50 Ohm Dummy Load

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Jim Martin:
Please explain for
the benefit of those
who don't know why a
50 ohm VHF dummy
load is not
necessarily 50 ohms
at HF.  Especially
for the benefit of
someone who believes
a 50 ohm resistor is
a 50 ohm resistor
regardless of fo.

Steve Jackson:
Actually, I rather imagine a 50 ohm nonreactive load at VHF is probably also nonreactive at HF.  It's the other way around that isn't often so.

Steve Katz:
I agree with Steve KZ1X.

A good 50 Ohm resistor is 50 Ohms from  DC to whatever its upper frequency limit is, determined by reactive components that start to impact its impedance.

I've never seen a "VHF" dummy load (that actually worked at VHF) that wasn't an even better dummy load at HF.


Bob Lewis:
A good 50 ohm VHF dummy load will be 50 ohms all the way down to DC so it will work fine at HF.

Now the resistors used in dummy loads can have some inductance in addition to the resistance and that can cause the impedance to be something other than the required 50 ohms, no reactance (50-j0). Special non-inductive resistors are normally used for the larger loads but they are not perfect. Usually, however, the inductance causes more problem at VHF than at HF so you often find loads that work fine at HF but don't work well at VHF and above.

Jim Martin:
C'mon guys, I'm kind of looking for some support here.  Had someone who couldn't get their rig's auto-tuner to load into a Radio Shack dummy load (supposedly for VHF-per this individual) but my dummy load worked perfectly with the same radio.  I suspect their 50 ohm load wasn't non-reactive.  May I say key word: Radio Shack?


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