[SOC] 'Scope question

[Fabian Kurz]

Hi Tom,

On Sun, Nov 11, 2007 at 08:48:59PM -0500, Tom McCulloch wrote:
>    OK, so I wasn't able to get good Voltage P-P readings on the scope for 
> any of the bands I tried (80 through 10 meters) -- they all seemed too low.

Can you give some actual values you measured (both with a watt-meter
and with the scope; which formula did you use to calculate the power
from the RF voltage)?

Are you maybe using a probe with a built in voltage divider? A ratio
of 1:10 is very common on standard probes. You can easily check by
measuring the voltage of a known source (like a battery or your 12V
power supply).
 
> 1) My scope is 20 Mhz, which I know is low -- is this a problem in 
> measuring RF voltage, or just a problem in calculating frequency?

Above 20MHz is indeed a problem in RF voltage, since the input has a
low-pass filter characteristic which has a certain cutoff frequency.
Above this, the signal is attenuated and your voltage readings are too
low. But on all bands up to at least 20m it's fine.

> 2)  Rather than open up the K2, I put a "T"-adapter on its RF output 
> (antenna) jack (outside the rig, on the back panel).  I put a 50 ohm dummy 
> load one side of the "T"  and left the other side open so I could stick the 
> 'scope's probe into it.  I put the ground clip of the probe onto the ground 
> post on the back of the K2.  Is using a "T"-adapter a bad idea?  Should I 
> have gone inside the rig and used the jack itself (with the dummy load 
> connected outside the rig, of course)?

It shouldn't make a noticeable difference at such low frequencies.

73,
-- 
Fabian Kurz, DJ1YFK * Dresden, Germany * http://fkurz.net/