, and servicing including television electronics”.  It covers everything from sounds waves through basic electronics, PA systems (including a little info on a WE theatre amp), transmitters, car and aircraft radio, troubleshooting – you name it, it’s in here.  Not a college text, this looks like it could be a handbook for the radio technician or advanced hobbyist of the 1940′s.  Lots of good vintage info!
  • Slim Jim antenna calculator & Slim  Jim information. I always thought that “Slim Jim” was just another term for J-pole. I was wrong. This site not only explains the difference, but has a calculator that lets you design your own. Now, I’m going to have to build one of these!

More interesting stuff from the Internet – 8/5/12

Here are some interesting items I’ve gleaned from Google+ and the amateur radio mailing lists that I’m subscribed to:

simple emf probeSimple EMF probe. EMF probes for professional use can be very pricey, but if you just want to get an idea of what’s happening with the electromagnetic fields in your vicinity, you could build this simple probe (see right) and connect it up to your oscilloscope.

Simple tilt-over tower. KD0ZZ has built a very simple tilt-over tower. Not only that, he’s also published the plans for it. If you have any questions, you can join the HamRadioHelpGroup Yahoo Group and ask your questions there.

WSPR Google+ page. If you are a Google+ user, you can follow this page to get news about WSPR.

 

 

Oriole on a tower and other links of interest

Oriole on a tower

Google sent me a link to this photo. I love orioles, and thought I’d share it with you. In her post, the photographer writes, “My son built the tower, which is 120 feet tall. Not sure how high the oriole was, but he was up there.”

Here are a couple of other links:

Minimum Discernible Difference. This is an attempt by AB7E to “quantify the difference in readability between two signals of comparable, but unequal strength.” The goal was to determine if it’s really worthwhile to modify an antenna to gain say 1 dB or 2 dB of signal strength. Very interesting discussion.

Build your own 100W,  20m, end-fed, half-wavelength antenna. Most end-fed, half-wavelength (EFHW) antenna designs that you see are for QRPers. The reason for this is that the voltages at the end of a half-wavelength can be quite substantial, even at QRP levels. This design, however, is rated at 100W. I have all the parts to do this, except for the enclosure, and plan to try this soon.

If you don’t have all the parts, and don’t feel like scrounging them, the Honolulu Emergency Amateur Radio Club, who published these plans, will also sell you a completely assembled antenna for $42. That seems like a pretty good deal to me, and profits go to support the club. They also have plans for a 40m version.

 

 

First Annual Antenna Party on the Air

Tripp Brown, AC8S, e-mail me yesterday:

I’m sponsoring the first anual Antenna Party on the Air the third weekend in in September! We’ll have hundreds, if not thousands, of stations on the air that weekend! So many antennas will be used by so many stations!

Whether you want to make just a few contacts, or, if you want to make a lot of contacts, don’t sit this one out! Get on the air and have fun talking about your antenna and learning about other’s antennas.

Here are more details:

Starts: 2300Z, September 21

Ends: 0400Z, September 23

Bands: 160, 80, 40, 20, 15, 10, 6, and 2 meters

Modes: AM, CW, FM, SSB

Purpose:
To get as many hams on the air as possible that weekend and to help hams learn about different types of antennas. The idea is for the participants to demonstrate their antennas and describe them to other hams, so that if they hear one they like, they will be able to put up one just like it.

Exchange:
Signal report, US state, Canadian province, or dx country, antenna being used, where it’s located, for example, on a balcony, in an attic, how high up it is, and, how much power you’re running. An example exchange might be:

you’re 59 Alaska, running a long wire, in a backyard, up 10 feet, running 200 watts.

Other Rules:

  • Stations should only be worked once per band, once per mode.
  • Only single station, single ops. This will get as many stations on as possible.
  • Power:  200 watts or less.
  • Must run only from a fixed location, such as, your place of residence.
  • Logging: no logs need to be submitted. Just take down any info about the other stations antenna that you need to know, such as what is given by the station. The station can give as much info on the antenna as needed, such as, how to build it, or, tell the station you’re working, that they can find directions on how to build and put up the antenna at a certain web site, or, to send you an email with a request on info about the antenna.

Extra Class Question of the Day: Feedpoint impedance, antenna efficiency, frequency range

One of the most basic antenna parameters is the feedpoint impedance. Why would one need to know the feed point impedance of an antenna? To match impedances in order to minimize standing wave ratio on the transmission line. (E9A04) The reason that it’s important to minimize the standing wave ratio, or SWR, is that if you’re using coaxial cables, minimizing the SWR will also help you minimize losses. If you minimize losses, you’ll radiate more signal.

Many factors may affect the feed point impedance of an antenna, including antenna height, conductor length/diameter ratio and location of nearby conductive objects. E9A05 For example, we say that the feedpoint impedance of a half-wavelength, dipole antenna is 72 ?, but that’s only really true if the antenna is in free space. When it’s closer to the ground than a quarter wavelength, then the impedance will be different. That’s why you have to tune the antenna when you install it.

Another antenna parameter that’s frequently discussed is radiation resistance. The radiation resistance of an antenna is the value of a resistance that would dissipate the same amount of power as that radiated from an antenna. (E9A15) Radiation resistance plus ohmic resistance is included in the total resistance of an antenna system. (E9A06)

If you know the radiation resistance and the ohmic resistance of an antenna, you can calculate its efficiency. You calculate antenna efficiency with the formula (radiation resistance / total resistance) x 100%. (E9A10)

Vertical antennas are sometimes criticized as being inefficient antennas. Soil conductivity is one factor that determines ground losses for a ground-mounted vertical antenna operating in the 3-30 MHz range. (E9A12) If soil conductivity is poor, ohmic resistance will be high. One way to improve the efficiency of a ground-mounted quarter-wave vertical antenna is to install a good radial system. (E9A11)

The frequency range over which an antenna satisfies a performance requirement is called antenna bandwidth. (E9A09) Normally, the performance requirement is an SWR of 2:1 or less. In fact, you’ll sometimes hear this parameter referred to as the 2:1 SWR bandwidth.

Finally, this section has a question that really doesn’t fit in here about folded dipoles. A folded dipole antenna is a dipole constructed from one wavelength of wire forming a very thin loop. (E9A07)

Handheld instrument analyzes antennas and cables, measures power

I came across this product announcement in the daily e-mail I get from EE Times. Wouldn’t it be great to have this analyzer? There is one drawback. It costs almost $7k. 

Site Master Handheld Cable & Antenna Analyzer S331L

 Features
  • 2 MHz to 4 GHz Handheld Cable and Antenna Analyzer, impact, dust, and splash resistant
  • Measures power from 50 MHz to 4 GHz
  • More than 8 hours of continuous battery operation
  • Standard built-in InstaCal™ module and Power Meter
  • FlexCal™ maintains calibration with frequency changes
  • Built-in one button Help function
  • 800×480 7” TFT touch screen display and multiple USB ports
  • Internally store >1000 files with fast preview of stored traces
  • Industry standard *.dat format compatible with Line Sweep Tools (LST) and HHST

You can download the product brochure to get an even better idea of what a cool meter this is.