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Thread: SWR meter - Do it yourself project

  1. #1221
    Join Date
    Mar 2015
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    Quote Originally Posted by Pollux View Post
    Well, it's much better, and I think this will be sufficient (if, and only if your capacitor and resistor are fit to work at such high frequencies, but I'm not qualified to tell this)
    You don't need to go SMD, Basically, as soon as the exposed wire length out of the coax cable is less than 1/16 of the wavelenght (~2.5 mm @ 5.8 GHz) their influence is negligible.

    Maybe you coud twist the black and red wires together, to avoid EM interferences to ride these wires, but I think this is already a good setup.
    Test it, if you get a VSWR reading that changes with the channels, you know it's working...
    i really appreciate the help you guys here are inspiring so much I'm going the last leg with a bias board I'm a total rookie so go easy on me. Just waiting on the trim pots I believe

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  2. #1222
    Join Date
    Mar 2015
    hi guys hoping you can verify my setup once again results are crazy different using a bias board. I'm hoping my setup it's correct with no Lipo in VTX and AA battery plugged in I get .29 DC. With a known good antenna and VTX powered up, it Zeros out. voltage does change with channel selection Trim Pot #1 @ 0.30 and Trim pot @ 0.00 Thank you in advance your a big help guys!
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    Last edited by gsx8299; 4th November 2016 at 04:16 PM.

  3. #1223
    Join Date
    Nov 2016

    i built the ibcrazy meter and am now wondering whether I damaged the diode in the processs (i think I read somewhere in there or on rcg that the diode doesnt like too much heat, correct?).
    I use a -16db .5-8GHZ Krytar 1508 coupler, SMD cap and resistor, and a Agilent HSMS-286K diode. Im measuring on 5.8ghz.

    The following behavior makes me think I might have messed up something:
    - When measuring forward and reverse voltage it makes a huge difference on what channel I set the vtx. Lower channels = higher voltage.
    - I get a non zero reverse voltage regardless of vtx. I tried 25mw, 200mw and 600mw. My intuition was that with 25mw and -16db coupling I shouldnt get voltage readings?
    - If I measure reverse voltage with a 50 ohm terminator I still get non zero readings, in particular 100mv-200mv.
    - In general: on 600mw 5945mhz I get a forward voltage of around 2.5V and a reverse voltage of 200mv to 1.4v depending on antenna.
    - I tested a self built 6lobe antenna and got a swr of 1.05. Seems to good to be true D:. I also measured a foxeer antenna which was even better. 3/4 no name bangood antennas performed pretty shitty. Suprisingly though I also measured a spironet antenna and an aomway antenna, which also performed pretty bad, this makes me think somethings up. I'll probably create a more thourough documentation on antenna performance this weekend.
    - Readings do change if I put my hand in proximity to the antenna, at least this part works as expected

    Happy about any comments about the behaviour, cheers.

  4. #1224
    Navigator Martin7182's Avatar
    Join Date
    May 2012
    Bussum, Netherlands
    Hi Yosh,

    All of that looks quite normal. If you get any voltage it means the diode is still working. With 25mw and -16db you should get lower values than with higher powers. You're right that there is minimal power needed to get the diode to work, but as long as you're above that level, the diode is more or less linear. A 50 ohm terminator can still cause some reflections. But note that you're also measuring the cable. Vr between 100mV and 200mV with 2500mV Vf means VSWR lies between 1.08 and 1.17. These values are to be expected.

    1.4V reverse voltage (with 2.5 Vf) is pretty bad; VSWR = 3.55. But such antennas exist. I would throw away that antenna and use the one with 200mV reverse voltage

    Your 2.5V Vf at 600mW looks normal. If I remember correctly, I also got around 2.5V at 100mW, but that was with a -10dB coupler. Your coupler is -16dB (6 times less output power) but you use 6 times more input power, so our voltages are equal.


  5. #1225
    Join Date
    Nov 2016
    Thank you Martin, good to know that my detector did work. Past tense because clumsy me just ripped out one of the cables, and I can't just resolder it because I encased the wholeshebang in super glue . Glad I bought 5 diodes...

    Anyways, one more thing that I noticed is that the two video transmitters that I tested both tend to drop in output power quite a bit over time if I don't cool them. One dropped from 2.5V to 1.9V, at which point I stopped the experiment because I didn't wanna fry the vtx. If i use a fan to blow air on its ok though. Does this get better if use an attentuator?

  6. #1226
    Navigator Martin7182's Avatar
    Join Date
    May 2012
    Bussum, Netherlands
    Super glue? What a funny guy you are!
    I use low power transmitters that don't need cooling. But other people are also using fans to keep their vtx steady. An attenuator doesn't help because that only converts energy into heat what otherwise was radiated by the antenna (and eventually converted into heat too). Your vtx isn't aware of any attenuator; it still sees a 50 ohms load. So with an attenuator, your vtx does the same thing as before.

    Perhaps you could try lowering the voltage that powers the vtx.


  7. #1227

    140usd Digital 5.8ghz SWR meter from OwlRC - how accurate is it?

    Got the OwlRC 5.8ghz SWR meter yesterday and did some empirical data comparison against readings taken with a high-end VNA.

    Red text below are the major discrepancies and Green text below are the minor discrepancies

    Hugo's crosshair
    5.8ghz OwlRC meter: Lowest SWR of 1.03 at 5657mhz (equivalent to reflection of -36.6dB)

    20ghz KeySight VNA: Lowest reflection of -17.5 dB at 5945mhz
    reflection of -13 dB at 5657mhz

    IRC 8db RHCP Patch
    5.8ghz OwlRC meter: Lowest SWR of 1.41 at 5775mhz (equivalent to reflection of -15.4 dB)

    20ghz KeySight VNA: Lowest reflection of -17.8 dB at 5830mhz
    reflection of - 15 dB at 5775mhz

    Realacc Triple Feed Patch
    5.8ghz OwlRC meter: Lowest SWR of 1.27 at 5765mhz (equivalent to reflection of -18.5dB)

    20ghz KeySight VNA: Lowest reflection of -26dB at 5645mhz
    reflection of -23dB at 5765mhz

    Refer to actual curves in the video below:
    Last edited by RC-Concepts; 15th August 2018 at 06:24 AM.

  8. #1228
    Is a SWR reflection bridge similar to a directional coupler?

    I am thinking of getting this (goo.gl/pEBvvq).

  9. #1229
    Hello all,
    BUilt the design in the first post.
    Just not sure about the capacitor..
    Value is correct but not sure about the type.. using that brown monolitic capacitors (value 333)
    Problem is, I do not read any voltages.. I changed the diode but same..
    There might be two possible problems.. The cap i am using doesnt work with such high frequencies...
    Or.. 802.11g modem output is not continuous to measure the swr or power values..
    I am using a 5dB directional coupler with the setup.
    Any comments welcome.

  10. #1230
    @gobris, the modem output is definitely hopping but you should be getting a voltage that fluctuates, most home models have a gain of 17db. Check your RF attenuator to make sure your attenuated output does not exceed your 5db directional coupler. The brown capacitor worked for me so I don't think that's the issue.

    Recently, I happened to find this old iOS Antenna Array Analyzer in the iTunes folder on my old computer. Mind you, it is by no means a PCB/microwave simulation software like CST or Altium, nor is it an antenna modeler like 4NEC2. What it does best, is the simulation of 3D radiation pattern for elements in an array formation. The elements it supports are basic antenna-types like patches and dipoles. Despite the simplicity, I found it particularly useful when working on the layout of multiple microstrip antennas on a PCB For anyone who is interested, you could download the iOS-based Array Analyzer from the link provided in the video description.

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