I need a 33cm and 23cm band SWR meter recommendation

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KJ4RIV, Jan 18, 2011.

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  1. KJ4RIV

    KJ4RIV Ham Member QRZ Page

    I have come to the conclusion that if I am going to continue to develop and build my own antennas, at the very least I need an SWR meter (or a wattmeter would do fine). The problem is that I can find almost nothing that covers these bands.

    I transmit 910MHz and1296 MHz at powers between 500 and 1500mW so I don't need a high power meter or anything. All of my connectors are SMA, but I'm sure there are adapters for just about any type of UHF connector.

    Can anyone make a recommendation? Modifying a meter is certainly an option. I'm pretty handy with a soldering iron.

    If this is in the wrong forum feel free to move it (but let me know where).


  2. VK2TIL

    VK2TIL Ham Member QRZ Page

    At UHF and beyond it's difficult to make an SWR meter as we know it.

    Bridges are easy though, provided care is taken in design and construction.

    Google will get you lots of designs including;





    You can buy RLBs;


    but they are simple & cheap to build.

    Here are some of the bridges I've built over the years;


    Your transmitter will serve as a signal source and a simple diode detector (carefully constructed with a good "microwave" diode) can be used to find the "null".

    The little blue bridge in my photo (technically not an RLB but an RF bridge) has the diode built-in; I don't know its upper limit but it should be a few GHz.
  3. AC0FP

    AC0FP Ham Member QRZ Page

    Bird model 43 watt meters have slugs in the 400 - 1000 MHz range with full scale powers down to 1 watt.

    fp :)
  4. KJ4RIV

    KJ4RIV Ham Member QRZ Page

    So how do you read one of these bridges? Do you fabricate a leader for a voltmeter out of coaxial cable? I can see it is supposed to read the voltage imbalance between a known load and the one in test.

    I have seen a few wheatstone bridge circuits placed inside a waveguide for resonant control, but no explanation on how to read VSWR. If I knew how to read it (what meter would actually work?) I'd have no problem building a bridge.

    Can you show me how you read yours?


  5. VK2TIL

    VK2TIL Ham Member QRZ Page

    You will find a lot of information with a Google on "return loss bridge"; here is a start;


    Good stuff there, including (but not limited to) "Manual Return Loss Measurements".

    Manufacturers like Eagle, Anzac and Wiltron have good app notes on their websites as RLBs are commonly used by professionals in the comms & satellite field.

    There are a couple of ways to use a bridge.

    One way is to connect it up (as in the literature that you will find) with an open or a short on the DUT (antenna in your case) port.

    Apply RF and note the detector reading; the detector must be 50-ohms (diode with a load resistor) and the reading should be power (dBm). If the reading is in voltage, convert to dBm in 50 ohms.

    Now connect the antenna to the DUT port and apply RF; note the detector power in dBm as before.

    The difference is the return loss (RL).

    RL and SWR are mathematically related; you will find lots of tables on the 'net.

    After a while doing this you will think in terms of dB and RL, not SWR.

    Another way is to terminate the DUT port in 50 ohms and note the detector output; it should be zero. Remove the termination and attach your antenna.

    Now adjust the antenna until you get zero or very small detector output; you will have matched your antenna to 50 ohms.

    The process for either of these methods takes longer to describe than it does to perform, at least after the first few measurements.
  6. VK1OD

    VK1OD Ham Member QRZ Page

    In the case of the RLB that use a high impedance detector...

    That calculation depends on the equivalent source impedance being 0 ohms. Transmitters don't usually comply with that requirement, so expect some error in calculated RL if that simple calc is used.

    Conversion of detector DC voltage to dBm is another potential source of error.

    For example, if you used a 50 ohm source such as a SSG, then the method above would give a figures of 8.29dB and 6.85dB for 25 and 100 ohm loads respectively, both have actual RL=9.5dB.

    Last edited: Jan 19, 2011
  7. KJ4RIV

    KJ4RIV Ham Member QRZ Page

    Good point, Owen. I was just going to use two known loads and compare them for calibration. A little error is fine. I'm not after perfection.


  8. VK1OD

    VK1OD Ham Member QRZ Page

    Alex, the null occurs at the right load impedance, just that at least for the designs that use a high impedance detector, calculation of Return Loss is a little more complicated than stated. The error becomes vanishingly small as VSWR approaches 1.

    You will see lots of designs for these things around on the net, and most explanations ignore the effect of source impedance and detector impedance, but they are relevant to measuring absolute RL.

  9. VK2TIL

    VK2TIL Ham Member QRZ Page

    W7ZOI thinks that both a 50-ohm source and a 50-ohm detector is required for an RLB; see Introduction To RF Design p.151 et seq.

    I will not volunteer to tell him that he's wrong!
  10. VK1OD

    VK1OD Ham Member QRZ Page

    He is quite correct.

    There are designs for an 'RLB' that use a high impedance detector, and the calculation of RL is not as simple. (Example: http://www.vk2zay.net/article/179)

    There are 'RLB' applications that use a transmitter for a source and implicitly assume that Zs of a transmitter is 50+j0, and that is not necessarily the case, so another potential source of error.

    Some designs have an inadequate common mode choke on the detector port, again compromising accuracy.

    Last edited: Jan 19, 2011
  11. VK2TIL

    VK2TIL Ham Member QRZ Page

    I didn't espouse high-impedance detectors in the context of measuring RL; I referred to;

    " ... the detector must be 50-ohms (diode with a load resistor) ... ".

    If the "comparison" method is used, the instrument is most-properly described as an RF bridge rather than an RLB.
  12. VK1OD

    VK1OD Ham Member QRZ Page

    Kerry, my point is that there are heaps of articles on the net that purport to describe Return Loss Bridges. Many use high impedance detectors, many fail to state that the source impedance is important to accuracy, and few deal with the design issues related to common mode choking of the detector. Whilst some espouse the use of simple detectors at low power, their linearity is an issue that needs to be calibrated into the measurements.

    I cannot ever recall seeing an article that had a mathematical proof of operation. That is not to say that there isn't any, just most articles depend on an act of faith on the reader's behalf or simply a qualitative description of the null condition and an unsupported statement that they measure Return Loss. Discussion of Return Loss as a negative dB quantity are a sure sign of loose thinking.

    They are not the 'no-brainer' often implied.

  13. VK2TIL

    VK2TIL Ham Member QRZ Page

    The mathematics is nicely laid-out in IRFD.
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