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Re: [Amps] AL 1500 P.O. Meter Function

To: "'MICHAEL HUGHES'" <>, <>
Subject: Re: [Amps] AL 1500 P.O. Meter Function
From: "Joe Subich, W4TV" <>
Date: Mon, 5 Oct 2009 00:26:35 -0400
List-post: <">>

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: 
> [] On Behalf Of MICHAEL HUGHES
> Sent: Sunday, October 04, 2009 9:50 PM
> To:
> 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.
> Mike,
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