Elecraft › [KX1]

Output Power Measurement

Classic

List

Threaded

5 messages Options

Ron Bower

Output Power Measurement

I have searched the archives and have not been able to find a definitive
answer.

I've now completed the construction of the basic KX1 and would like to have
a fairly reasonable measurement of the output power on 20M and 40M

I have a very decent dummy load but do not have a very accurate RF Power
meter. I do have a decent DMM and could measure the voltage drop across the
50 ohm dummy load - I'm not sure how accurate the DMM would be at 7 or 14
MHZ.

Is there a recommended method to measure the power output without an RF
power meter ?

Thanks in advance,

Ron, AC2C

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

Jack Smith-6

Re: Output Power Measurement

You might use a diode, such as a 1N914, 1N4148 or (preferably) a 1N5711
to rectify the voltage developed across the load and then measure the DC
out of the diode. With a high Z load such as a DMM, the DC voltage will
be close to the RF peak voltage.

A standard DMM, even a good one, will not provide usable results above a
few hundred KHz, unless it's a specialized instrument intended for RF
measurements.

Jack K8ZOA

Ron Bower wrote:

> I have searched the archives and have not been able to find a definitive
> answer.
>
>
>
> I've now completed the construction of the basic KX1 and would like to have
> a fairly reasonable measurement of the output power on 20M and 40M
>
>
>
> I have a very decent dummy load but do not have a very accurate RF Power
> meter. I do have a decent DMM and could measure the voltage drop across the
> 50 ohm dummy load - I'm not sure how accurate the DMM would be at 7 or 14
> MHZ.
>
>
>
> Is there a recommended method to measure the power output without an RF
> power meter ?
>
>
>
> Thanks in advance,
>
> Ron, AC2C
>
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

Don Wilhelm-4

Re: Output Power Measurement

In reply to this post by Ron Bower

Ron,

Use a diode detector. A 1N4148 diode will do fine up to about 12 watts.
Connect the anode of the diode to the center conductor of your coax *at*
the dummy load (use a coax TEE adapter if you have no other access).
From the cathode end of your diode, connect a 0.01 uF capacitor to the
shield side of the coax.
Your DMM leads connect across the capacitor.

Measure the forward voltage of the diode with your DMM if it has that
function, otherwise, assume that the diode forward voltage is 0.6 volts.

With this probe, the DVM will read the peak RF voltage. You can
calculate the power from the DMM reading as follows:

The peak voltage across the resistor is equal to the DVM reading plus
the diode forward voltage, so add them.
You may then calculate the RMS value of the result above.
Then square that result and divide by the resistance.

Or you may accept (or derive) the formula will be as follows with no
intermittent RMS calculation.
P = (Vdvm+Vdiode)^2/2R = (Vdvm+Vdiode^2/100 for a 50 ohm dummy load
OR, if you assume the diode drop is 0.6 volts
P=(Vdvm+0.6)^2/100

73,
Don W3FPR

Ron Bower wrote:

Jack

Re: Output Power Measurement

In reply to this post by Ron Bower

Ron,

I just went through this scenario. With the help of Don Wilhelm I was able
accurately measure my power out on my KX1 and K3.

I first used a homebrew load with "free" resistors given away at a club
meeting. They only worked at low frequencies. At 20 meters the SWR to far
off to be of use!

I bought an Elecraft DL1 because it uses one of the correct types of
resistors for this application and it has a built in VDC test point!

The only thing to be wary of is that the DL1 test point is at the 25 ohm
point in the series of resistors!!!

Don's formula (and mine) differs from that found in the DL1 manual.

Elecraft:

P(in watts) = ((Volts * 1.414) +(.15))^2 / 50

Don:

P(in watts) = ((Volts) + (diode drop))^2 / 25

Jack:

P(in watts) = (((Volts) + (diode drop) * 2)) / (square root of 2)))^2 / 50

There are 3 points to be aware of.
1. Use the square root of 2 and not the 1.414 figure.
2. Measure your diode drop.
3. Combine the drop with the VDC before doing any other calculations.
4. If you do not use Don's formula, DIVIDE by the square root of 2.

The Schottky diode in my DL1 measures .330 mv.

A 1N34a (germanium diode) has a drop of about .3 volts. A Schottky's drop
is a little more but offers more linearity!

In any case, the VDC voltage must be divided by the square root of 2 to get
the RMS. In other words the VDC(0-Peak) is 1.414(approximately) times the
RMS voltage

If you measure the VDC at the 50 ohm point then my formula becomes:

P(in watts) = ((VDC + (diode drop)) / (square root of 2))^2 /50

After getting an accurate measurement of my KX1's power I found that the KX1
ATU was showing values that were too high. By adjusting C9 and R1 I was able
to get the KX1 and the DL1 to agree!

BTW, I also used the DL1 to calibrate my K3 at low power. Now the K3, my DL1
and my Versa Tuner V all agree!

-----Original Message-----
.....snip.....
I've now completed the construction of the basic KX1 and would like to have
a fairly reasonable measurement of the output power on 20M and 40M

I have a very decent dummy load but do not have a very accurate RF Power
meter. I do have a decent DMM and could measure the voltage drop across the
50 ohm dummy load - I'm not sure how accurate the DMM would be at 7 or 14
MHZ.

Is there a recommended method to measure the power output without an RF
power meter ?

Thanks in advance,

Ron, AC2C
.....snip....

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

Ron Bower

Re: Output Power Measurement

In reply to this post by Ron Bower