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Bird Model 43 Reflected Power using Drake DL300 Dummy Load

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AA6P, Jul 12, 2015.

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  1. AA6P

    AA6P Ham Member QRZ Page

    I'm curious if anyone has an explanation for the following test results.

    A Drake DL300 Dummy Load is connected directly to the output of a Bird Model 43 Wattmeter. The Plugin is rated for 100 Watts from 2-30 Mhz.

    The input of the Wattmeter is connected to an IC-706MKIIG transceiver with 18 inches of RG-8X coax.

    All bands from 160 meters through 10 meters are tested after adjusting forward power to 100 watts on each band.

    Reflected power on all HF bands is approximately 4.5 watts. That represents a VSWR of 1.54:1. The dummy load is specified to be 1.1:1 or less.

    A DC resistance measurement on the dummy load shows 43 ohms (DVM) and 45 ohms (VOM). If my calculations are correct that would result in a VSWR of about 1.11:1 if there were no other factors involved. There is very little change in the DC resistance after the element heats up.

    Any thoughts on why the reflected power is reading 4.5 watts?

    Thanks, Kent

  2. KI6LZ

    KI6LZ Ham Member QRZ Page

    Need to check for reactance.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The 43 is a pretty dumb meter. The main housing as I'm sure you know is just a line section and a 30uAdc meter and has no active components.

    All the 'work' is done by the element, or 'slug.' And if it reads accurately for forward power, it should read accurately for reflected power also, as it's the same coupler and detector either way.

    I'd also try an antenna analyzer or similar instrument on the dummy load to see if it's reactive.
  4. AA6P

    AA6P Ham Member QRZ Page

    Thanks for the suggestions. I just remembered that the accuracy of the Bird Model 43 is stated to be 5% of the Full Scale reading. That would be plus or minus 5 watts in this case.

    The lowest marking on the 100 watt scale is 2 watts which makes you wonder about the plus or minus 5 watt accuracy. I was hoping that relative readings would still be fairly accurate.

    I think the general recommendation is to measure values only in the upper parts of the scale. That would suggest that a 10 watt plugin would give better accuracy for the reflected power. You would also need to be careful to not measure forward power with the low power plugin.

    73, Kent

  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That's all quite true. I have three 43 meters and about fifty elements for them, accumulated over a period of decades, and you're right, it's 5 percent of full scale for accuracy, and even that is only if the elements are within calibration. You can have them calibrated, but it's not so cheap to do so.

    However, I must say in about 45 years of Bird 43 ownership, I've actually never seen one measure more reflected power than there actually was. Of course, it's possible. I've just never seen it. Usually, it's 'lower.'
  6. AA6P

    AA6P Ham Member QRZ Page

    I wonder if we can dismiss reactance as a factor because the reflected power was the same from 2 Mhz to 30 Mhz.

    The Bird 5% full scale accuracy doesn't seem like it would apply to the lowest marks on the scale. In other words, how can plus or minus 5 watts apply to the first markings at 2 watts and 4 watts?

    We could also look at this problem another way. Let's say a reasonable SWR for the 43-45 ohm dummy load from all factors is 1.25:1. That would require 1.2 watts in reflected power. The reflected power after things warmed up was closer to 4.2 watts. That's certainly within the 5% tolerance.

    However, I also wouldn't expect the reading to be noticeably higher than the actual figure.

    73, Kent

  7. SM0AOM

    SM0AOM Ham Member QRZ Page

    Unless the DL-300 is not damaged, which seems to be ruled out by DC measurements, the remaining cause for the high reflected power reading would be directivity error in the Bird slug.
    They are specified for a directivity of more than 25 dB which provides reasonably accurate SWR readings down to about 1.3:1 using a meter scale with 0-100 graduations.

    However, if the slug is damaged, its directivity may be reduced. If it is, say, 15 dB the meter will indicate 3 W reflected power at 100 W forward even if there is none from the load.
    The way to check is to substitute known good RF load or Bird slug and see what reflected power readings they give.

  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    Close enough for government work.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    It does apply, even the Bird manual tells you that.

    For a 100W element, accuracy is stated at 5% of 100W = 5W at any point on the scale, down to zero.

    So it is indeed possible to have a 2:1 SWR (which would be ~11% reflected power, or 11W REF with 100W FWD) and the meter could read anything from 6W to 16W reflected and be fully within its specified accuracy. That's why for serious SWR measurements, you'd normally use a more sensitive element for reflected power; and Bird did and still does sell dual-element couplers with a dual-meter (cross-needle meter, so it's two meters inside one case) for those who frequently measure SWR. Of course, now they also (and have for many years) sell digital meters as well.
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    I was doing it wrong. I was hooking my meter up backward to read reflected power. :p

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