Capture Effect and Low Power Repeater Use

1. Capture Effect and Low Power Repeater Use By: David E. Rudd, CET WC AI4JI

2. What is Capture Effect? In referring to radio, capture effect, or FM capture effect, is a phenomenon associated with FM reception wherein only the stronger of two signals at, or near, the same frequency will be demodulated.

3. It’s What? Capture effect is defined as the suppression of the weaker signal at the receiver limiter; where the weaker signal is not amplified, but attenuated.

4. How does it happen? When more than one signal is nearly equal in strength, or the signals are fading independently, the receiver may switch from one signal to the other and exhibit picket fencing.

5. What in the blue blazes is Picket Fencing? Originally from amateur radio, the phrase was used to describe the way an FM transmitter will cut in and out as it nears the capture threshold of the receiver, thus chopping the speech of the transmitting operator. It is not clear if the phrase was intended to describe the loss of the speech, or if it actually referred to the chopping sound itself, which imitates the noise produced by dragging a stiff object across a picket fence.

6. Kidz Today.. • Picket fencing is slang for the chopping effect sometimes heard by cell phone users at the edge of a cell's coverage area, or (more likely) by the landline user to whom the cellphone is connected. "Picket fencing" refers to the way portions of speech are stripped from the conversation, as if the listener was walking by a picket fence, and hearing a conversation on the other side that changes audibily depending on the position of the pickets relative to the listener.

7. Back to Technical Reality.. The capture effect can occur at the signal limiter, or in the demodulation stage. Some types of radio receiver circuits have a stronger capture effect than others. The measurement of how well a receiver can reject a second signal on the same frequency is called the capture ratio for the specific receiver being measured.

8. How do we measure it? Capture Effect is measured as the lowest ratio of the power of two signals that will result in the suppression of the smaller signal.

9. What about Ancient Modulation? Amplitude Modulation, or AM radio is not subject to Capture Effect. This is one reason that the aviation industry, the military, and some others, have chosen to use AM for certain voice communications rather than FM. Thus allowing multiple signals to be heard on the same frequency.

10. The Capture Effect Battle While we are listening to our repeater and conditions are favorable for ducting, a repeater from Alabama comes booming in on our frequency. When our repeater un-keys you hear them in the background. Our repeater has won the capture effect battle. When you are listening to the morning CRS net and in the middle of someone grumbling about Brain Donors, all of a sudden the Alabama repeater comes in loud and clear; well, they won the battle. Their signal was so strong, it overpowered our local repeater’s signal at your receiver.

11. You can relate to this.. While we are happily using our repeater and some individuals in another state decide to point their antennas in our direction, crank up their power, and operate in a manner against the rules; you will hear what sounds like our folks dropping in and out, or being muffled. That is Capture Effect and our repeater’s receiver trying to switch between the stronger of the two signals arriving at the antenna.

12. Low Power Repeater Use The FCC Rules state that an operator will use the MIMIMUM amount of power to conduct reliable two way communications.

13. What is RELIABLE? Reliable [ri-lahy-uh-buh l] –adjective that which may be relied upon; dependable in achievement, accuracy, honesty, etc. (reliable information.)

14. Full Power? To engage in reliable communications, a signal report of 2 and 2 or better is all that’s needed. Rarely should anyone need full power when operating within 20 miles of the repeater, unless you are mobile and down in a deep valley with a poor antenna!

15. QRM and QRN QRM is the Q-Code for Interference. I am Being interfered with. / Are you being interfered with? (relating to another transmission) QRN is I am troubled by static. / Are you troubled by static? (relating to atmospheric noise)

16. QRM When you hear someone using the repeater and you intentionally key over them or cause their transmissions to become unreliable, you are causing QRM. When you fire up that amp, point your beams and give your call over everyone else in an attempt to break a pile up, you are causing QRM.

17. QRN QRN is caused by lightning, Sporadic E, Meteors, the Aurora Borealis, one of Arnold’s GA Power facilities, cracked insulators on a power poles, etc. Natural and unintentional man made noise / radiation.

18. Dealing with local QRM Log it! Write it down, get as much info as you can. Date, Time, Operator(s), Location, any information you feel is pertinent. Get it to the trustee. Do Not discuss it over the air! Don’t hint about it, don’t try secret squirrel codes, ignore it, just go QSY or QRT! If someone knows it’s working, they will continue!

19. The Amateur’s Code Paul M. Segal, W9EEA (1928)  The Radio Amateur is: • CONSIDERATE..... never knowingly operating in such a way as to lessen the pleasure of others. • LOYAL..... offering loyalty, encouragement and support to other amateurs, local clubs and the American Radio Relay League, through which Amateur Radio in the United States is represented nationally and internationally. • PROGRESSIVE..... with knowledge abreast of science, a well built and efficient station, and operation beyond reproach. • FRIENDLY..... with slow and patient operation when requested, friendly advice and counsel to the beginner, kindly assistance, co-operation and consideration for the interests of others. These are the hallmarks of the amateur spirit. • BALANCED..... Radio is an avocation, never interfering with duties owed to family, job, school or community. • PATRIOTIC..... with station and skill always ready for service to country and community.

20. Good Amateur Practice • Giving a little ground--even if you have a right not to--in order to help preserve Amateur Radio and not cause it to get a bad name or hasten the day when it becomes obsolete. • Respecting band plans, because they make it possible for every mode to have a chance. • Not transmitting a 6-kHz bandwidth signal when there are lots of people on the band. • Not acting like an idiot just because you were interfered with. • Being aware that we all love Amateur Radio, and there's no need to damage or disgrace it just to save face. • Keeping personal conflicts off the air. Settle your arguments on the telephone, the Internet or in person. Just keep them off the air. • Cutting a net or a contester a break, even if you don't have to and even if you have no interest whatsoever in nets or contesting. • Operating so that if a neighbor, niece or nephew or news reporter hears you, that person will be impressed with Amateur Radio. • Realizing that every right carries responsibilities, and just because you may have a right to do certain things doesn't mean it's right to do them in every circumstance. • You don't "own" or get preference to use any frequency even though you've been on the same spot every morning for years shooting the breeze with Arnold. • Not operating so that whoever hears you becomes sorry they ever got into Amateur Radio in the first place.

21. WCERC Club Rules • NO Radio or Club POLITICS! • Don’t do or say anything to another member for which you should apologize. • Have Fun. • If you dish it out, you better be able to take it; sooner or later, somewhere, somehow, someone will get you back. • If in doubt, refer to rules 1, 2 and 3! • Enjoy the Hobby and Friendship.

22. Thank you for your time, patience, understanding and participation. We have one of the best and growing clubs in the entire area, and it’s because of our quality members! The End