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

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  1. #1

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

    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).

    Thanks,

    -Alex

  2. #2
    Join Date
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    Default

    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;

    www.nerg.asn.au/NERGNEWS/NN200512.pdf

    www.nerg.asn.au/NERGNEWS/NN200907.pdf

    http://www.wolfgang-wippermann.de/

    http://www.qsl.net/n9zia/wireless/appendixF.html#11

    You can buy RLBs;

    www.amtronix.com/rlb.htm

    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. #3
    Join Date
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    Bird model 43 watt meters have slugs in the 400 - 1000 MHz range with full scale powers down to 1 watt.

    fp
    [FONT=T93Y][COLOR=#0000ff][B]ACFP

    [/B][/COLOR]
    [/FONT]

  4. #4

    Default

    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?

    Thanks,

    -Alex

  5. #5
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    You will find a lot of information with a Google on "return loss bridge"; here is a start;

    http://www.wetterlin.org/sam/

    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. #6

    Default

    Quote Originally Posted by VK2TIL View Post
    ...
    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.
    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.

    Owen
    Last edited by VK1OD; 01-19-2011 at 03:12 AM.

  7. #7

    Default

    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.

    thanks,

    -Alex

  8. #8

    Default

    Quote Originally Posted by KJ4RIV View Post
    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.

    thanks,

    -Alex
    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.

    Owen

  9. #9
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    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. #10

    Default

    Quote Originally Posted by VK2TIL View Post
    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!
    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.

    Owen
    Last edited by VK1OD; 01-19-2011 at 06:14 AM.

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