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what is your vswr at the antenna?

Discussion in 'CB Antennas' started by bob85, Jul 10, 2013.

  1. bob85

    bob85 Supporting Member

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    imagine you and a buddy just erected your 1-10k antenna up the hill behind the house,
    you have 500ft of rg213 running from the shack to your antenna,



    you have no or insignificant common mode current on the braid and the plugs are fitted correctly,

    your buddy sits in the tree & hooks his vswr meter right at the antenna feed-point,
    he always measures at the feed-point, he won't have none of that coax stuff messing with his readings,

    you run back to the shack & hook up your identical vswr meter to the rig with a double male connector, tune to 27.205mhz and key the mic,

    1.2:1 says your meter in the shack,

    what vswr does your buddy's meter read ?
     
    #1 bob85, Jul 10, 2013
    Last edited by a moderator: Jul 14, 2013

  2. Robb

    Robb Yup

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    Did the buddy test the SWR with a 1/2w length of coax at the antenna?
     
  3. bob85

    bob85 Supporting Member

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    no he hooked the meter to the antenna with a double male.
     
  4. W5LZ

    W5LZ Crotchety Old Bastard

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    It's impossible to answer that question. The SWR at the antenna's feed point could be almost anything. And with that length of a feed line the SWR seen at the transmitter -could- result from no antenna connected at the other end of that feed line.
    - 'Doc
     
  5. The DB

    The DB Sr. Member

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    A little under 2:1?

    Sorry doc, wrong answer. If you know the attenuation rate of the feedline it is a simple matter of a calculation, if you know the equations.


    The DB
     
    1 person likes this.
  6. Robb

    Robb Yup

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    Well; I know that a piece of coax that is 500 ft long is going to absorb most/if not all of the return current.
    So measuring it at the shack would be a useless gesture.

    The proper SWR for a resonant 1/4w vertical antenna should be ~1.4:1 which is ~36 ohms at X freq. An Interceptor 10k is a 5/8w with a beta match which can change the capacitive reactance to get a lower SWR at the antenna. Since you pointed out that there was little CMC on the coax; then it should be close to resonance.
    That isn't my answer; that's just me writing/thinking out loud - lol.

    Thought that if a 1/2w length of coax was used - with velocity factor correction for length - will give an accurate reading since it will be exactly 180 degrees out of phase with whatever the antenna SWR is - which should be identical to the antenna SWR. Isn't that the same as hooking the SWR meter to the base of the antenna?

    Dunno what the answer is; but I would use either the 1/2wl coax or the SWR meter at the base of the antenna to measure/find out. Like Doc said, an open-ended piece of coax @ 500 ft long can measure a SWR of ~2.4:1. Maybe lower. I dunno. Will have to agree with Doc; there is no real way to tell . . .
     
    #6 Robb, Jul 10, 2013
    Last edited: Jul 10, 2013
  7. The DB

    The DB Sr. Member

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    I'm changing my answer. I was converting SWR to a percentage of reflected power using a chart that heavily rounded in my initial answer, below I actually calculated it out.

    I came across two different loss numbers for RG-213, one is 1.1 per 100' and the other is 1.14 per 100'. As I'm not sure which is closer to the one in use in the scenario I ran the calculations for both.

    1) Assuming 1.1 dB loss per 100' I calculate 1.47:1 SWR.
    2) Assuming 1.14 dB loss perr 100' I calculate 2.02:1 SWR.


    The DB
     
  8. The DB

    The DB Sr. Member

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    That 36 ohm figure is for a quarter wavelength vertical over a "perfect" infinite horizontal groundplane. A 5/8 wavelength base antenna such as the I-10k is very different. For one, most 5/8 wavelength base antennas allow you to tune R and X individually, theoretically giving you the ability to achieve a perfect match at resonance while still having minimal losses.

    With an infinite impedance on the far end, bob85's SWR meter (the one on the radio side) would show 1.73 or 1.78 depending on the feedline's loss characteristics instead of the 1.2:1 that he measured...


    The DB
     
  9. bob85

    bob85 Supporting Member

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    db
    thanks for having a go,

    i used this one for coax loss

    Welcome to Times Microwave | Coaxial Cable - Attenuation & Power Handling Calculator

    and this one for return loss vs swr

    Return Loss & VSWR Conversion

    and the method from the bottom of this white paper

    http://bird-electronic.com/en/Resou...-Measurements-Using-In-Line-Power-Meters.ashx ,

    rob,
    forget 1/2wave lumps of coax mythology,
    it is perfectly acceptable to measure vswr at the rig end so long as you know the loss in the coax and have insignificant common mode current on the coax braid,
    you can measure the coax loss with your watt meter before you run the coax up the hill,


    doc is been his usual silly self.
     
  10. W5LZ

    W5LZ Crotchety Old Bastard

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    As it's originally presented, there are too many 'unknowns' and assumptions for a definite answer. In a 'best case' situation, the SWR seen at the transmitter end would be dealing primarily with the losses in the feed line. In a 'worst case' situation, the resulting SWR measured in the same place is the result of both feed line losses and impedance mismatches. That opens the door to an almost infinite range of possibilities. (No, it isn't really an 'infinite range', but it is quite 'broad'.)
    - 'Doc
     
  11. The DB

    The DB Sr. Member

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    No argument with some of this, we are talking about a lot of loss, weather it is bob85's calculation of 5 dB or either of the data sheets I looked up, which are even higher, we are talking about a 68% to 73% signal loss even before the signal gets to the SWR meter at the antenna. However, just because there are high losses does not mean you cannot calculate the SWR at the antenna itself from a reading at the radio end.

    The hardest part to achieve this has been getting the loss characteristics of the feedline. Some of us have equipment that can measure this, unfortunately that equipment doesn't do me any good in a hypothetical situation as was presented so I have to resort to what information I can find, I used data sheets for 30 MHz, which in and of them selves will have a margin for error when dealing with 27 MHz.

    However, once you have this information it is a matter of picking an arbitrary number to use for the forward power (it doesn't matter what it is as SWR does not change with power, I like to use 100), then calculate the reflected power based on that and the SWR reading you have.

    Once you have the effective forward and reflected powers in watts for one side of the feedline it is simply a matter of factoring in loss from the forward signal and gain for the reflected signal to get the forward and reflected power figures at the antenna end, then convert these back into an SWR.

    Unless you have so much feed line loss that your SWR meter shows 1:1 at your end you can figure out the readings on the far end.

    Now, if the nearby reading was 1:1 I would have agreed with you, not enough information.

    The biggest potential problem I see here is most SWR meters aren't exactly the most accurate devices in the world as most read lower than the actual SWR. When dealing with SWR in these conditions the difference from a bad reading is greater on the far end.

    The next biggest issue at hand, assuming you don't have fancy equipment that can measure feedline loss directly, is getting accurate loss measurements for a given feedline as sources such as data sheets don't always agree, or provide information at the specific frequency of operation, and even if they do there is always a margin for error in the real thing. These factors also skews the results in the real world.

    Even accounting for both of these potential problems, you can still calculate the antennas's SWR to within a reasonable margin of error.

    I guess in the end it all has to do with a proper understanding of the language of transmission lines. :whistle:


    The DB
     
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  12. bob85

    bob85 Supporting Member

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    i find the hardest part of understanding transmission-lines is filtering out the technobull, im still on a learning curve,
    looking at what respected sources tell us is a good start imho.
     
  13. Robb

    Robb Yup

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    The source I use is here:

    The Real SWR Page - Explained by WC7I - Will high SWR burn up my radio?
     
  14. The DB

    The DB Sr. Member

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    There is one detail I have an issue with on that page, however, the part of it that I have an issue with has nothing to do with reflections on the transmission line, and is in relation to the antenna itself, namely, just because the antenna absorbs power from the transmission line does not mean all of the absorbed power is radiated.

    He cites many of the sources that I learned from myself.


    The DB
     
  15. bob85

    bob85 Supporting Member

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    you won't find any of that 1/2wave coax voodoo on there rob, not if maxwell & cebik have looked it over and given it the thumbs up.
     

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