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Amplifier Build that presents a high SWR to my radio. Need to reduce

Discussion in 'Radio Circuits, Repair & Performance' started by KC1FIH, Jul 28, 2021.

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  1. KC1FIH

    KC1FIH XML Subscriber QRZ Page

    I purchased a 70 Watt Amplifier Kit and built it
    70 watt amplifier.jpg

    Unfortunately, the kits do not come with any instructions. I have managed to find a variety of web sites where people built the amp and offered perspective. The kit seems to be shipped in varying configurations. In any case, I built it and it does not work properly. It basically amplifies, but the input SWR as seen by my Ham radio is high. I am using a 50 Ohm dummy load to test output.

    Here is a link to one site describing the amp/more info

    https://www.qsoshack.com/make-a-45-watt-linear-amplifier/

    There are many other sites describing it and the build.

    The picture below shows the input transformer labelled as having a 2T:0.5T turns ratio.
    Input Transformer.jpg


    When the amplifier is active, the input signal from my radio goes thru an inductor(the transformer) to ground. My radio sees a high SWR (at 3.9 MHZ).

    The resister labelled R6 (18 Ohms in my build)/Vertically placed was not described on some web sites, but others said to include it. The resister is over the secondary side of the transformer. Some said to use 21 Ohms, some said 15 ohms. A table was shown of SWR over various choices of R6 choices.

    The resister is 20210726_151220.jpg over these two contact areas.

    What circuit components are needed to reduce the SWR as seen by my radio. I am going to try a 15 Ohm resister on the secondary instead of my 18 and see how that works. One person mentioned to use a variable cap in series with the transformer input and tune it.

    I wish to have a decent SWR on the 20 and 40 meter bands (at the very least). Like to have 1.5 or less.

    I have a nanovna and can test with that. If I can do a sweep and show the smyth chart or swr, would that help?

    I am also contemplating modeling this in LTSPice but wonder about my modelling skills as the schematic is somewhat less descriptive.


    I feel that this should boil down to a calculation of what component I need and where it should be placed as determined by a smyth chart. I dont really know those charts , but the nano might be helpful . I just want this swr 1.5 or less by adjusting the circuit with the rigt compoent(s) and have it stay like that on as many bands as possible.

    The amplifier has its own problems in terms of cleanliness of signal and heat dissipation. Please offer some pointers as to the route I should take.

    I have been reading up on transformers and how power is conserved and the secondary resister will show up on the input side thru the turns ratio, but wonder how that can fix this over multiple bands.
     

    Attached Files:

  2. K6BSU

    K6BSU Ham Member QRZ Page

    It is obvious your input transformer doesn't look like 50 + J0 to your exciter. You could try adding a pi resistive network at the input. Calculate for about 3 dB attenuation.
     
    G4COE likes this.
  3. KC1FIH

    KC1FIH XML Subscriber QRZ Page

    You mean, implement something described here...

    https://www.electronics-tutorials.ws/attenuators/pi-pad-attenuator.html

    If I do that, won't power be dissipated in the network rather than going to the transformer coil? If I calculate and implement this, how will this handle the different bands? I mean, do different frequencies affect the resistance the network is designed to match, requiring new resistance values?
     
  4. G3YRO

    G3YRO Ham Member QRZ Page

    Yes it will ! But resistors (if they're not inductive) will present the same impedance at all frequencies.

    If you have plenty of excess drive, you could connect a 60 ohm resistor to ground, and then a series 100 ohm resistor to the input of the Amplifier board. Then your rig will then always see a good SWR into the Amp.

    I imagine it's quite hard to design an Input Transformer that allows the Exciter (your rig) to see a 50 ohm impedance from 1.8 to 30 MHz (which is what you ideally want to achieve) . . . clearly the one in your Kit doesn't achieve that.

    Out of interest, what SWR are you seeing on each band between your rig and the Amplifier? (you didn't say)

    Roger G3YRO
     
    Last edited: Jul 28, 2021
  5. WA1FOK

    WA1FOK Ham Member QRZ Page

    Hi
    looks like you are using resistors that are not carbon, they are made by using a spiral pattern which will look like an inductor. Try using carbon composition resistors.
     
  6. KC1FIH

    KC1FIH XML Subscriber QRZ Page

    I am using Metal Film resisters. It sounds like that might be a problem. The SWR graph of when I used a 15 Ohm resister is below. I also tried an 18 Ohm. They are basically the same. I made this graph with my Nano VNA. I am wondering if using a more powerful exciter , like my VTX, will change the readings. I might have to try the nano on the resister and see what that shows.


    15 Ohm SWR.jpg
     

    Attached Files:

  7. G3YRO

    G3YRO Ham Member QRZ Page

    I can't make any sense of the Units on your Graph . . .

    So what actual VSWR are you seeing into the Amp on each band?

    Roger G3YRO
     
  8. KC1FIH

    KC1FIH XML Subscriber QRZ Page

    That is a graph of SWR by frequency. The vertical axis is swr. The horizontal frequency. From the nano vna, it says that it never really gets higher than 5:1. I also says that the min swr is at 4.84 MHZ at 1.17:1. I just order a Chinese antennae tuner kit that i suppose will match it perfectly. I feel that these swr values will change when using the radio as an exciter as the nano doesnt sent much power?
     
  9. KC1FIH

    KC1FIH XML Subscriber QRZ Page

    I ordered this kit from Amazon
    https://www.amazon.com/gp/product/B07TSJM4RR/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

    It is labelled a [HF Transceiver Kit Manual Antenna Tuner Tune DIY Precision Kit 1-30 Mhz for HAM Radio QRP DIY Kit]

    Will this fix my problem? I would insert it between the radio and the amplifier instead of radio and antennae. The antennae is resonant where it needs to be.

    It uses a T-topology network and matches between 30-300 ohms.
     
  10. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Interesting project, made difficult by having no schematic.

    Comments, your approaches suggested above are not appropriate for various reasons.

    Circuit Goals:
    -Present 50 Ohm impedance to radio on each band.
    -Provide XX Ohm impedance to input side of circuit on each band. (after the transformer).
    Caveat: I do not know if the input impedance of this solid state circuit varies by band, such as happens with tube amplifiers.

    "Technician" Guesstimate Method:
    -Place a 50 Ohm swamping resistor across (parallel) the Input from radio. Use a wattage appropriate for the power you are inputting.

    More scientific approach:
    -Using the VNA measure the Impedance of the input terminal on each band with the amplifier powered. Units should be Impedance, NOT SWR. Spot measure at center of band are sufficient.
    -Disconnect the output side of the transformer. Measure the Impedance on each band [at the output side], as above.
    -List the measurements for each in a table.
    -You now know what Impedance is require for the amplifier input on each band.
    -You now know what Impedance is being presented to the radio on each band.
    -You can now evaluate whether the transformer turns ratios are appropriate to meet the two goals above.
    -You can calculate the necessary transformer ratio to provide the correct complementary L - Impedance, and if any C - Capacitor needs to be added as an LC Network.
    -If the amplifier input impedance varies by band, you would need to either 1. Select a single band to match or 2. Include a switch to select matching networks for each band.

    CAVEAT: I'm not an engineer. Just providing ideas for research.
     

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