[Amps] Dummy Loads & Wattmeters

Steve Katz stevek at jmr.com
Wed Dec 2 09:51:53 PST 2009


It's likely been overloaded at some point and the Globar should be changed.


-----Original Message-----
From: David C. Hallam [mailto:dhallam at rapidsys.com]
Sent: Wednesday, December 02, 2009 9:44 AM
To: Steve Katz
Cc: AMPS List
Subject: Re: [Amps] Dummy Loads & Wattmeters


I have measured the Cantenna with 4 different ohmmeters, HP-410B, 
HP-410C, a DVM, and VIZ multimeter.  They all read the same, +/- 1 ohm.

Who knows what the Cantenna has been subjected to.  I bought it at a 
hamfest several years ago.

David
KW4DH

Steve Katz wrote:
> Yep, there is.  An accurate power measurement with the model 43 is P(actual) = Pf (indicated) - Pr (indicated).  With a 70 Ohm load you should have a VSWR of 1.4, a return loss of about 15.6 dB, or about 3% reflected power, so the Bird should read "high" by about 3% when viewing indicated Pf.
> 
> But I'd look into why the load measures 70 Ohms!  Check your Ohmmeter if you can to verify its accuracy, and if that seems good, probably the load's been overheated and the resistor should be replaced.
> 
> WB2WIK/6
> 
> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]On
> Behalf Of David C. Hallam
> Sent: Wednesday, December 02, 2009 9:16 AM
> To: AMPS List
> Subject: [Amps] Dummy Loads & Wattmeters
> 
> 
> A little education, please.
> 
> I am working on restoring an old transceiver and linear.  I have a Bird 
> 43 wattmeter and a Heath Cantenna dummy load.  The wattmeter is for 50 
> ohm impedance and I measure the DC resistance of the Cantenna at 70 ohm.
> 
> Is there any effect on the wattmeter reading from this mismatch?
> 
> David
> KW4DH

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