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Difference in RF power in UHF and VHF: technical question

Discussion in 'Ham Radio Discussions' started by W6OAK, Jun 13, 2018.

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  1. W6OAK

    W6OAK Ham Member QRZ Page

    I have a dual-band transceiver capable of putting out 50 watts of RF power on VHF and UHF. I recently bought a standing-wave and power meter to take a look at my transmission line.

    I expected to see some power loss, but the difference in forward power between VHF and UHF surprised me.

    In VHF, the meter reads about 42 watts of forward power. But when transmitting in UHF, I get no more than a little under 30 watts. (SWR reads at about 1.0)

    Why the difference? Can someone help explain?
    Thanks in advance.
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Perfectly normal! The efficiency, of the final amplifier stages, drops off as the frequency increases.

    This was true even in the "goode olde dayes" with tubes instead of transistors. For example, the Collins S-Line / KWM-2- series is rated at 100-watts output on 80, 40, and 20-meters, 90-watts output on 15-meters, and 80-watts output on 10-meters. This for the same power input reading.

    Also, what is the reflected power reading? You have to subtract the reflected power reading from the forward power reading to get the true power output from the transmitter. For example, if you are reading 42-watts forward power on 2-meters but have a reflected power reading of 12-watts, then the actual power output of the transmitter is 30-watts. On 70 cm, if you are getting a forward power reading of 30-watts and a reflected power reading of 1-watt, then the actual power output is 29-watts. You need to use a good 50-ohm dummy load, which will have, technically, no reflected power, to really get a good idea as to how much power you really have.

    Just what particular SWR bridge / wattmeter are you using to make the power measurements? Quite a few of the units available are just not that accurate.

    Glen, K9STH
    N2EY likes this.
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    What radio do you have ? What is your power supply voltage ?

    Most radios are rated less power out on UHF than VHF.

    Have Fun.
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Exactly what is the radio and also exactly what is the wattmeter?

    And how about those Golden State Warriors? Congratulations!:)
  5. AG6QR

    AG6QR Subscriber QRZ Page

    Good thoughts so far, but also consider that coax loss generally goes up with frequency. Probably not too significant for a short jumper, but you didn't say how much coax of what type is between the radio and meter.
    WZ7U and K3KIC like this.
  6. K3KIC

    K3KIC Ham Member QRZ Page

    Another thing to check, at least one of my radios had different settings for output power depending on band. Are you sure the radio is actually set to 50 W on both bands?
    N0TZU likes this.
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    I have noticed that many radios are rated for input power, Not output power.

    Reminds me of the good old days and power limits.

    Big numbers sell radios, Amplifiers, Power supplies and antennas.
  8. KP4SX

    KP4SX XML Subscriber QRZ Page

    Tell us the type of rig and meter.
  9. KA0GKT

    KA0GKT Ham Member QRZ Page

    I agree with Glen.

    Rather than test into an antenna, test into a good quality 50-Ohm resistive load sufficient to handle the transmitter power output.
    W4NNF likes this.
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    I agree 3.

    Testing into a 50 ohm load will tell the truth, As close as possible.

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