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Thread: What is acceptable SWR?

  1. #1

    Default What is acceptable SWR?

    I have a comet 790 dual band antenna and the swr is perfect on 2m but 2:1 on 440. Will this damage the radio or is it acceptable?

    Thanks for your help.

    Pierre
    KD2EPQ


    Sent from my iPad using Tapatalk

  2. #2

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    1:1 is perfect impedance match between antenna and feedline, 2:1 means circuits begin to reduce trasmitted power and 4:1 is impedance mismatch.

  3. #3

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    Quote Originally Posted by Jeanguypataterubberboot View Post
    I have a comet 790 dual band antenna and the swr is perfect on 2m but 2:1 on 440. Will this damage the radio or is it acceptable?

    Thanks for your help.

    Pierre
    KD2EPQ


    Sent from my iPad using Tapatalk
    An acceptable VSWR is 1.5:1 for any frequency band no matter if it's in a mobile or a base application. I'm wondering what you're using at 440 MHz / 70 cm for determining the VSWR. Many VSWR meters are not accurate at 440 MHz normally only good to 150 MHz and depending on where you're inserting the VSWR bridge into the coaxial line will produce different readings. The ultimate piece of test equipment would be a Bird Wattmeter with a 50 or 100 W UHF slug which would provide the true RF Forward & Reflected power in watts and you would want to see only 5% Reflected RF power.

    Depending on the transceiver's manufacture of that radio it may have a circuit to cut back the RF Output to prevent damage to the transistor finals. Sometimes if you have trimmed the coaxial cable to shorten the length, it could have been cut at an odd multiple (electrical) wavelength which would cause a similar high VSWR problem but I don't know how long your coaxial cable is. You will need to know what type of coaxial cable you are using i.e. RG-58 for the Velocity Factor to do this formula.

    Formula:

    http://www.qsl.net/w4sat/velfact.htm

    RG-58 Coaxial Cable 66% Velocity of Propagation

    Once you have measured the physical length and calculated the electrical length, you can determine how much more coaxial cable that might be required for the 440 MHz operation. You're essentially making the coax to match the antenna. I wouldn't consider transmitting on the 440 MHz band till you know that you have a low VSWR.

    73,

    Dan

  4. #4
    Super Moderator 5B4AJB's Avatar
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    2:1 is OK for anything under 10W, higher than that, things start getting hot and smoking...

  5. #5

    Default What is acceptable SWR?

    Well somebody correct me if I'm wrong but I think 2:1 is acceptable. And like someone else stated hear it is hard to get a accurate UHF reading with the average VHF UHF SWR meter. It's a decent chance that you actually have a better SWR them 2:1.
    Last edited by K5MOW; Thu 24th Jul 2014 at 12:10. Reason: spelling
    Have a great day.
    73
    K5MOW

  6. #6

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    The acceptable mismatch depends entirely on the transmitter in use. Generally, the design of the PA dictates how much excess, reverse power it can dissipate so you'll need to look at the specs for the kit you're using to be sure.

  7. #7

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    Thanks guys. I will be keeping it at 5 watts until I am sure the set is low enough. The radio is an Icom ID-5100.
    Wa9wvx thanks for the technical info I'll be checking the coax length. I don't have a bird meter. All I have is an mfj analyst but it doesn't cover 440

    Thanks again.

    Pierre
    KD2EPQ


    Sent from my iPhone using Tapatalk

  8. #8
    Super Moderator 5B4AJB's Avatar
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    Couldn't find a picture on Google of my one - I guess it's too old
    Can't remember where or how much I paid for it either, 20 at a rally is my guess.


    [edit] this was the table I was thinking of, seems you'll have about 10% reflected power...
    Last edited by 5B4AJB; Thu 24th Jul 2014 at 22:44.

  9. #9
    K7KBN's Avatar
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    "Powre"?
    "Impedanec"?

    ...and what's "500 hm"? Oh, wait...never mind
    73
    Pat K7KBN
    Semper ubi sub ubi.

  10. #10
    WB9UDA's Avatar
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    Quote Originally Posted by K7KBN View Post
    "Powre"?
    "Impedanec"?

    ...and what's "500 hm"? Oh, wait...never mind
    funny...... but I read it just like you did!

    That looks like a really nice meter. I wish I had one.

    The Bird meter is the best, but the price would make a grown man cry!
    I did have one, once, years ago.


  11. #11
    K7KBN's Avatar
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    Whether the Bird is "best" depends on what and how you're using it. It's not a lab standard for accuracy (only 5%, which drops to 8% if you have the "43P" version and you're reading PEP). Quite a few ham-grade SWR meters will meet or beat the Bird 43/43P for accuracy when used as the manufacturer specifies (and the manufacturer specifies which bands/frequencies his product is to be used for).

    And all measuring devices and instruments require periodic calibration and adjustment. Even the Bird AND its slugs.

    Where the Bird 43 shines brightest is in its ruggedness. It does stand up to daily use and abuse without many complaints.
    73
    Pat K7KBN
    Semper ubi sub ubi.

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