Antennas and SWR Revisited!

Size: px

Start display at page:

Download "Antennas and SWR Revisited!"

Berniece Fleming
5 years ago
Views:

1 The Kaw Valley Amateur Radio Club Newsletter THE TRANSCEIVER May 2014 Editor: Doug Dunton ARRL Affiliated Since Club Officers President: Paul Mills Secretary: Susan Sims Treasurer: Paul Bayless Antennas and SWR Revisited! Why revisit this subject again? There have been frequent misunderstandings of this subject. Where do you tune an antenna? You tune an antenna at the antenna. Tuning is usually done by changing the length of the antenna. Short antennas are tuned by making them appear longer than physical length. The most common method of making an antenna look longer is to add a loading coil. What will the SWR of a resonate antenna be? It will be greater than 1:1, unless we do something to make it 1:1. A half wave dipole that is ½ wavelength or greater above ground will have an impedance around 75 ohms. This will give us a SWR of around 1.5:1 If we turn it into an inverted-v the impedance will approach 50 ohms at some point, thus an SWR of near 1:1. However these changes will change antenna pattern and takeoff angle. A quarter wave ground plane will be around 25 ohms unless we angle the radials downward. So, at resonance, a ground plane antenna could have a SWR of as much as 2:1. It is usually desirable to angle the radials downward for best performance and feed line match. Yagi antennas will have a very low impedance, thus a high SWR unless some matching is done. Where is the best place to match antenna impedance? The best place to match antenna impedance is at the antenna or its feed point. Our radios are designed to attach to 50 ohm antennas with 50 ohm feed line. Therefore the best place to cure these mismatches is at the place where the mismatch occurs. This becomes more important as SWR increases, and as line loss increases. (See: Understanding SWR by Example November 2006 QST page 37)

Why does my SWR increase when I talk? There could be several reasons for an increase in SWR as you talk. First, remember that SWR cannot be read with no power applied to the antenna system.

2 Why does my SWR increase when I talk? There could be several reasons for an increase in SWR as you talk. First, remember that SWR cannot be read with no power applied to the antenna system. Your antenna system could have some component that is failing under power. Another could be a defect in your SWR meter, especially those built into radios. Often samples of forward and reverse power are taken at different times. If a forward sample is taken at a point of low modulation and the reverse power sample at a point of high modulation, the SWR could be improperly calculated. When this happens, you may see SWR values jumping around as you talk. Don t panic, verify. Why does my external SWR meter read different from the built in meter on my radio? Because you do not have a perfectly matched antenna system, and the two meters are located at different places in the system. This leads to myths that antenna feed lines must be a certain length. (There are some antenna designs where certain feed line lengths should be avoided.) Also meter accuracy varies. Or if you are using an internal antenna tuner, the SWR really is different at the two points. I have an SWR on the low end of a band of 1.7:1, high end is 1.8:1, and center of band is 1.4:1. Should I use an antenna tuner to improve this? No. The tuner will only add to your losses. Your SWR is not high enough to hurt your radio, so leave well enough alone. I plan on talking about ESR at the next club meeting. What is ESR? If you really want to know, come to the meeting. See you there! Have a wonder filled month. Until next month 73 de AC0HY Reminders: Annual 2014 Membership form at KVARC Application Ham FAQ: For Hams new to the area or are just licensed - Ham FAQ KVARC Club meeting May 2nd 7:30pm. This will be at the Topeka Shawnee County Library. Topeka, KS. Eyeball QSO starts about 7pm Regular test session at June 14 at Topeka Library at 1pm. Pre-register with Paul at ham.test@kvarc.org

3 Regular test session at May 9th at Carbondale City Hall at 7pm. Pre-register with Paul at Tuesday night ARES net on at 830pm NEW NET! - Thursday night Carbondale net 800pm on KB0WTH repeater Wednesday night simplex net on at 730pm run by KB0WOW Dan Field day June 28 th and 29 th and the 27 th if we decide to do some setup on Friday evening. More details to come EINSTEIN ON RADIO "I am often asked how radio works. Well, you see, wire telegraphy is like a very long cat. You yank his tail in New York and he meows in Los Angeles. Do you understand this? Now, radio is exactly the same, except that there is no cat." Attributed to Albert Einstein Other info: Training Topic What is a Communication Emergency? One of the primary missions of the Alpha-1 ARES covering Shawnee/Wabaunsee county, is to provide communications during an emergency. This training topic will address the question, then, what is a communication emergency? A communication emergency exists when a critical communication system failure puts the public at risk. A variety of circumstances can overload or damage critical day-to-day communication systems. It could be a storm that knocks down telephone lines or radio towers, a massive increase in the use of a communication system that causes it to become overloaded, or the failure of a key component in a system that has widespread consequences. Most cellular phone systems are designed to handle only about 6-10% of

4 the subscribers at any one time. This works well in normal situations and is economical for the company. But when a crisis happens, they quickly become overloaded as everyone(the other 90%) tries to talk at once. Examples are easily found. Violent storms and earthquakes can knock down communication facilities. Critical facilities can also be damaged in normal circumstances: underground cables are dug up, fires occur in telephone equipment buildings, or a car crash knocks down a key telephone pole. Hospital or 911 telephone systems can fail. Even when no equipment fails, a large-scale emergency such as a chemical or nuclear accident can result in more message traffic than the system was designed to handle. Some emergency operations occur in areas without any existing communication systems, such as with backcountry searches or fires. (1) With such endless possibilities, then, it is important to recognize that we as amateur radio operators are a valuable resource to the communities we live in because we possess the skills as emergency communicators, the equipment and the frequency spectrum to meet the needs of such communication emergencies. These skills require training and practice. Training available through ARRL's Licensing, Training and Education section. We all must be prepared to assist with Emergency Communication in the event of a disaster.. Why? Because EmComm is a lot more disciplined and different than general 2 meter communication and Net check-in. 1 American Radio Relay League Amateur Radio Emergency Communications Course, An Introduction to Emergency Communication, (EC-001), Learning Unit 1, Topic 1: Introduction to Emergency Communication course. Doug Bluthardt AC0RS

Please send any equipment you have for sale and I will include in next month s issue Elmer-ing Attached is a form for letting me know what you might be interested in having help

I have mentioned this new item to a few people, I would like to suggest adding a get together some other time of the month as a learning session.

5 Please send any equipment you have for sale and I will include in next month s issue Elmer-ing Attached is a form for letting me know what you might be interested in having help with. Or If are available to help someone. Please fill in as appropriate and back to me or bring to the club meeting Friday. I have mentioned this new item to a few people, I would like to suggest adding a get together some other time of the month as a learning session. I am open to suggestions as to a day and time. I can try to schedule time at the library or if a different location is available I would take that as a suggestion also. So what do you think? Let me know by or at the club meeting. ARRL Affiliate since 1926

7 Need / willing to be an Elmer Info Call Name How to contact Phone Other Location City Long/Lat Grid square want/need help Interests level of expertise Willing to help Antennas CW Digital General Info Radio to buy APRS ATV Level of expertise (1-5) 1=beginner, 5=expert Give to Doug(WD0DBS) or to wd0dbs@arrl.net

The Kaw Valley Amateur Radio Club Newsletter THE TRANSCEIVER. June Club Web Page:

The Kaw Valley Amateur Radio Club Newsletter THE TRANSCEIVER June 2018 Club Web Page: http://www.kvarc.org How Can Anyone Be Bored in Today s World? Each and every day brings so many adventures that I