Dummy load question

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MiXiN
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Dummy load question

Post by MiXiN » Mon Jul 12, 2021 4:07 am

I'm into CB'ing and have just myself a 1KW Zetagi DL61 dummy load for testing my amplifiers.

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 ohm.

Is this normal?

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

Here is what's inside my dummy load.
dummy_load_dl61_2220222.jpg
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sinus trouble
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Re: Dummy load question

Post by sinus trouble » Mon Jul 12, 2021 11:47 pm

It seems strange that the load would drop to such a low value? How hot was it getting?

It could be that your SWR meter is reacting to the change in load? As load impedance drops, Current and temperature rises in turn levelling off readings?

Most SWR meters are not too accurate, Especially on a high power reading setting.

To be brutally blunt, Zetagi are not renowned for high quality! I had a similar product by MFJ which failed!

I guess as a CBer? Keying up occasionally may be the limits of that dummy load.
I am as stupid as I look! :|

MiXiN
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Re: Dummy load question

Post by MiXiN » Tue Jul 13, 2021 3:34 am

sinus trouble wrote: Mon Jul 12, 2021 11:47 pm It seems strange that the load would drop to such a low value? How hot was it getting?

It could be that your SWR meter is reacting to the change in load? As load impedance drops, Current and temperature rises in turn levelling off readings?

Most SWR meters are not too accurate, Especially on a high power reading setting.

To be brutally blunt, Zetagi are not renowned for high quality! I had a similar product by MFJ which failed!

I guess as a CBer? Keying up occasionally may be the limits of that dummy load.
It was getting pretty hot, as I was feeding it with just over 200W for a few minutes.

While checking SWR, I was also checking the power meter & there was no reduction in power; both SWR & power remained constant over the duration despite what I mention about the dummy load resistance dropping.

I was under the impression that resistors increase their resistance when heated up, not lower resistance - but the main thing I can't get my head around is the pretty much perfect SWR & constant power despite a seemingly bad mismatch.

By the way, I checked power & SWR on a few SWR/Power meters & they all read the same +/- 5%. One of the meters was a Daiwa CN-901HP which is a reasonable quality hobby grade meter.

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sinus trouble
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Re: Dummy load question

Post by sinus trouble » Tue Jul 13, 2021 11:07 pm

MiXiN wrote: Tue Jul 13, 2021 3:34 am I was under the impression that resistors increase their resistance when heated up, not lower resistance
Thats what would be expected? Your dummy is going the wrong way? :lol:

The MFJ version i had suddenly jumped up to around 150 ohm which sent the SWR haywire! :lol:

It seems a mystery what is going on? What is that thing infront of the dummy resistor?
I am as stupid as I look! :|

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Re: Dummy load question

Post by MiXiN » Wed Jul 14, 2021 1:45 am

Those in front of the dummy load appear to be 7 x metal washers spaced equally apart & they're connected to the centre of the SO239 socket, one side of the Resistor.

On either side of the washers are copper strips connected to the outer (ground) of the SO239 socket, and they both connect to the other side of the Resistor & metal case.

I've found that bending the copper strips away from the resistor decreases SWR at VHF, so I'm wondering if this is some type of crude matching setup or transmission line?

Albert H, Analyser, RF-Head, Yellowbeard, etc' - can you tell me what's going on here?

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