SWR vs Field Strength?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W5DXP, Dec 12, 2019.

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  1. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    I have worked on a good number of power amplifiers, most are broadband. The load Z for max output power from all of them is never 50 ohms and if it is it is by chance. All of them are designed to supply a specified power into 50 ohms and into a specified SWR (sometimes up the 3:1 but mostly limited to 2:1) but they can pump out a lot power above the specification into some non-50 ohm loads if the power control loop is not closed. Who knows exactly what load the final device sees when it goes thru output filters and t/r switching. (in the mil and other radio services I bet none of the transmitters run without a power control loop. I never checked my ham band transceiver to see how its power is controlled)
    When I measured return loss and transmatch output current, I used a good 50 ohm source impedance device (not a power amplifier but a good leveled sig gen and with good RL bridge).
    What Cecil brought up is not related to what the PA can pump out.....at least I don't think so at the moment:)

    If you want to see what the Z is for max output is for you transmitter, re-run the test and then measure the Z looking into the tranmatch. Should be the conjugate of what the transmitter's output Z is for max output...correct?
  2. W5DXP

    W5DXP Ham Member QRZ Page

    It is related to: Did W5OLV tune his antenna better or worse or equal to the way I tune mine?
  3. KM3F

    KM3F Ham Member QRZ Page

    I have adjusted a Tuner to 50 ohms and 1.1 to 1 match with an Analyzer, then tuned the amplifier into it.
    Seems the amplifier acts differently for tuning (AL 80B).
  4. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Before you used the antenna analyzer to adjust the transmatch, how did you tune the AL 80B (I think it is tube amplifier with a pi network)?
    I have another old Ranger (6146 final / pi network). Sold the first one in the late 60's. I first adjust the transmatch for a low SWR with my RL bridge (as you did with the ant analyzer) to establish a reasonable impedance for the Ranger. Then do the dip-the-plate current routine and increase the loading with the pi net until the plate current reached 120 to 150 mA when dipped. It would be like a dog chasing its tail if I didn't do it that way with a transmatch in the system:). In the 60's I used dipoles (468/F) and never worried about SWR. The pi net could get the Ranger's plate load correct with a wide range of antenna Z's.
  5. KM3F

    KM3F Ham Member QRZ Page

    This is the finer points of adjusting the Tuner and amplifier.
    The tuner can yield, within reason, infinite combinations that will offer a 1.1 to 1 match.
    I have used a calibrated Noise Generator to do the same function.
    Are they all correct?
    No but all within limits the amplifier can load into, there is very little on air difference.
    If one graphs the tuner setting over each band and does the same with the amplifier setting into those Tuner settings, you can return to those setting at all times and be spot on (no on air tune ups) providing the antenna has not changed.
    What I have noticed is the amplifier can work over a wider frequency, again within some limits, without retuning if care is taken to use the best matches.
    Also Tuner Q can enter this and affect the amplifier setting over a change in frequency.
    It gets a bit complicated when nit picking is done, when close enough, is usually good enough.
    Get to far off, I think IMD could increase from nonlinearity in the amplifier.
    When I tune up on SSB now for the last 10 years, I pulse tune using the radio's paddle dot keyer and peak reading watt meter function in the Tuner. Very fast and min on air time causes very little QRM to others plus very easy on the amplifier tube being pulsed at 50% duty cycle.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I have two field strength meters, one of which is a professional/lab type instrument (surplus, of course! -- this thing was over $3000 if new) with calibrated antennas that cover about one octave each; the other is a small homebrew diode detector that isn't calibrated at all.

    While the lab instrument provides actual "data," they both agree on what is better or worse -- exactly.

    It's fun to make F/S measurements, and even close by you get results -- in the farther field, using a simple detector and small antenna may require running serious transmitter power to provide indications. I added a preamp to the simple detector once but it drove me nuts because I live fairly close to a 1250kW SW BC station which is actually aimed almost exactly at me, and the F/S indication from that transmitter surpassed the F/S I could produce at 1000 feet. It's difficult to use untuned detectors when you're close (even a couple of miles) to a BC station.
  7. BILLYBOB560

    BILLYBOB560 QRZ Member

    If you have an Antenna Analyzer you can put it at the Transmatch input and measure this impedance, no?

    Curious what it might be (for a "50 ohm" rig.)

    Suspect it may CHANGE with power level (if the rig's Protection circuits are active or not)...

    Last edited: Dec 14, 2019
  8. KT1F

    KT1F Ham Member QRZ Page

    I don't think transmitters are expected to have an output impedance of 50 ohms. I don't mean whether or not it's a "perfect" exact 50 ohms. I mean there's no reason for it to be close to 50.

    Some power amplifier stages have a power efficiency greater than 50%. Think about that.
  9. W9XMT

    W9XMT Ham Member QRZ Page


    Good examples are some AM & FM broadcast transmitters, which can have modulated r-f power output to total a-c input power efficiencies of greater than 85%.
  10. W5DXP

    W5DXP Ham Member QRZ Page

    Given that energy cannot be destroyed, what happens to the energy (joules) in the harmonics of a class-C amp?

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