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Tapping Into The Load

Created by Scott Reaser, K6TAR on 2019-10-28
Tapping Into The Load
Scott Reaser, K6TAR

You can make a minor add-on to the MFJ 260C 300 Dry Dummy Load that extends the utility of this box. A 60 db attenuation pad output is added. This tap is great for a scope connection. Nice.

Figure 1 shows how I have added a circular mount BNC receptacle to the box. I show in an internal close up, Figure 2, how I have added a 1:1000 divider from the usual SO-239 output to the BNC. I used 56 and 56K ohm parts. (Because I had them).

Figure 3 shows the modified box in action looking at a SSB signal.

Figure 1, BNC Tap Added to Dummy Load

Figure 2, Internal View of Divider Resistors

Figure 3, Modified Dry Load In Action, SSB Signal
K9MHZ 2019-11-06
Re: Tapping Into The Load
Good for using carbon resistors. Any idea if modern metal film resistors would become inductive(?)
W9WQA 2019-11-03
Re: Tapping Into The Load
great,simple idea. keep it simple,no need to do any different. not making lab tests for the space shuttle!!
K9MHZ 2019-11-02
Re: Tapping Into The Load
Hey Gary, sorry for the dumb question. By “flat frequency response”, are you saying that it’s a flat attenuation as well as no distortion of the sample across the HF freqs?

Thanks, good tip!

Brad, K9MHZ
NA6O 2019-10-31
Tapping Into The Load
Good job, Scott... a very useful addition to any dummy load!

But you should add frequency compensation. Stray capacitance in parallel with the 56k will reduce the attenuation as freq increases. In fact, it may only be around 40 dB at 30 MHz. What you want is R1C1 = R2C2 where R1 is 56k, R2 is 56, and C1 and C2 are their respective parallel capacitance. As a starting guess, we can estimate that there might be C1 = 1 pF. Then what you need is C2 = 1000 pF in parallel with the 56 ohm resistor. Freq response will then be roughly flat thru the HF bands. This is exactly how compensation works on your 10X 'scope probe.

You can measure this. If you have only a 'scope for measurements, run your xmtr at constant power on 80m then on 10m. See if the output is roughly the same in both cases. If not, adjust C2. If you happen to own a VNA, or a spectrum analyzer with a tracking generator, then you can directly measure the overall freq response and optimize it very accurately. Then you will also know the exact attenuation factor, too.

Gary NA6O
VE3KKQ 2019-10-31
Tapping Into The Load
Great writing and interesting "How to" article, well done, Scott.

Seems I'm too early to read all the "expert" opinions on what is wrong with this article.
KB5ZSM 2019-10-31
Tapping Into The Load
Like it and now I know where my scope
Went... LOL
KC7MF 2019-10-30
Re: Tapping Into The Load
Great idea
K9MHZ 2019-10-28
Tapping Into The Load
Like it!