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I'm hoping the smarter folks here can help me create some dummy loads with higher SWR's.
I'd like to do some temperature delta measurements without putting signals on the air since I want to include some high duty cycle scenarios at the typical 4-5 Watts outputs of my QMX's and QDX's.
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I Googled this idea but only got hundreds of hits on the general topic of SWR.
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Perhaps SWR's of just 2 and 3.
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I think I should limit it to those values just because there is plenty of evidence that even short bursts of TX into very high SWRs or open loads is a generally bad idea without super fast protection built into the TX circuit chain.?
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Thanks in advance,
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GREG? KI4NVX
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Hi to get a swr 2, just use a T bnc and put 2 dummy load on it, then they will be in parallel, 25 ohm. The other is to put 2 dummy load in serie, but that is not so easy, to get 100 ohm. 73, oz8pz Stig
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Den 27. feb. 2025 kl. 15.27 skrev Greg McCain via groups.io <gamccain50@...>:
? I'm hoping the smarter folks here can help me create some dummy loads with higher SWR's.
I'd like to do some temperature delta measurements without putting signals on the air since I want to include some high duty cycle scenarios at the typical 4-5 Watts outputs of my QMX's and QDX's.
?
I Googled this idea but only got hundreds of hits on the general topic of SWR.
?
Perhaps SWR's of just 2 and 3.
?
I think I should limit it to those values just because there is plenty of evidence that even short bursts of TX into very high SWRs or open loads is a generally bad idea without super fast protection built into the TX circuit chain.?
?
Thanks in advance,
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GREG? KI4NVX
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This is simple, make several soda
water dummy loads.
Here is Mine.
I made mine 1 to 1 SWR. it is dead flat from 160 thru 2 meters.?
Probably Higher, but don't know because don't have equipment any
higher.
1500 watts+ into it with mo problems also.
Costs Nothing.
Made simple, the two wires are 12 ga copper, about 6 inches long.?
Open at the far end.
Simple center conductor goes to one wire, Braid to the other.
Fill jar with water.
Measure SWR. It WILL have a SWR.? If the SWR is what ya want youre
done.
Naturally the impedance will be higher than 50 ohms.
Add tiny amounts of baking soda to the water, make sure it is
fully mixed, even wait a while to ensure it is completely
dissolved. and measure again. the impedance will have dropped.
Keep adding till you get what ya want. only do TINY amounts. it is
very easy to overshoot what you want.
If you do simplay dump out some solution and add plain water to
dilute it some.
simple...
I haven't tested this idea, of making it variable impedance by
varying the amount of water, in the jar, or varying the amount the
wires are inserted into the solution.? I think it might make it
impedance variable?
Joe WB9SBD
On 2/27/2025 8:26 AM, Greg McCain via
groups.io wrote:
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I'm hoping the smarter folks here can help me create some
dummy loads with higher SWR's.
I'd like to do some temperature delta measurements without
putting signals on the air since I want to include some high
duty cycle scenarios at the typical 4-5 Watts outputs of my
QMX's and QDX's.
?
I Googled this idea but only got hundreds of hits on the
general topic of SWR.
?
Perhaps SWR's of just 2 and 3.
?
I think I should limit it to those values just because there
is plenty of evidence that even short bursts of TX into very
high SWRs or open loads is a generally bad idea without super
fast protection built into the TX circuit chain.?
?
Thanks in advance,
?
GREG? KI4NVX
?
?
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Use 4x3W 100Ohm non-inductive resistors (a series of 2 parallels, or a parallel of 2 series, is the same) soldered on a BNC connector. They can dissipate 12-15W peak, and would not get hot with 4-5W.
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72/73 de Enzo M0KTZ
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On 27/02/2025 14:55, Stig oz8pz-oz8wsp-oz8wsr via groups.io wrote: o get a swr 2, just use a T bnc and put 2 dummy load on it, then they will
be in parallel, 25 ohm. The other is to put 2 dummy load in serie, but that is
not so easy, to get 100 ohm. And note that each of those ways will have a different effect on the PA. You could make different SWRs by adding L and C all giving different loading conditions with a SWR of 2.0 73 Alan G4ZFQ.
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Hi Greg,
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How about usin an antenna matching unit. You should be able to set the SWR where ever you want it.
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Cheers,
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Mel. M0KMD
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I should of added, connect your dummy load to the antenna port of the ATU.
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Mel. M0KMD.
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Many thanks everyone.? Lots to ponder. A couple to try with what I have on hand.
On Thu, Feb 27, 2025 at 11:02 AM, Mehmet Dinch via groups.io <msdinch@...> wrote: I should of added, connect your dummy load to the antenna port of the ATU.
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Mel. M0KMD.
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Soda/salt water dummy load?!? Why didn't anyone tell me it could be this easy and cheap?
A web-page about the idea:
Thanks for the tip, Joe!
72 de Todd W2TEF
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Been using one since my Novice days.
1975.
Was created not knowing any better because the vaporizer, ya know
the old like hankscraft
vaporizer for when you are sick? And if your water was too "Soft"
no minerals,
and it wouldn't boil in tme manual it told ya to add saly or soda
till it would start to boil.
Think about it, many super high voltage high power tubes use
distilled water as the cooling medium. and were like talking 10
thousand volts in contact with the water.
But pure water just H2O is an incredible insulator.
The Mayo jar if it had pure H2o in it would give over 40 meg ohms
resistance!
Being a stupid teenager ham, I thought what's happening and
figured when addind the soda to the water it is lowering the
resistance and ya know for makong power and all that current draws
etc. bla bla bla. so thats how I made my first one in 1975.
Joe WB9SBD
On 2/27/2025 10:54 AM, Todd W2TEF via
groups.io wrote:
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Soda/salt water dummy load?!? Why didn't anyone tell me it could be this easy and cheap?
A web-page about the idea:
Thanks for the tip, Joe!
72 de Todd W2TEF
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6 dB attenuator - open and shorted output.
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... or 3 dB attenuator - open and shorted output.
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I bought a couple of Hans' dummy load kits (because of their great design) and populated them with different values of resistors.
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For a 100 Ohm load: 10ea. 1K 1% 2W resistors.
For a 200 Ohm load: 10ea, 2K 1% 2W resistors.
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On Thu, Feb 27, 2025 at 10:18 AM, Joe WB9SBD wrote:
Made simple, the two wires are 12 ga copper, about 6 inches long.? Open at the far end.
Simple center conductor goes to one wire, Braid to the other.
I was a contractor to Fort Monmouth, home to the Army's E-Labs until the fort closed.??
They had a megawatt-level dummy load for testing power supplies.? It was essentially a swimming pool filled with copper sulfate solution, with a pair of huge graphite electrodes.
There was a crane which moved the electrodes in and out of the water to vary the load resistance.? Just a bigger version of WB9SBD's.
73, Don N2VGU
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Be careful with this, the VSWR you set will likely be wildly different at the harmonics than at the fundamental, and it will cause a different response than a resistive mismatch would, which is the same VSWR at (almost) all frequencies.?
This can be easily checked/verified with a nanoVNA, just set the markers to the fundamental, and to the harmonics which will likely look terrible.? Recall that one of the touted advantages of tuner is harmonic reduction.
The tuner method might for example excite an oscillation which might not occur with a resistive mismatch.
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Another method is to use a short or open after an attenuator, a 3dB pad will yield 3:1VSWR for example.?
You can reverse the phase by swapping open and short, or vary the phase by varying a tank circuit through resonance as a load.? You can get almost 180° phase change in this way.?
We used this at transistor manufacturers for load-pull testing and could get 20:1 VSWR with a low-loss tank at 2 meters.
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I have set up tests at 4:1 using terminations and tees, I could have gone further but I ran out of tees.....
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73, Don N2VGU
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The above was in reply to a suggestion of using a tuner for VSWR testing.
73, Don
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You can read the VNA results at . It looks legit.
This is the finest piece of hillbilly engineering I've seen since the Wouff Hong. :)
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I put a 50 Ohm dummy load on the antenna side of my ZM-2 tuner, and then connect rig to the transceiver side of the ZM-2.? I can then dial in a range of complex impedances by changing the capacitors and achieve a particular SWR.? Same can be done for a different kind of manual tuner.?
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If you want a resistive-only load to achieve a particular SWR, you can use a resistor across the BNC port.? For example, ~100 Ohms will give an SWR of 2:1, as will ~25 Ohms.? You would likely need to put some resistors in parallel to achieve your desired power rating.? Then you would need to calculate the individual resistor value so that R/n gives you the desired resistance.??
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73
Mike N9OHW
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Sounds to me like the problem is that a 2:1 VSWR could be several possible combinations of capacitive, inductive, and resistive impedances and those have different effects...
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So maybe define the problem a little more clearly?? Do you want a purely resistive 2:1 VSWR of 100 ohms?? Or 25 ohms?? Or some kind of combination of resistive and reactive impedances?
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The most thorough test would probably include some testing with each condition but I don't know how much of a difference it would make.
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A tuner would let you modify inductive and capacitive reactance to get the VSWR you want
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Different value resistors would give you different resistive impedances.
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Not sure what the salt water dummy load would be but my hunch is that it would be resistive impedance.
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Does this sound right?
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Todd W2TEF
Donald S Brant Jr
Chuck AI7SY