Dummy Load Query

[Dereka5]

I've recently got myself a 1KW Zetagi DL61 dummy load for testing various amplifiers, etc'.

My Zetagi DL61 dummy load shows 48 ohms when checked cold/unused with an ohmeter, but when it heats up after use, checking it again with an ohmeter, the resistance can go as low as ~35 ohms.

Is this normal?

Even though the resistance/impedance of the dummy load varies so much, the SWR stays under 1.1:1 throughout HF, and under 1.3:1 at VHF (2M band). SWR stays steady at these levels despite such aforementioned drastic changes of resistance/impedance over the duration.

Surely an impedance/resistance of ~35 ohms would exhibit a SWR of around 1.6:1 or thereabouts, so am I missing something?

I say this because a 100 ohm or 25 ohm impedance would be a 2.0:1 mismatch.

Can anybody shed some light on why I'm seeing no mismatch at 35 ohms when heated, and also, is it normal for this "big resistor" or similar to drop in value when heated up?

Here is what's inside my dummy load.

 

[sp5it]

It's a crap. Just like that.
Get proper dummy load.
That's how mine 1kW dummy load looks like.

Mike

 

[groundwire]

Cheap poor quality resistor.

 

[Dereka5]

Thanks for the replies.

I know Zetagi is poor, but I got this very cheaply at £20 (circa $30).

I don't intend testing it with 1KW, as my 220W amplifier (Zetagi B550p) is more than ample for my needs. Besides, my antenna will only take 250W before it's toast.

I've tested this Zetagi dummy load at 220W for 3 minutes, and the SWR remained at 1.1:1 - but I'm trying to establish just why the SWR stayed good even though the dummy load resistance dropped to around 35 ohms after heating up.

Should a resistor of the type in my dummy load drop in resistance like this, and why is the SWR still showing as good (1.1:1) when it does?

I'm looking forward to somebody with some technical "knowhow" to please give an explanation as to how this is happening.

 

[Crawdad]

Am nothing close to an expert, but.........
RF (AC) impedance and DC resistance are unequal animals. RF impedance cannot be measured with a DC resistance measuring VOM. Despite the heating, your dummy load's impedance may not be changing enough to alter the VSWR greatly.

Hopefully, the experts will be along to correct me and we'll both learn something.

73