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Checking SWR long post

Discussion in 'Ham Radio - CB - Trail Communications' started by explr1, April 16, 2003.

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  1. explr1

    explr1 Active Member

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    SWR (standing wave ratio), is a measurement of how efficiently your antenna system will radiate the power available from your radio. In simple terms, your radio would like to radiate all of its power, but can only do so if the other components cooperate. Bad coax and mounts, or inefficient antennas and ground plane can cause system bottlenecks. The easiest way to understand the concept is to think of it in terms of water flow. That is, if you put a one inch faucet on a two inch pipe, your potential output will be restricted by the one inch outlet. So goes antenna systems. Setting your antennas SWR will reduce the restriction of radiated power.

    If all radios only transmitted on one channel, it would be a much easier task to design antennas. As it is, on CB alone, there are 40 channels to contend with. Mobile antennas can only be made to resonate at one specific frequency (channel). The goal of the antenna manufacturers is to build the antenna to resonate at a frequency in the middle of the available band (channel 19 on CB) and make it broad- banded enough to keep the off-frequency related SWR at the two extreme ends of the band below 2.0:1. It should be noted that if you communicate on one or two adjacent channels anywhere within the band, you can tune your antenna to achieve optimum performance on those channels. Most people, however, prefer to use the entire bandwidth when tuning.

    THINGS YOU WILL NEED
    Knowledge of what not to do .... read previous sections.
    Properly installed antenna system (mount, coax and antenna) that was made for the type of radio you will be using and has been tested for shorts and opens in continuity. (See "Testing Continuity")
    Functional radio.
    SWR meter. (See "SWR Meter Hook-Up")
    Short piece of coaxial cable (jumper) with PL-259 connectors on both end.
    SWR METER HOOK-UP
    The SWR of the antenna, without feedline, can be measured by placing the SWR meter in-line at the antenna instead of at the radio. However, the coax can help or hinder performance. In the end, your SWR should be checked at the radio end because all components will be a part of the final operational system being used.

    SWR TESTING REMINDERS:
    Remember to check for continuity, shorts and opens in your coax and mount installation first.
    Take measurements in an open area with the vehicle's doors and hatches closed.
    All measurements should be taken with antenna tip on, unless you do not plan to use the tip in normal use.


    THE SET UP
    If already connected, disconnect the coaxial cable from the radio. Connect the coax cable that normally connects to the back of the radio to the SWR meter connector marked "Antenna" or "Ant". Now, connect one end of the jumper cable to the back of the radio and the other end to the SWR meter connection marked "Transmitter" or "Xmit". Your SWR meter is now in series (in-line) with your radio and antenna.

    You should now have your vehicle in an open area, with all doors closed. Turn your radio on and tune to channel one or the lowest channel on your radio. If your radio has side band operation, make sure you are in AM mode before doing SWR tests.

    The following assumes that your SWR meter has a standard set of switches, knobs and meters. That is, there will be at least one switch with the marking Forward (FWD) in one position and Reference (REF or SWR) in the other. There will also be a knob or sliding controller marked "Set" or "Adjust". Most meters come with full instructions. If the common configuration does not match your meter you will need to rely on the meters manual for assistance.

    With the radio on the lowest channel (1 on CB) and the SWR meters switch in the Forward (FWD) position, depress the transmit switch (key up) located on the microphone. While holding the unit in this transmit mode, adjust the meter needle to the set position using the Set or Adjust knob on the meter. As soon as the needle is in alignment with the corresponding mark on the meter face, flip the switch to the Reference (REF) position. The meter is now showing your SWR on channel one. Note the value and quickly release the microphone switch. Record this reading on your paper to the nearest 1/10th. i.e. 1.8, 2.3, 2.7, 1.4, etc.

    Now, switch your radio to the middle channel (19 on CB). Place the meter switch in the Forward (FWD) position, depress the microphone switch and adjust the meter to place the needle on the Set position of the meter face. Once in the set position, place the meter switch in the Reference (REF) position and note the reading. Release the microphone switch and write this value down to the nearest tenth of a point. Note: If your antenna system is closely matched to the radio you may get little or no movement from the meter needle on this channel. This is normal.

    Finally, place your radio on the highest number channel (40 on CB). Place the meter switch in the Forward (FWD) position, depress the microphone switch and adjust the meter to place the needle on the Set position of the meter face. Once in the set position, place the meter switch in the Reference (REF) position and note the reading. Release the microphone switch and write this value down to the nearest tenth of a point.

    With these three readings, you can determine many things about your system. For instance ...

    If SWR on channels 1, 19 & 40 is below 2.0, your radio can be safely operated on any channel without causing damage to the radio's circuitry.
    If SWR on all channels is above 2.0 but not in the "red zone" (normally over 3.0), you may be experiencing coaxial cable reaction (bad quality, wrong length, etc.), insufficient ground plane, or have an ungrounded antenna mount.
    If SWR is in the "red zone" on all channels, you probably have an electrical short in your coax connectors, or your mounting stud was installed incorrectly and is shorted. DO NOT USE YOUR RADIO UNTIL YOU HAVE FOUND THE PROBLEM.

    If SWR on the lowest channel is higher than it is on the highest channel, your antenna system appears to be electrically short. See the following section title "Adjusting Short Antennas".
    ADJUSTING LONG ANTENNAS
    If the SWR on channel 40 is greater than that on channel 1, your antenna is considered to be "LONG" and reduction of physical height and/or conductor length will correct this situation. Depending upon antenna model, this entails screwing down the tunable tip (Illustration #1: Firestik II, Firefly), or, removing the tip, making short slits in the plastic covering and unwinding and clipping off wire (Illustration #2: Firestik, Road Pal). Firestik Designer Series antennas require loosening the allen screws and lowering the metal whip

    ADJUSTING SHORT ANTENNAS
    If SWR on channel 1 is greater than that on channel 40, your antenna is considered to be "SHORT" and increasing the physical and/or electrical length of the antenna is required to correct this situation. Because we make our antennas extra long, readings which indicate "Short" normally stem from ground plane deficiency (lack of vehicle metal surface for the antenna to reflect its signal rom). This condition is often corrected by adding a spring and/or quick disconnect to increase the physical height. Ground plane deficiencies can also be compensated for by using dual (co-phased) antennas or special no-ground-plane antenna kits.

    NOTE: The shorter the antenna, the more sensitive it is to adjustments. For example, removing two wire turns on a 4 foot antenna might move the SWR by 0.3; the same amount removed from a 2 foot antenna may move the SWR by 1.0. Make smaller adjustments on shorter antennas.

    DUAL ANTENNAS
    Measurements and determination of short or long conditions are the same as the single antenna procedure. However, when tuning co-phased antennas, if you adjust one antenna, it is advisable to adjust the other in equal amounts to keep them in perfect balance.

    For more great info check www.firestik.com FAQ and tech sections.
     
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  3. Tbars4

    Tbars4 Moderator Emeritus Elite In Memoriam

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    ...just curious and wishing to find a good answer...on a swr meter would you say a reading of 1.5 would be optimal with a single antenna???...excuse me if i missed it but what would be the optimal reading for single or dual antenna's??? :scratch:
     
  4. High_Order1

    High_Order1 Active Member

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    what that guy copied from the website is saying to you that whatever is jacked into your radio needs to be less than 1:3:1. So, 1.5 would be unadvisable to operate.

    The reason you use two antennas is to clear up a propagation problem caused by not dead centering a single aerial. Like twin carbs on a bike, though, they have to stay balanced or the radios' performance will suffer. Its' not a case of 'one is good so two must be better'. The best CB antenna for a vehicle is a 102" whip, but unless you're a Trooper, that giantass antenna can be more trouble than its' worth. And that's where all those other antennas come from. They compromise formfactor with sensitivity and radiation pattern.

    -Shawn
     
  5. Tbars4

    Tbars4 Moderator Emeritus Elite In Memoriam

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    ...this is what i was refering to with a single antenna, being under 2...copied from above...i was looking for the optimal range with an swr as i believe it to be 1.5...:D

    If SWR on channels 1, 19 & 40 is below 2.0, your radio can be safely operated on any channel without causing damage to the radio's circuitry
     
  6. High_Order1

    High_Order1 Active Member

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    what those numbers mean is this: 1:1 means that every milliwatt of radio output is getting out of the antenna. The antenna, mount, feedline, and radio are all matched and happy.

    You'll never see it. There are longwinded explanations, but basically some compromise has to be involved when you are dealing with a radio that can transmit on multiple frequencies, feedline and antenna issues, etc. etc. You can get a degree in that.

    Any number above 1:1 means something is out of whack, and for all of the power leaving the radio, some of it is being reflected back. This is a bad thing. Not because like some people think, that the returning energy damages the final stage transistors, but because all of the reflected energy is a symptom of an untuned antenna. An untuned antenna detunes the transmitter, causing it to sink a lot more current than it was designed to handle, and that poofs it.

    So I say all that to say that 1:1 is optimal, but unworkable. Personally, 2:1 means I go back and keep working. I regularly see 1.2 and 1.3. If the best you can do is 1.5, that should be fine. For some reason, I thought I saw that you wrote 1:5. Solly:confused:

    -Shawn
     
  7. Tbars4

    Tbars4 Moderator Emeritus Elite In Memoriam

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    ...as to this post and my swr meter, they do not read 1:1 or 1:3:1...so is the lowest reading you can get on an swr meter the best, trying to match high and low channel , to the lowest reading???
     
  8. High_Order1

    High_Order1 Active Member

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    I am guessing your meter will look sorta like this:

    [​IMG]
    You want it below that number 1.5 if at all possible.


    -Shawn


    Dawns on me we prolly shouldn't be cluttering this sticky. Think you should start a new thread? (apologize to the moderators in advance)
     
  9. Tbars4

    Tbars4 Moderator Emeritus Elite In Memoriam

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    ...simple question with a simple answer..:D yes, that is the same meter..the reason i asked the question here is so if anyone else came across this thread, they would get better use if they new what the optimal number was instead of just below 2 on the swr meter...i too wanted the info after thoroughly reading this lengthy thread..thank you for the answer and posting the standard swr pic...:salute:
     
  10. rizzjc

    rizzjc Scubajoe - Radio Mod Staff Member Moderator Elite Explorer

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    Optimal is 1:1, as stated, that means 100% of your power is going out the antenna, and not back to the radio. 1:1 is not practical, but anything under 1.5:1 is considered pretty good.
     
  11. CobraMountaineer

    CobraMountaineer Active Member

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    My digital tells me 1.08:1 on my dual band... I guess that is good??? LOL

    N3BXH
     
  12. The DB

    The DB Active Member

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  13. rizzjc

    rizzjc Scubajoe - Radio Mod Staff Member Moderator Elite Explorer

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    Thanks for posting!
     

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