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Mobile SWR Meter Placement. Which one?

Discussion in 'CB Antennas' started by 530, Jun 1, 2019.

  1. 530

    530 Well-Known Member

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    From what I have read over the years the swr meter should be at 3 different places, lol. I have read at the radio to read whats happening back to it, after an amplfier, and the last at the end of the coax by the antenna itself. I have the radio and 15.2ft of LMR400UF coax going to an amp and then another 15.2ft coax going to the antenna. It is direct wired at the antenna which is a puck mount. I do not have a good meter like an mfj and have been tuning the antenna with the amp inline and off, the channel 1 and 40 match way with the meter close to the radio(4in jumper). This is the way I have seen all over and the most of, but also have seen the other two ways mentioned.
    Where would the best place be for the swr meter to tune the antenna with an amp inline?


     
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  2. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Ideally I would put one between the transmitter and amp and then another right after the amp. If I could only choose one then I would put it right after the amp. You never know what might happen to the antenna or coax and this would be cheap insurance against damage to the amplifier.
     
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  3. 530

    530 Well-Known Member

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    I did do it before after the amp and there wasn't much difference at all from the reading at the radio. I'd ike to know which is best or second best as I can't do it at the antenna, which I heard is the best? lol. Ill try it again though. Still would like to know which is correct. I have seen many and from techs and ham people and have literally seen these 3, and all saying its the right way lol.
     
    #3 530, Jun 1, 2019
    Last edited: Jun 1, 2019
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  4. M0GVZ

    M0GVZ Sr. Member

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    One between the transmitter and the amp just for testing the amp is presenting a 50 ohm impedance on its input and then remove it. After that then between amp output and antenna and leave that in place.

    You would use an ANTENNA ANALYSER at the antenna feed point to check its tuning before disconnecting and connecting up the feed line if you have one but not a SWR meter because it doesn't really tell you much. You do not leave a SWR meter at the antenna end permanently. Makes absolutely no sense to do that. Any issues that happen will be seen at the other end of the coax.

    Basically you have a SWR meter to ensure that the SWR is low enough to prevent damage to the PA of whatever is transmitting.
     
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  5. CB590

    CB590 W9WDX Member

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    I did transmitter to amp. I run amp off first.. then kick it on after radio is checked for SWR.. doesn't help my meter tops at 300 and I'm constantly swinging up near that with the TS350..
     
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  6. Dr_DX

    Dr_DX Active Member

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    Looks like your coax is cut for 27.385 = 1/2 wave length @ 15.2". Which means you will get an accurate SWR reading at the input end. IMHO, You should always tune your antenna to the last thing in line - your amp - with it on. The output impedance of your amp will probably be different than the output impedance of your radio. The most accurate way to do that is to use a stripline SWR meter and a double male UHF so you can screw the meter directly into the back of the amp without using a jumper. My favorite meters for that are:

    GC Electronics 18-155
    Vanco SWR-1
    Royce 2-100
    Carter Craft 90-300
    Philmore FS-45 (late model)

    They were all made in the same factory in Japan and it is good quality meter - 2-30Mhz, 1kw power, easy to see minute movement below 1.5. You can find these on fleabay all day for <20$ shipped. 100's of thousands of them have been sold over the years - there is a good chance you have one laying around. Tune the antenna with the amp on. Once you have it as low as you can get it, remove it from the back of the amp and screw the antenna coax back in.

    Now screw the meter into the back of the radio, and with the amp on, check what your SWR is going into the amp (if you have a swr meter in the radio, use it). If you don't like the SWR, adjust the input match on the amp, NOT the antenna. If it is quite high and it won't adjust down, then you can disconnect the sniffer circuit in the amp and add a small relay in the radio and build a p.t.t. circuit and key the amp that way. This removes the small coupling cap out of the rf circuit and as a bonus you don't have to run SSB delay on the amp. The amp will key when you key, and unkey when you let go & no chattering.
     
  7. 530

    530 Well-Known Member

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    I did try the meter after the amp with a 4in jumper of the same size cable I had lmr-400 UF and that was awhile ago I don't remember the exact numbers but they were almost as low or at least the same as at the radio and I remember being completely satisfied cuz I never checked it again although I did do it with the amplifier off because that's everything I read at the time. Thank you for bringing up the idea of the male to male connector I really actually never thought of that. I haven't checked yet but I am guessing the meters you have posted have the side female connectors as I doubt I could do it with the rear mounted female connectors. Thanks again.
     
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  8. The DB

    The DB Sr. Member

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    You don't need an electrical half wavelength of coax to accurately measure SWR. While impedance changes with the length of coax (and is where you actually need a half wavelength of feedline), SWR does not, unless you have a problem with your antenna. The only exception to this is all coax has loss, which will, over the length of the coax, lower the SWR measured. As the length of coax in this case is pretty short, this shouldn't be an issue. A lot of people used to "tune" their antenna by cutting coax, i.e. changing the coax length. If changing the coax length appears to let you tune the antenna's SWR then you have a problem with your antenna system, generally a faulty ground connection.

    Also, the amount of power does not, in and of itself, affect SWR. If your SWR changes when you turn your amplifier on, then something else is happening. 95% of the time you are putting out harmonics and other spurious emissions that are outside the bandwidth of the antenna and thus will be reflected back as they will see a high SWR when they get to the antenna. This is a common problem with amplifiers made for the CB band.


    The DB
     
  9. 530

    530 Well-Known Member

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    Ok so for an update. Tested SWR at all points and all are at 1.0(excuse me if thats not the right way to type it) on 1 through 40 at radio and at a meter where the amp should be, except after amp, waiting for meter...
    Quick question adjusting the input match on the amplifier is that something I could do myself without any high-tech meters? What I have done lately is matched swr on 1 n 40 with the amp out of line. 1.0 on both at the radio. I then hooked an swr meter where the amp would be and its the same 1.0. When i put the amp inline at the radio the swr is 1.3 on 1 and 40 and drops a bit to 1.2 when i talk, on amps low power. Waiting for a new meter for after amp check, I have a side connecting meter but it only handles a hundred Watts.
     
    #9 530, Aug 24, 2019
    Last edited: Aug 24, 2019
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  10. CB590

    CB590 W9WDX Member

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    To be honest if your amp is on and your doing 1:2-1 and 1:3-1 your doing fine.

    Get a meter to handle the higher watts and run with it..

    On sideband you may see RF harmonics playing in at the higher wattage fluctuating.. don't stress too much unless you constantly banging 1.5 SWR or higher range which I don't think you'll see..

    At 140w from my RM203 (using a whistle) I am seeing a 1:2.5-1 SWR.. so I'm not stressing it..

    IMG_20190824_100724563.jpg
     
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  11. 530

    530 Well-Known Member

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    I run anytone 6666 or a 955 or a 979 with a Texas Star 500dx non-variable the last time I had it on and mfj meter at the end line with about 30 Watts pep I was doing if I remember correctly it was a while ago it was 275 RMS swinging to 575+ Peak. Here's a video of the amp I have, he used to be a friend of mine's before he passed away and I had a 667 V and he was getting the 500 worked on but he needed mine for his radio and I really needed his amp for my radio and we just traded...
     
    #11 530, Aug 24, 2019
    Last edited: Aug 24, 2019

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