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SWR question

Discussion in 'Communication (CB | GPS | HAM)' started by 88sub4x4, Dec 18, 2006.

  1. 88sub4x4

    88sub4x4 1/2 ton status

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    I just installed a wilson 2ft fiberglass antenna in the middle of my roof on my Jeep. Hooked up to a cheapo radio shack unit. only about 2 ft. from antenna. I tuned the SWR's and it is only 1.25 on the meter. the needle barely moves. Is there such a thing as too low? The old antron I had mounted on the rear bumper would only get down to 3, and a wilson 2000 on the bumper wouldn't get below 7, thats why i went roof mounted. though a better ground plane ould be better, but this seems low. Somebody school me on this please....
     
  2. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    SWR can be as low as you can get it .

    The other antennas , if they are over 3.0 swr all the time , something is wrong , usually center touching ground somewhere .
     
  3. 88Silverado

    88Silverado 1/2 ton status

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    A 50 Ohm resistor will have a 1:1 match but wont radiate too good.

    Considering the 1/4 wavelength is around 9 feet and your antenna is around 2 feet there a substantial amount of loading in the short antenna. The SWR is probably right, but its radiation effectiveness is pretty poor.

    Basic rule of thumb is if its below 2:1, dont worry about it. The difference in performance by getting the SWR lower is not noticeable.
     
  4. readymix

    readymix 3/4 ton status

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    The SWR on mine is 1.2:1 without the amp and around 1.5:1 with the X-200.
     
  5. 88sub4x4

    88sub4x4 1/2 ton status

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    OK, you guys are confusing me...what is this 50 ohm resistor and 1/4 wavelength? Should I have gone with a taller antenna? They are only $1 per foot more.
    Should I put the 6ft antron up there, or maybe try the wilson 2000 up there. I went short cause i use this wheeling and didn't want to hit every freeking tree
     
  6. readymix

    readymix 3/4 ton status

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    No No.....A 50 Ohm resistor will not have a 1:1 match.

    Impedance matching is far more complicated than resistance measurements. Resistance is a DC measurement and RF is not DC at all.
    Capicatance, inductance, MHO's, Reactive Capicatance.....blah, blah, blah.

    Now if you were to put a 50 Ohm termination/load built for that purpose it would work, and be pretty close to 1:1.

    Just get a good antenna, ground it well, and adjust lendth for SWR readings and you will be fine.
     
  7. 88sub4x4

    88sub4x4 1/2 ton status

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    Umm, so am I good with my current setup at 1.25 on swr meter? I grounded the **** out of it, so i know thats good.
     
    Last edited: Dec 19, 2006
  8. 88Silverado

    88Silverado 1/2 ton status

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    Yes...a 50 Ohm resistor will provide a 1:1 SWR on the output. Look inside a dummy load, theres nothing special in there except a pure resistor. No tuning networks etc, just a resistor. Thats why they are so broadband is because there is no tuning networks in them. In fact the tuned outputs of the transmitter using RLC networks etc use plain resistors and measured in DC Ohms. I have prescision 50 Ohm loads here at work for our network analyzers that go to 40 GHz and they will read 50 Ohms with a DC Ohm meter.

    Yes for impedance matching theres many more complex factors involved. We can go into a long discussion on why you should check the VSWR directly at the antenna connector and Not at the end of a coax and why you should never trim you coax to obtain a 1:1 match but thats for later :wink1:

    The shorter the antenna from natural resonant lengths the greater the loss due to tuning networks, lower radiation resistance and reactance and the lower the efficiency. Therefore a 2 foot antenna for CB with a 1:1 match will not perform as well as a 9 foot antenna with a 1:1 match. The efficiency and gain will be lower

    Agree :waytogo:
     
  9. 88sub4x4

    88sub4x4 1/2 ton status

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    :confused::confused::confused::eek::eek::eek::what::what::what:

    I guess I'll just keep the setup I have for now, cause you guys have totally confused the sh*t out of me!!! I have been trying to learn this stuff little by little, but it seems I get more lost with every project I do......
     
  10. readymix

    readymix 3/4 ton status

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    OOPS. I was off in right field when I posted that. A BNC type dummy load will equal out like you said.

    I work with RF everyday so I do in fact know what I am talking about. The problem is that I deal with waveguide and not Coax so when I am talking tec. I thinking always refers back to what I deal with regularly.
    COAX and waveguide function alike but the termination loads are designed differently. Hopefully I did not add too much confusion to the mix here.
     
  11. 88Silverado

    88Silverado 1/2 ton status

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    88SUB...Dont worry. Sounds like your installation will be OK ;)

    Readymix, what do you do? Im an EMC engineer for the U2 program at Lockheed. Spent many years as a tech and engineer in our antenna group.
    Use to do alot of material measurements in waveguides measuring their electrical properties. Yes waveguide loads, terms, shorts, opens, etc are a bit different than coax.
    We use to say anything below 500MHz was DC :haha:
     
  12. readymix

    readymix 3/4 ton status

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    U2.....Way cool.

    I am a CIWS Phalanx 1B and MK 86 GFCS Mod 8, 9, 10 & 12 tech in the navy. Klystrons and TWT's in those systems.

    I have experience with MK23 TAS, SPS-48E, SPS-40, NSSMS, Tomahawk, Harpoon, RAM, and all other kinds of shipboard perif. weapons systems.
     
  13. 88Silverado

    88Silverado 1/2 ton status

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    Way Cool...:waytogo:

     
  14. W7NB

    W7NB 1/2 ton status

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    With antennas longer is allways better when you are below 1/2 wavelength. The "loading coil" at the base of the antenna is used to tune the shorter stub. It is resonant - IE the low SWR, but it is not very efficient. Think about how poorly a cb walkie talkie works with the rubber ducky antenna. They still put out 4 watts, but only have a range of a mile or two due to the crummy antrnna. The same radio hooked up to a good mag mount antenna will perform virtually identically as a typical mobile radio.

    Now to answer your question - witha 1.25 to 1 SWR you are just fine. You won't hurt your radio, and for a trail rig the short antenna is fine to talk with others in you group. It simply wont "get out" as well as a 5 footer will due to the difference in efficency. I carry a short natenna with me and put it on if we are bashing around in the brush so I don't tear up the big one - works good for the trail. If we have an emergency I have three other radios in the rig so I don't use the CB for that anyway.
     

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