Placing An External SWR/PWR Meter Inline Between The Antenna And Radio

Discussion in 'General Technical Questions and Answers' started by K4SRF, Mar 17, 2021.

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  1. K4SRF

    K4SRF XML Subscriber QRZ Page

    Any real reason, benefits, problems, thoughts or comments about running an external SWR/PWR meter inline between the antenna feedline and the radio(s), especially if the radio(s) have their own built in SWR/PWR meter? (This conversation came up at a recent meeting.) Some are all for it, others just the opposite, so I am gathering other's opinions. What are the pros and cons?

    I have noticed that if I do this, my readings are drastically different. For instance, it can be 1.0:1 on the analyzer, but show 1.4:1 on the external meter and worse, 1.9:1 on the radio's built-in SWR meter. (Putting the feedline directly to the radio will result in the same SWR reading on the radio as I got on the analyzer, 1.0:1.)

    The jumper coax I am using is a Times Microwave LMR 400 and has tested perfect. I know that there will be some loss, but this seems extreme to me. It could be that I have a "less-than-perfect" external meter. It is a MFJ-849, but it shouldn't be causing this much of an issue I would think.
     
  2. K3LI

    K3LI Ham Member QRZ Page

    Assuming you have a quality swr/pwr meter, believe it, not the radio. You could also borrow one and see if it matches yours. The meters in the radio I never look at.
     
    PU2OZT likes this.
  3. W9WQA

    W9WQA Ham Member QRZ Page

    your question...

    Any real reason, benefits, problems, thoughts or comments about running an external SWR/PWR meter inline between the antenna feedline and the radio(s), especially if the radio(s) have their own built in SWR/PWR meter?

    its fine, i do it, no reason not to...
     
  4. KN4XJ

    KN4XJ Premium Subscriber QRZ Page

    I believe that what you describe is the typical way of doing it. rig <-> cable <-> meter/switch/whatever <-> cable <-> antenna. what analyzer are you using? you are correct to question the mfj.
     
  5. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    I like having an external meter inline. It provides a ready reference that a signal is being transmitted, Pwr, SWR and fast recognition of feedline/antenna problems.

    Regarding SWR meters, the old axiom applies: "The man with two watches never knows the time".

    -Pick one and use it for your adjustments.
     
    KP4SX and WG7X like this.
  6. KM3F

    KM3F Ham Member QRZ Page

    The answer can get quite complex as you will see during the replies that will be forth coming.
    Simply said, the SWR at the end of the coax will be the same when connected directly to the radio without the exernal meter.
    Once you add a coax jumper to length to get the external meter inline, the meter sees a differnt forward and reflected value.
    Why is because the impedance at the coax is not a pure 50 ohm resistence but has frequency senitive 'reactance compoants' that change value with the position the meter is placed to include the radio.
    For example, a 50 ohm flat non reactive dummy load should not act the same way as an antenna connected to a coax feed line but read the same no matter the extra jumper length because the reactive compoants are not present. In this case, only the coax 'losses' would be inplay.
    Coax losses are also in play in the antenna system and combine with the reactive parts
    The coax losses make the antenna SWR look better with extended length. Not something that is actually better, as losses are lost signal to heat.
    This is especially sensitive at vhf and uhf frequencies.
    This is about the easiest I can make it for you to begin understanding.
    A look into the coax looking at the antenna load with VNA will show the reactive values all along the frequency span and change as you sweep the band. If the antenna has matching coils, there will be reactive values present. This is what is called the +/- j part of the load.
    I't can't be avoided in antennas 99% of the time.

    Good luck.
     
    KP4SX and K4SRF like this.
  7. AI3V

    AI3V Ham Member QRZ Page

    My .02$

    My radio has a built in power/vswr meter.

    It, the vswr meter, is only "accurate" at the full power of the transmitter.

    I use a open wire fed doublet, and a manual "antenna tuner". It, the "tuner" has a vswr meter that I can adjust the sensitivity such that I can adjust the "tuner" with a watt or 3 from the transmitter,

    This is very handy to prevent transmitter damage due to high vswr while tuning.

    The 2 meters agree to a few "points", say 1:1 vs 1:3

    Once upon a time I had a dual slug Bird coupler, with a panel meter in a nice cabinet, and about a dozen "slugs"

    Probably be close to $2,000 to replace.

    One day I realized they did Jack diddly to make my signal stronger, or my receiver more sensitive.

    So I sold them and bought gear that did.;)

    Rege
     
    KI5WW likes this.
  8. K1XH

    K1XH Ham Member QRZ Page

    My question is why WOULDN'T you use an external meter?
    I am more likely to believe a calibrated and tested quality meter such as a Bird 43 Thruline meter than I am to believe the one in the rig...
     
  9. K4SRF

    K4SRF XML Subscriber QRZ Page

    An MFJ 269-D
     
  10. AI3V

    AI3V Ham Member QRZ Page

    Ok, answer this:

    What would the maximum reduction in signal strength due to vswr meter errors be, between the meter built into your new hf rig, and the expected improvement in said signal by buying a "Bird"


    And how much louder would you be if instead you bought a amplifier or a antenna instead of the "Bird"?

    It is my opinion that a "external meter" is generally a poor place to $pend money.

    Rege
     

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