HOMEMADE RF DUMMY LOAD FOR HF ONLY

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Hi All..! Thanks for all the comments, appreciate it, and yep I agree I must of had a brain fart going on because this would of been fine for HF, but for 2.4Ghz which was my aim, it fails miserably. Therefore I will change the title of the video. On a side note I have measured the resistor directly to a VNA, please check my post on my community page or my feed. You can see that it does not plot as expected. In my mind these do not cover from DC to 6 Ghz, regardless of my mistake with the connection between the N-Type and the resistor.
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Опубликовано:

3 апр 2023

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Комментарии : 50

@TechMindsOfficial Год назад

@germanjohn5626 Год назад

with careful layout and design this style of resistor will work all the way up to 3+Ghz. But it is not easy to do and involves compensating and matching. Some of them have a maximum frequency of 500Mhz. I can send you a care package to experiment with name brand high quality parts should you be inclined to try again.

@christopherleubner6633 Год назад

The best way to make this would be to make a strip line on copper clad board to feed the resistor. Attach a short length of microwave coax to feed it and use double sided copper clad board.

@MrApolloTom Год назад

Sorry a couple of people felt they had to say it so harshly. We're all learning as we go, and this is what helps us all learn. "An ignorant person is one who doesn't know what you just found out" as Will Rogers said.

@chrisharper2658 Год назад

As others pointed out, you can't use a piece of wire from the N connector to the resistor and expect it to work much beyond the HF band. Use a good quality coax right up the resistor connection including the ground connection. Obviously the coax needs to be rated for the given frequency but at least for a dummy load losses aren't important.

@matsbengtsson3756 Год назад

You need a 50 ohm transmission line between connector and resistor. That wire will cause a severe missmatch on any frequencies above HF as it is a serial inductance. The resistor is not the problem.

@cheesedoodlefeeder Год назад

Well the thick wire is an inductor at the higher freq. Maybe use coax to keep it a transmission line .. I built a dummy load for hf using a series and parallel combo of similar resistors. I used copper sheet and then put the assembly into a mineral oil filled paint can. Even with copper the resistors got so hot that the solder melted connections on a few of them. I didn't realize how bad it was until the finals in the amp I was testing zorched out! So I don't trust those resistors and would only use them in the future at maybe a fraction of their specified power ratings!

@galileo_rs Год назад

There is nothing wrong with the resistor, you have placed a transmission line of probably 150Ω in series with it.

@juergenschimmer960 Год назад

You might also be able to calculate the length and impedance of the connecting wire based on the readings of the VNA

@TechMindsOfficial Год назад

@marsgal42 Год назад

I'd mount the resistor like a high-power RF transistor with a 50 ohm microstripline between the coax connector.

@2EOGIY Год назад

This build needed to add a very short, constant distance between the signal and the ground. For much better results, connect the resistor directly to the socket. Make sure that the soldering tab is parallel to the project box surface. If possible, use connector bolts to mount a resistor.

@101blog Год назад

Fun little video ...a shorter wire (Preferably Coax) would be useful as this is an antenna at higher frequencies. Also if you ramp up the power you 'll probably find that Heatsink not as effective as it could you'll need a metal to metal contact to the heatsink and resistor or atleast scrub the metal or have the bolts go all the way through the heatsink.. Anyway enjoy the series keep it up

@Peter-nf3wp Год назад

On higher frequencys say above 1GHz the distance to resistor from N connector is crusial , keeep it as short you can , for even higer frequncies try to solder the resitstor tap directly to the N connector tip.

@njjensentube Год назад

Hi ... the wire is your problem ... for a start I would mount an SMA connector directly on the resistor ,and then measure it with your VNA. This is to understand the performance of the resistor by it self. If the resistor is ok then as others have suggested use a transmission line on a pcb to connect between n-connector and the resistor ... thanks Niels

@TechMindsOfficial Год назад

@GateKommand Год назад

Thanks for another informative and interesting vid my friend.

@MakeDoAndMend1 Год назад

Connector should be direct to resistor. But OK for hf if using a wire. No good for vhf. You must have watched my video. Cheers from old George

@kevinkc3onohelijeepworld953 Год назад

When my buddy did his dummy load using strip line resistors he used 100ohm in parallel and a tiny bit of capacitance to balance the circuit. I’m good on qrz if you want to shoot me a email I can forward you his design and exact details 😉

@lusatianus1199 Год назад

Building a dummy load for HF can be done much easier. Just put an connector at the lid of a jar and solder some copper wire to it. Fill the jar with water, add salt step by step - check the resistance using the NanoVNA. 3/4 teaspoon of salt to 400ml water is the rule of thumb. I built one of these, with 400ml it can handle 300W easily and ist usable up to 30MHz.

@handyandy4x4 Год назад

Try the resistor closer to the socket , shorter wire to resonate . I build a paint can dummy load and had similar effect so shortent the tails did improve it ..

@mikewillis1592 Год назад

Hi Matt, Time for a tech in on RF and transmission lines. I can explain what's wrong, but others will below. The resistor will be fine

@TechMindsOfficial Год назад

@mikewillis1592 Год назад

@@TechMindsOfficial I very much doubt they look like a resistor at 6 GHz. I would be surprised if they were much good above 1 GHz but they will be usable to some extent. I will have a look at your pages. Incidentally, it is easy to make what you want with that and some suitable coax, but you must not use the dicast box as the heatsink as it has poor thermal conductivity. What I would do is cut a hole in it and put a bit of copper plate and then the heatsink. The copper plate will spread the heat but it will also allow you to solder the coax to it. It doesn't need to be very thick for this but it will improve the grounding. If you look at my posting on the dual pallet 70cm PA you can see something similar for the dump resistor on the power combiner.

@lmamakos 7 месяцев назад

You might also consider polishing the surface below the resistor smooth to increase the thermal conductivity between the resistor body plate and the mass of metal. The metal-to-metal contact will have much better thermal conductivity than relying on the heat sink spooge to fill in the tiny gaps between the two hunks of metal.

@Trent28888 Год назад

The bewt way is put a socket on a 90 degree bracket and mount it as close as possible,the resistor is fine but its also actin like a stub on 70cm

@hamscanner Год назад

Be interesting to see the inside of the dummy load that you fried

@germanjohn5626 Год назад

I have used plenty of those resistors, some become capacitive in the UHF plus region and commercial outfits compensate / match them for the frequency range in use. They need to be connected either with semi rigid cable or mounted directly at the connector preferable on a copper heat spreader. RF is not DC so please don't use long wires. You have a huge mismatch there at higher frequencies. One way to deal with it is to move the resistor directly to the connector on a piece of copper stock with the center of the connector shortened and connected directly to the resistor. The other is to make a board with a 50 ohm trace (not easy to do). Also please remember a die cast box is great for shielding but a lousy medium for high RF current.

@TechMindsOfficial Год назад

Thanks John! I did try connecting the resistor to the VNA directly and I posted the result, seems that these resistors don’t fully support its specs. Thanks for your comment though, appreciated.

@hoggif Год назад

Very long leads make a huge loop. Just think of it all as a one loop inductor made of long wire and back to connector through casing. I'd either use a direct coax from connector to resistor (both power and ground) or create a some kind of 50 ohm strip line if it suited better. (to be more exact long wire also has quite some capasitance so it is more or less LC circuit you did build but I'd assume inductance being the larger problem) You get away with long leads on HF but at UHF or especially microwaves, they are just no good. One possibility could be mounting the resistor near the socket and having a short direct connection from socket to resistor. This would assume the enclosure conducts enough heat. BTW, I disagree with the wisdom UHF connectors are no good for 2m or 70cm. I have measured some adapters and leads I have with my 1.5GHz spectrum analyzer and the ones I measured were just as good as say N-to-sma adapted similar lines with only minor difference.

@christianholmstedt8770 Год назад

Um, you need to actually design a 50ohm impedance circuit to connect the resistor to. The resistor is fine. The circuit design is not at all suitable for RF.

@danwalkley304 Год назад

I was jumping up and down also, that wire from socket to resistor was a hf antenna, lol

@ptarsob Год назад

In fact you are using an inductior, your long wire, to connect the resistor to it. That is a big mistake. You should have used a piece o coaxial cable, directly connected to both ends of both, the load and your female connector. 73 from PY1JDX, Paulo

@zapallalla Год назад

I really like your content but I would consider your audio. It seems to be overcompressed (from the intro music to to your voice). You don't need to do that. It gives a bit cheap vibe. Just have your voice as normal level is enough. I guess the intro music is a bit over used stock track? You have such good and deep content so I just wish best for your channel!

@MrApolloTom Год назад

Yes I agree

@humaxf1 Год назад

Great content as usual... but I also have to agree that the audio sounds over processed.

@DannyBokma Год назад

I hope you were recording this on the first of april and were aware that you were fooling yourself 😂.

@TechMindsOfficial Год назад

@JimAndyAllyn Год назад

Wow. Just wow. I've watched enough of your videos that I thought you knew better than that. You can't connect the resistor to the N connector to the resistor with that INDUCTOR that you called a "wire." If there is anything between the connector and the resistor, it should be a microstrip transmission line on a printed circuit board. Better yet, mount the connector to the edge of the heatsink with the pin just above the flat surface, mount the resistor to the flat surface of the heatsink, and solder the tab of the resistor directly to the pin of the connector.

@TechMindsOfficial Год назад

@PY1SAN Год назад

Long wire.

@jensgoerke3819 Год назад

If I was to build a small dummy load I'd use 20 1k carbon resistors in parallel and solder them directly to the connector - a bit fiddly, but it would keep the cabling really short. Oil for cooling would circulate through the box, so heatsinks on all the other sides would be ideal.

@xszl Год назад

Use wired resistors for that ;-)

@jensgoerke3819 Год назад

@@xszl I know that's old school, but so am I 😉 2 to 5 layers of resistors around the center contact of the connector, their other leads forming a cage, grounded on solder tabs on the connector mounting screws, leaving enough room for the oil to circulate. Think of a sea urchin with the spines the ground wires of the resistors.

@xszl Год назад

@@jensgoerke3819 Just looked it up, you were right, you need carbon for that. I should have known better, I made them for CB a long time ago. Geezz...thats what 40 years does to your brain I guess...