I have one of the old-style dummy loads that's essentially a resistor in a can of oil. Now that I'm focusing on QRP, and never using more than 100 watts, its 1500 watt capability is overkill. I may have to consider this smaller dummy load.
It is shocking how many people don't have them, thousands tied up in radios but no dummy load. As someone previously mentioned in the comments a high power attenuator is more useful if you have the choice and very useful when taking measurements. There are a lot of used high power attenuators and loads on the market, personally that would be my choice, not a cheap "HF only" load. Buy with care though, some will have been damaged by impact or overload. For those that are tempted with high power attenuators, beware not to connect them "backwards" the low power port is easy to damage. I have a lot of smaller loads, but the 2kW Bird attenuator sits under the bench and does not get in the way.. 73
Thanks for your info. Yes, I am tempted with high power attenuator and would probably buy one at a good price/quality :). On the other hand , dealing with 100W and more, I'm a bit cautios about the galvanic connection between the transmitter, attenuator and the measuring device. If something goes wrong, the full 100W ( or 1000W ) may end up on my oscilloscope or a spectrum analyser :) So I think, the inductive RF-tap with no galvanic contact with the transmitter output might be a safer way to go. But it's OK , IMO, on the lower levels of RF . I use attenuators a lot with my 5-10W transceivers with no issues, so far. 73! Linas LY2H
I found out the hard way how fragile that resistor contact is :( Anyways dummy load is a very useful tool, especially a bit smarter one (with a diode and capacitor) that can be used as a crude power measure with a voltage meter. Very interesting to see what was inside that Chinese dummy load, wasn't expecting that kind of construction.
48 Ohms (at DC) looks bad, but if you do the math (or measure it, as you did) it's not bad at all. The SWR is equal to the system impedance divided by the actual impedance (where the larger term is the numerator to get a number greater than 1). For 48 Ohms that's an SWR of 1.04 like you measured. Now if you consider the reflected power, which is calculated by multiplying the forward power by (SWR - 1)^2 divided by (SWR + 1)^2, you get a very small number. For 100W forward power, the reflected power is just 3.8 mW (that's 3.8 milliwatts!) or -34dB (not accounting for any line losses of course). For me the bigger concern with dummy loads is the frequency stability when moving across bands. Ideally, the resistor would be purely resistive from DC to daylight, however in reality this is never the case. Also keep in mind that the resistor you use to calibrate the VNA determines the accuracy of your measurements. The NanoVNA is a fine device, but I wouldn't trust the provided calibration load any further than the lower end of the UHF band. When you're approaching giga-hertz frequencies, even the connectors start to gain some complex impedance.
@@Linas_LY2H Thanks! I'm happy that you made a video about a seemingly boring device. I actually bought my first 100W dummy load recently, albeit a different manufacturer. The one I got even has a -40dB output, which I can leave unterminated when using it as a dummy load. I think most people buy a dummy load without thinking about the things you mentioned in the video, so I appreciate the effort! 👍
Mechanically that's a nice design. But RF wise it will radiate all over the local area if you put 100 Watts into it. A dummy load must be entirely RF shielded in a Faraday cage, i.e. a metal box.
I have some 15watt cylindrical dummy loads bought from Maplin. When testing my ft817 at 5 Watts, it slowly gave a swr warning. Could these loads be warming up and changing resistance? I cannot trust these loads. G4BTI.
Thanks for sharing your experience. It doesn't sound for me like normal behaviour of a 15W dummy at just 5W load. If this dummy load is comprised of many smaller resistors, I would check the soldering quality and for defective resistors . If it's made of a single resistor, then it would take to replace this resistor, if possible. Good luck! 73! Linas LY2H
That Zetagi dummy load takes me back to my very early days of CB before I got my licence. 73
I have one of the old-style dummy loads that's essentially a resistor in a can of oil. Now that I'm focusing on QRP, and never using more than 100 watts, its 1500 watt capability is overkill. I may have to consider this smaller dummy load.
I have two… and used them once to calibrate my KX2. But I do see the need for it, as much as any piece of gear in the shack. Thanks Linas.
👍Thanks for video. I think checking a dummy load with a NanoVNA is a good idea.
It is shocking how many people don't have them, thousands tied up in radios but no dummy load. As someone previously mentioned in the comments a high power attenuator is more useful if you have the choice and very useful when taking measurements. There are a lot of used high power attenuators and loads on the market, personally that would be my choice, not a cheap "HF only" load. Buy with care though, some will have been damaged by impact or overload. For those that are tempted with high power attenuators, beware not to connect them "backwards" the low power port is easy to damage. I have a lot of smaller loads, but the 2kW Bird attenuator sits under the bench and does not get in the way.. 73
Thanks for your info. Yes, I am tempted with high power attenuator and would probably buy one at a good price/quality :). On the other hand , dealing with 100W and more, I'm a bit cautios about the galvanic connection between the transmitter, attenuator and the measuring device. If something goes wrong, the full 100W ( or 1000W ) may end up on my oscilloscope or a spectrum analyser :) So I think, the inductive RF-tap with no galvanic contact with the transmitter output might be a safer way to go. But it's OK , IMO, on the lower levels of RF . I use attenuators a lot with my 5-10W transceivers with no issues, so far. 73! Linas LY2H
I found out the hard way how fragile that resistor contact is :( Anyways dummy load is a very useful tool, especially a bit smarter one (with a diode and capacitor) that can be used as a crude power measure with a voltage meter.
Very interesting to see what was inside that Chinese dummy load, wasn't expecting that kind of construction.
Maybe you should try to test with another meter or two.
I have a few different ones I've collected over the years.
Great Job Linas. Happy Weekend &/73 de Your Friend Günter from Germany 💯👍🙋♂
A common mode choke
48 Ohms (at DC) looks bad, but if you do the math (or measure it, as you did) it's not bad at all. The SWR is equal to the system impedance divided by the actual impedance (where the larger term is the numerator to get a number greater than 1). For 48 Ohms that's an SWR of 1.04 like you measured. Now if you consider the reflected power, which is calculated by multiplying the forward power by (SWR - 1)^2 divided by (SWR + 1)^2, you get a very small number. For 100W forward power, the reflected power is just 3.8 mW (that's 3.8 milliwatts!) or -34dB (not accounting for any line losses of course).
For me the bigger concern with dummy loads is the frequency stability when moving across bands. Ideally, the resistor would be purely resistive from DC to daylight, however in reality this is never the case. Also keep in mind that the resistor you use to calibrate the VNA determines the accuracy of your measurements. The NanoVNA is a fine device, but I wouldn't trust the provided calibration load any further than the lower end of the UHF band. When you're approaching giga-hertz frequencies, even the connectors start to gain some complex impedance.
Thanks for your contribution! With your explanation, this "boring" device is becoming a higly educational and interesting topic! 73! Linas LY2H
@@Linas_LY2H Thanks! I'm happy that you made a video about a seemingly boring device. I actually bought my first 100W dummy load recently, albeit a different manufacturer. The one I got even has a -40dB output, which I can leave unterminated when using it as a dummy load.
I think most people buy a dummy load without thinking about the things you mentioned in the video, so I appreciate the effort! 👍
Looks a great kit. Do you know if this could be sent to the uk? M0TTL...
I think, they should be shipping outside EU too, but please check the HFKits website. 73! Linas LY2H
Mechanically that's a nice design. But RF wise it will radiate all over the local area if you put 100 Watts into it. A dummy load must be entirely RF shielded in a Faraday cage, i.e. a metal box.
I have some 15watt cylindrical dummy loads bought from Maplin. When testing my ft817 at 5 Watts, it slowly gave a swr warning. Could these loads be warming up and changing resistance? I cannot trust these loads.
G4BTI.
Thanks for sharing your experience. It doesn't sound for me like normal behaviour of a 15W dummy at just 5W load. If this dummy load is comprised of many smaller resistors, I would check the soldering quality and for defective resistors . If it's made of a single resistor, then it would take to replace this resistor, if possible. Good luck! 73! Linas LY2H
I still have very old resistors in tin of oil hi. 73 my friend. Zl3xdj.