[Elecraft] Elecraft RF Probe

[Larry Phipps]

The diode probe measures peak voltage if it's a standard diode/cap 
detector... and it needs to handle the peak inverse voltage, which would 
be 100V ( 70.7v RMS) across a 50 ohm load at 100W. A series pair of 
1N5711 Schottky diodes should give you 140V PIV, which should be safe. 
The drop will still be quite low... lower than a single silicon diode, 
but obviously twice as high as a single Schottky.

If you go that route, you should put a 1 meg resistor across each diode 
to balance the voltage across them.

Larry N8LP



Stan Rife wrote:
> 	Thanks, Ron, and everyone. I am trying to measure the RF voltage at
> the dummyload, and calibrate the K2 RF meter. The probe is fine at 20watts,
> what everyone has recommended I check it at. I was just trying to read it at
> 100watts to verify that the K2 meter tracked with the output. That's when I
> took out the diode. 
>
> 	Tom and Don both warned me about this, but I just had to push the
> envelope. I'll just set it at 20watts and let it go at that. I'll see if I
> can find some 1N34A diodes locally.
>
> 	The voltages range from 25v to 70v. 71 volts equals 100.8 watts, if
> I remember my calculations. 
>
>
>
> Stan Rife
> W5EWA
> Houston, TX
> K2 S/N 4216
>  
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ron D'Eau Claire
> Sent: Monday, May 22, 2006 9:20 PM
> To: elecraft at mailman.qth.net
> Subject: RE: [Elecraft] Elecraft RF Probe
>
>
> As you make the RF probe better able to handle high RF voltages you will
> also cause it to be less accurate at lower RF voltages.
>
> The common RF probe diodes have a turn-on voltage of about 0.2 volts. Below
> that, they are almost open circuits and have no output. Higher PRV diodes
> have turn-on voltages of closer  to 0.6 volts - three times that of the more
> sensitive diodes. So it takes three times the RF voltage to even begin to
> show a DC output and you'll need to be well above the turn on voltage to
> show an output that accurately indicates the RF applied. Put two or three
> diodes in series and you'll need 1.2 to nearly 2 volts  to just start
> showing some indication of an output.
>
> Bottom line, don't try to use such a probe for low RF levels.  
>
> Ron AC7AC
>
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