On 6/9/2011 8:12 PM, Carl wrote:
> That is making the assumption stretch that the measuring instrument is
> correct. As I said earlier the 259 has problems with the digital display
> readout section and we havent heard back yet what the analog VSWR meter on
> it reads.
Last I knew and it has been long time, accurate, calibrated instruments
were expensive.
Course it was cheaper to get them calibrated than to purchase the
equipment to do the calibration and then sent it in for calibration.
I do hear tell there are some pretty good ones becoming available at
reasonable prices, although they are a tad more than what MFJ sells.
73
Roger (K8RI)
73
Roger (K8RI)
> Carl
> KM1H
>
>
> ----- Original Message -----
> From: "Commander John"<crazytvjohn@yahoo.com>
> To:<Amps@contesting.com>
> Sent: Thursday, June 09, 2011 4:11 PM
> Subject: Re: [Amps] Dummy load varying R
>
>
> This seems a easy one. The load is not pure resistance. It has resistance
> with reactance which was measured at +6j. Reactance resistance changes with
> frequency. a opposite reactance would cancel it out at the frequency of
> interest.
> john
>
>
>
>
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>
>
>
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>
> W9ZY
> .... Commander John ....
>
>
>
> --- On Wed, 6/8/11, Al Kozakiewicz<akozak@hourglass.com> wrote:
>
>
> From: Al Kozakiewicz<akozak@hourglass.com>
> Subject: [Amps] Dummy load varying R
> To: "amps@contesting.com"<amps@contesting.com>
> Date: Wednesday, June 8, 2011, 10:22 AM
>
>
> Maybe not quite amps related, but this is the best forum I know of for RF
> expertise ...
>
> Without belaboring irrelevant details, I'm calibrating an LP-100A wattmeter
> I just built. With a brand new MFJ 300W dummy load I measure 50.1 ohms with
> my old Fluke DMM. The dummy load is connected to the coupler with about 6
> feet of RG-8U. It appears as though the resistance of the dummy load
> increases with frequency, to the point where it is 65 ohms in the 20m band
> and the LP-100 can no longer compensate.
>
> After fiddling with the coupler with no joy I decide to connect my MFJ-259
> to the cable and verify that the resistance is actually 50 ohms across all
> the HF bands. Turns out that it's not. In the 20m band the impedance at the
> end of the RG-8 has risen to 66+6j.
>
> This is as simple a setup as I can imagine - a 50 ohm resistor in a metal
> box and 6 feet of new 50 ohm coax cable. Tonight I will test again without
> the cable if I can find a M-M UHF adapter. But I'm puzzled as to what the
> issue could be.
>
> Any ideas?
>
> TIA
>
> Al
> AB2ZY
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