Based on the schematic of the amplifier, the PO function is
not a wattmeter but a simple capacitive voltage divider and
rectifier on the RF output. A circuit like that has no
directivity and is easily confused by reflected power and
common mode currents on the feedline.
If you are concerned about accurate power measurements,
get a good wattmeter and use it in a matched circuit -
SWR = 1:1 - and use common mode chokes between the meter
and antenna. A capacitive voltage divider/rectifier will
only provide accurate readings with no reflected power to
"boost" or "buck" the voltage in the line. For example,
if the SWR is 2:1, "power" calculated by using a voltage
measurement can range from 25% to 400% of the actual
power depending on the distance to/from the antenna (load).
... Joe, W4TV
> -----Original Message-----
> From: firstname.lastname@example.org
> [mailto:email@example.com] On Behalf Of MICHAEL HUGHES
> Sent: Sunday, October 04, 2009 9:50 PM
> To: firstname.lastname@example.org
> Subject: [Amps] AL 1500 P.O. Meter Function
> I need some help. Upon receipt of my new AL 1500 I noticed
> that the Multi Meter P.O. "Power Output" function indicated
> 1,100 watts output on 40 meters and similar low output
> readings on 15 meters. In all cases the load is a Hygain
> Hytower with VSWR at 1.5"1 or less on the frequencies tested.
> After talking to the factory they had UPS pick up the
> amplifier and it along with the tube were returned to
> Ameritron. I was given priority as it is a new unit. My
> contact at Ameritron couldn't find anything wrong with the
> amplifier and promptly returned it to me after asking
> question regarding my line voltage and whether or not I may
> have had the ALC connected. My drop is connected directly to
> the electrical box, 12 gage wire and the AlC was not
> connected. I asked the Technician on more than one occasion
> regarding the P.O. Multimeter and the accuracy of the Power
> Output and was assured that it was an accurate indication of
> power output.
> After receipt of the Amp it behaved exactly as it did when it
> left my home. Over the weekend I inserted an additional
> piece of Coax, RG 8214, approximately 24' long in line with
> the antenna and the amplifier and got significantly different
> numbers. I was driving the amplifier in all cases with a FT
> 1000mp Mark V Field with 50 watts indicated output power.
> The P.O. Meter now indicated somewhere between 1,500 and
> 2,200 watts depending on the band. All were checked with the
> exception of 160 for which I have no load at this time.
> I also used a scope, TEK 2235 connected to my EWE antenna to
> monitor the transmitted RF of both the driver and the
> amplifier. Using these numbers I extrapolated the gain in
> decibels, converted it to gain and multiplied the gain by the
> driver power (50 watts). Considering the drive, 50 watts the
> results seemed reasonable, if not somewhat optimistic with
> power outputs approaching 1,500 to 1,800 watts.
> My guess is that the power meter in the AL 1500 is really a
> "Relative Power Output" meter, not a Power Meter. The power
> meter reading should not be dependant on where you place the
> meter in the transmission line, or at least I would not think so.
> I am not complaining about the amplifier, overall, I would
> make the purchase again, based on cost/benefit.
> My guess is that this is a RF sniffer circuit which is good
> enough for a tune up process but not what I was expecting.
> If it is truly a watt meter then there is another problem.
> Thanking you in advance for your thoughts.
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