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IC-7100 Built in SWR Meter Problem - A mathematical solution?

Discussion in 'General Technical Questions and Answers' started by W2PX, May 16, 2018.

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  1. W2PX

    W2PX Ham Member QRZ Page

    I have an IC-7100 as my main radio. I plan on replacing it down the road with 2 separate radios (dedicated HF and dedicated VHF/UHF) but that's a while off. Until then, I would like to try and get this design flaw mitigated where the internal SWR meter falsely reads a high SWR and therefore limits output power.

    On 2m my SWR is near perfect (1.1:1) saccording to the internal meter. However, on 70cm, the internal SWR meter thinks it is seeing a high SWR above 3:1 and as a result it reduces the output power. The higher I turn the power up, the more pronounced the effect. For example, if I turn the power down to 10%, the SWR reads below 2:1. As I turn the power up, the internal meter reads a higher SWR. It's obviously a calculation issue internal to the radio. My external meter shows SWR below 2:1 across the band.

    This is a known issue with this radio and is linked to feedline length. In fact, my research indicates many all-band radios have this problem on VHF/UHF. The manual clearly calls out this potential issue and states it is due to feedline length so I thought I could mitigate the problem by calculating feedline length in half wavelength increments based on wavelength and velocity factor and was able to reduce the error but no where near enough.

    Has anyone had an experience with this issue and is there a way to find the right length of coax without wasting PL-259's on each experimental coax length until I find the one that works?

    Jason, W2FPV
  2. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Umm, no.

    The metering function of your radio, and the impedance of your antenna/feedline system at the radio connection are independent and changing one will not change the other.

    -If the radio metering function is in error, then it must be corrected

    -If you want lower SWR of your antenna system, then you must adjust your antenna, and/or use transformer functions like adjusting coax length/impedance or using baluns/transformers. Note: If you are using coax of the same impedance as that of your radio and antenna, e.g. 50 Ohm coax, then changing length will have no effect.

    I refuse to believe that Icom has deliberately fielded a malfunctioning SWR meter function, and that they have suggested that adjusting coax length is the solution.
  3. W2PX

    W2PX Ham Member QRZ Page

    It's clearly state in their manual so they in fact did release the radio like this. Page 6-13 of the Advanced Manual states the following note:

    This transceiver can measure SWR in the 144 MHz or
    430 MHz bands.
    Depending on the length of the connected coaxial
    cable, or installation condition, the measurement may
    be different from the actual SWR of the antenna in
    these band.

    It only affects the VHF/UHF bands. HF is not affected by this.

    If you go to eham reviews of this radio, one of the users went through the trouble of testing multiple lengths of coax until he found the one that works.
  4. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Well then, it seems like you have left the realm of engineering and physics and entered the land of magical devices, trial and error, and random luck. No one can give you advice since science no longer applies!

    Get out plenty of coax, cable cutters, a box of connectors and start snipping and soldering until you find the random length that works! Good luck! :)
  5. W2PX

    W2PX Ham Member QRZ Page

    LOL thanks. I'm hitting the repeaters I want to without issue with the power set to 33% right now so I think I'll just let it be and use it as motivation to get my dedicated radios sooner rather than later. At least I know my TRUE SWR is good so there's no risk of blowing my finals as long as I keep the external meter calibrated and monitored.
  6. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Hooboy. Seeing is believing. Here is the manual page:

    Attached Files:

  7. K7JEM

    K7JEM Ham Member QRZ Page

    All that means is that as the frequency goes up, the actual SWR at the antenna is different than at the radio. On UHF, you should never see an SWR greater than 1.5 to one, if you have any sort of feedline length. If you are seeing 2 to 1 at the radio, there is some sort of problem with the antenna, or installation.

    If you want to try something, add about 4" of cable. That is about a QW at UHF and should show some change.
  8. G0GSR

    G0GSR Ham Member QRZ Page

    Jason, the only way to determine if the internal SWR meter is accurate at VHF/UHF is to connect a good dummy load directly to the radio and verify that the meter reads a low VSWR.
    I have not looked at the 7100 circuits but if the SWR meter is using the same directional coupler for HF and VHF/UHF, its quite likely that its performance will be very inaccurate.

    For a given antenna, the VSWR does not change dependant on where on the line it is measured. At VHF/UHF the VSWR reading will improve noticeably as the feeder length increases but only due to increasing feeder loss.

    Last edited: May 17, 2018
  9. G0GSR

    G0GSR Ham Member QRZ Page

    Oh dear, more misinformation from the CB world...

    Last edited: May 17, 2018
  10. AI3V

    AI3V Ham Member QRZ Page

    Couple points,

    A 2:1 vswr on 70cm is telling you that you have a bad antenna system.

    It's the high vswr that causes a reduction in power, not the vswr circuit.

    I would expect two different meters to read 2:1 and 3:1, inexpensive meters don't give lab grade measurements on bad antennas.

    The vswr appears to get better at low power because the meters are calibrated for full power. This is normal.

    Changing feedline length has a proportional increase in effect conpaired to low frequencies, NOTHING in a 70 cm radio system is 50j0 (Edit: this is why your two vswr meters read so different, neither one is exactly 50 ohms, non reactive. At the same time, neither is your antenna, feedline,connectors, or transmitter itself)
    (Edit part 2, even if the 2 vswr circuits are exactly identical, the fact that you are adding and subtracting feedline length when you add and remove the external meter may, and probably will cause the internal meter to show a different reading, because you are changing the length of a feedline[meter] that has a impedance that is different than the rest of the system)

    Want to really get confused?

    Search "GE Z match". Your local police department probably has a "antenna tuner" in the transmitter of their repeater.

    Last edited: May 17, 2018
    K7JEM likes this.

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