[AMRadio] MFJ-815B Watt-Meter with AM


Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Fri Feb 9 21:13:05 EST 2007


A lot depends on your voice, and the transmitter.
I get 4x only if I push it, most times on most radios
I get 3x the carrier power.

With mfj, I am surprised it worked after you used it on AM.

Brett
N2DTS

 

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net 
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jim candela
> Sent: Friday, February 09, 2007 8:48 PM
> To: Discussion of AM Radio in the Amateur Service
> Subject: [AMRadio] MFJ-815B Watt-Meter with AM
> 
> 
> 
> Hi,
> 
>    I recently bought a used MFJ-815B wattmeter. The price was 
> decent, so I
> took it home, and checked it out. The reviews on the net 
> (eham) were often
> giving it a 1 out of 5. I think they were overly harsh. Mine 
> works, and
> seems to agree with my Byrd 43 on 80 meters when I used a 50 
> and 500 watts
> slug. My dummy load is pretty good (50 ohms resistive), but I 
> still showed
> 1.3:1 SWR. There is an adjustment capacitor in the MFJ that nulled the
> bridge to 1:1 just fine.
> 
>   Then I ran some AM, and switched to PEP mode. A voice 
> modulated AM signal
> with peaks upward to 100% (as seen on scope) only showed 
> about 2X PEP power
> above the carrier. I recall years ago using a Mirage 
> wattmeter that had a
> PEP function, and it would read 4X the carrier with 100% 
> modulation via sine
> wave. The big difference between the MFJ and the old Mirage 
> was that the
> Mirage had active electronics within, and the MFJ is all 
> passive. It's hard
> to do a peak detect and hold without transistors or OP-amps.
> 
>   Then I looked carefully at the circuit card within the MFJ, 
> and there on
> the silkscreen was several missing resistors, and a missing 
> transistor. It
> looked like the MFJ once had active circuitry powered from 
> the 12 volt meter
> light power. Does anybody know about this, and why MFJ may 
> have created this
> design, and then left these parts out of the meter? Maybe 
> those parts were
> used in a more expensive model?
> 
> Regards,
> Jim
> JKO
> --
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> 9:04 PM
> 
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