SWR Issue - Antenna or Radio

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by VK4JE, Sep 5, 2021.

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  1. VK4JE

    VK4JE XML Subscriber QRZ Page

    Good Afternoon,
    I have an FT-847 connected to a GAP 20M Mono the GAP 20 is installed at ground level
    I have a LDG RT-100 installed at the base of the GAP 20
    The GAP20 and the FT-847 are 25M apart and are connected with LL400 Coax

    Operating on 10, 12, 15, 17, 20, 30 Metres at 100 watts the SWR is below 1.2 constantly and will stay there until the cows come home.
    On 40M I can occasionally tune it to 1.5 and 100 watts output, however after a few minutes of operation the SWR goes to 5 and the FT-847 throttles back to around 20 Watts as it enters self protection mode.

    I am planning on obtaining a dummy load next week and running FT8 on 40 Metres into it to see if the FT-847 fails or not.

    I am not sure if I am asking too much of the ATU and the GAP 20 Mono, however given the outstanding success on the 10 - 30 metre Bands I am not sure. You can see my contacts on my QRZ page so things are not all that shabby.

    I would be grateful for constructive feedback and comments.

  2. W9IQ

    W9IQ Ham Member QRZ Page

    It sounds as if the SWR or common mode current on 40 meters is too high and causing the tuner or balun/common mode choke to overheat.

    - Glenn W9IQ
    UT7UX, WB5YUZ, SWL37632 and 1 other person like this.
  3. VK4JE

    VK4JE XML Subscriber QRZ Page

    Hi Glen,
    Thanks for the reply, if I get to the bottom of the issue I will let you know.
  4. W1VT

    W1VT Ham Member QRZ Page

    It is usually ferrite overheating that causes a slow rise in SWR with a time constant in minutes.

    I have seen an inductor get hot enough to melt solder but don't think the SWR drifted as a result of that heating. But that was with an amplifier, not 100W.
    SWL37632 and AG6QR like this.
  5. SWL37632

    SWL37632 QRZ Member

    Concur with the prior comments.

    My experience is the same when using ferrite or iron powder inductors used in ATU's.

  6. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    I think the problem comes from trying to force a GAP 20 Mono to operate on 40m. Frankly, it's not a good idea for several reasons, especially if you are using a bit of feedline 1/2 wave long on 20m, as called for by the manufacturer.

    I wouldn't expect any tuner to match that efficiently, ever.
  7. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    You are overheating the PA. You need to reduce power as recommended on Page 47 of the Manual. The FT modes are very high duty cycle, particularly FT8 and FT4. You just aren't giving the radio enough time to stay cool.

    1. FCC Part 97 requires us to use "The minimum power necessary" to communicate.
    2. There is only 3dB difference between 50W and 100W. 1/2 a nominal S-meter mark. 100W to 25W is 6dB and only 1 S-meter mark.
    3. Use a reduced power level of perhaps 25-30w and only increase power for individual contacts that are very weak. When completed, reduce power again!

    -Stay cool, and don't fall into the mindset of "All knobs to the right!".

    -FT8 is not for 'mindless operators". Become a virtuoso on your radio control panel. Learn to use the different settings to the greatest advantage!

    FT847 Power.png

    FT-847 Operators Manual
    Last edited: Sep 5, 2021
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That wouldn't cause the SWR reading to change.

    I've used most solid state rigs ever sold for amateur use and never saw one that would damage the PA if SWR was even 10:1 or 20:1; they just fold back in output power until eventually there isn't any...but that's not damaging. The only failures I've seen were due to open or short circuits (VSWR infinity), and even then most won't fail, they just can't produce any power.

    If SWR changes over time with an antenna that's not moving and a fixed transmission line and no loose connections, something in the antenna system itself is overheating or otherwise overstressed. Ferrite core inductors in auto tuners are a very likely culprit. Get them hot enough and not only will tuning change (creating an SWR shift as measured by the radio) but the ferrite can be permanently damaged (takes a lot of heat) and won't recover.

    I agree trying to use a 20m antenna on 40m is very stressful for the tuner unless the tuner's at the "radio end" of a lossy transmission line. With a lossy line, the line will absorb most of the power and nothing should be damaged.
  9. VK4XJB

    VK4XJB Ham Member QRZ Page

    The fcc has not thing to do with it.

    At the moment my guess is something over heating and changing value. Had this happen with a 4:1 balun with the swr remaining fine at 100W but 200W was enough to push it over the edge with the swr rapidly climbing as a core heated. Assuming the radio passes the planned dummy load test I am wondering how some crude matching at the antenna to make life easier for the tuner would go. As an example on my 30m vertical I have a shunt inductor that is sized such it only has a small effect where I normally use the antenna and on 80m it lowers the swr enough I can use the vertical there without stressing anything too much. The swr is about 5:1 instead of the right hand side of the meter like it was before hand. Low enough I am not worried about frying anything and quickly on air next time a gum tree drops a branch and takes out the main lower band antenna. Again.

    A voltage node at the tuner is another thing I like avoiding. Already have burn marks on 1 capacitor where it arced.
    KA0HCP likes this.
  10. VK4JE

    VK4JE XML Subscriber QRZ Page

    Thank you for all of the comments and suggestions.
    Clearly I have some homework to do over the next few days.

    Hopefully the Dummy Load arrives this week so I can being in a constructive way.


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