Switchable Pi Attenuator with Dummy Antenna, using PCBWay PCB's
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- Опубликовано: 8 сен 2024
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GERBER FILES.
I have added a shared link. This one has the two slight changes made to it, to allow for two paralleling components of the antena parts, as explained in the video.
drive.google.c...
I always use an attenuator when aligning radios, and a switchable one to add to my workbench was always in the plans. I decided to add a dummy antenna to this, and the result is great. Using PCBWay's boards also makes the job look extremely professional, and easy to build.
The sponsorship notwithstanding, the views and opinions that I express in this video are my own. That was the first condition that I set for accepting their offer.
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By the way I'm a big fan of your channel. You inspired me to make a Capacitor tester and a Bulb limiter. I recently updated an old communication receiver from 1947 (Hammarlund HQ 129X). I admire your high output and enjoy it very much.
Glad you’ve got something from the channel. I learnt tons of stuff from others, and enjoy giving something back to the community.
Great idea, I just finished building your Bluetooth module this afternoon from the PCBWay boards I ordered using your drawings, this works extremely well. This Attenuator/ dummy antenna looks like another very useful addition. Thanks very much for sharing this with us.
My pleasure
Please help my circuit diagram audio splitter 2 input left and right out put 8 chennal gives me schematic
It would be interesting to know what the impedance transform characteristics are for the dummy antenna across the bands of interest? Armed with that information you can then terminate the scope with the correct resistance for various input frequencies. Also of course at higher frequencies there will likely be significant ‘bleed through’ when using a large attenuation. Solution is shielding between each attenuator section or build a separate fixed attenuator for large values that can be housed in a separate box. A very interested subject!
My first thoughts were similar to Jwahar below. You appear to have added a tank circuit as the dummy load. This will react differently to various frequencies, which it seems to be doing in your experiment. Keeping the dummy purely resistance with no inductance or capacitance should provide a flat frequency response. I do like the concept and the manufacture looks great.
Makes sense, but an antena does in fact react differently to different frequencies, so we expect that effect anyway. The concept of a dummy antenna was adopted in order to allow for tests under “standard” conditions, to allow for comparisons between different radios. This concept is well described here:
ax84.com/archive/ax84.com/static/rdh4/chapte37.pdf
The inductors will be filtering off the frequency for the dummy antenna.
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Excelente Manuel! La única sugerencia sería armar todo físicamente en serie, en este caso la etapa de antena y la salida a la derecha del atenuador de 24db, de manera de tener la mayor distancia entre entrada y salida de RF para mejorar la aislación entre ambos. Esto es de relevancia en las frecuencias más altas, inclusive se puede colocar una chapita (blindaje) entre el atenuador de 12db y 24db. Aclaro que trabajé muchos años con enlaces de microondas (Thomson-CSF), de allí mi "visión" desde frecuencias muy elevadas :). Felicitaciones por lo minucioso y buena calidad de todos tus videos.Un cordial saludo desde Buenos Aires.
Gracias Carlos.
I agree with your logic, but I wanted to fit it in one of my pre-cut aluminium cases, and did not want to use a 20cm one.
Thanks.
Useful Item..Good Job !
Thanks
At 7:45, that's the famous 400Ω Dummy load.
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about 400 OHM but your scope has still the 50 OHM input impedance so there is a impedance mismatch that causes the jump in voltage
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Best would be to peak the front ends with the intended antenna wire actually connected. Place a short wire in the generator output Bnc across the room.
I do that occasionally. Decided to give this method a try.
Can't you use a resistor and a multi-turn potentiometer instead of the 3 resistors? Also, I don't understand why the need for 2 blocking DC capacitors...
3 resistors cost 3 cents 😊
Strictly speaking, you don’t need two blocking caps. But with the voltages I have flying around some of these radios, I want to be absolutely sure that the D.C. doesn’t get past the socket, regardless of which way around I connect it.
Viva, vou fazer este atenuador. Para além dos ficheiros Gerber, pergunto de tem a lista de componentes ? Obrigado.
Nao tenho. Mas os esquemas no vídeo dão toda a informação.
Sim já consegui tirar os prints screens.
The dummy antenna might look authentic to the radio under test but at 455KHz the 125pF capacitor will have an impedance of 2798 Ohms, so the last section of the Pi attenuator will see around 3K, which is not what it should be i.e. 50R. The approximate output impedance of the dummy antennae will also be around 3K so then feeding a 50R coax cable with a 50R termination will be unmatched and cause a significant reduction in voltage. It might be useful to consider adding an UNUN (unbalanced impedance convertor for RF - like a BALUN); there is an interesting article here www.instructables.com/id/How-to-Make-a-Balun-Or-Unun/ on how to make a 50R to 200R RF transformer which would terminate the Pi attenuator correctly and provide something like a typical radio antennae at the output and maybe then adding the dummy antennae if felt necessary. I could be completely wrong about this as I'm no expert, perhaps a real expert could comment?
Makes sense, but an antena does in fact react differently to different frequencies, so we expect that effect anyway. The concept of a dummy antenna was adopted in order to allow for tests under “standard” conditions, to allow for comparisons between different radios. This concept is well described here:
ax84.com/archive/ax84.com/static/rdh4/chapte37.pdf
@@electronicsoldandnew Yes that's an interesting article on dummy antennas, which as you say will keep the radio happy and enable more accurate setting of the input tuning, but unless the last stage of the pi attenuator is terminated with a 50R load then the decibel attenuation for the last stage will not be as expected and this will affect all the preceding pi-attenuator stages, throwing away all the effort to get the resistor values as close to the calculated values as possible. To keep the attenuator dB correct the last stage needs a 50R resistor switching across its output whenever the dummy antennae is engaged e.g. a 6dB Pi attenuator only has an attenuation of 2dB if it terminated with 3K or the approx input impedance of the dummy antennae. If you add a 50R resistor from pin 1 of SW5 to ground it would come into effect when the dummy antennae is engaged and keep all the attenuator stages correct.
Ah, I see what you mean. Makes sense.
Great video, I’m making one now. Just wondering if you came up with a solution to the dummy antenna issue. I noticed a comment about adding a resistor to ground on switch 5, did you end up doing this?
No. It’s actually correct the way it is.
Your dummy antenna simulates an antenna, tuned for a certain bandwith. No antenna is capable of receiving all signals in equal strength.
Absolutely.
Thanks for sharing. I think your Dummy Antenna problems are due to impedance of the LC components of the dummy antenna which are of course sensitive to frequency of the signal. Most people build purely resistive dummy loads in the Amateur Radio world for this reason, to get a relatively bad banded constant load across the Amateur Band of frequencies. Also please can you share your schematic and/or gerbers. I could not find it in any of the links in your videos description section. cheers,
I have added a shared link. This one has the two slight changes made to it, to allow for two paralleling components of the antena parts, as explained in the video.
I was scratching my head about the load at the end. From what I can see the impedance sequence seem to be 50 ohms - 50 ohms then 50 - 50 and 50 - 50 and again 50 - 50, the it goes straight in to a 3,000 ohm section. Shouldn't there be some form of matching to go from 50 ohms to 3,000 ohms ? Like an audio output transform is there to match a 8 ohm speaker to the high output impedance of an EL84. Oh and the oscilloscope appeared to have a 50 ohm termination too which would bring in another mismatch.
Or ... have missed some thing really big here and look foolish ? :-}
That should be 'have I missed some thing really big ...'
Everything is 50 ohm all the way through until it gets to the injection point on the radio. If the injection point impedance is higher than 50ohms, no problem, because we’re not really transmitting great power levels. If your load is higher, you don’t drag the signal down.
Now bear in mind that we’re not interested in exact signal levels here, only relative, so precise impedance levels at the injection point don’t really interest us.
Think of it as using a signal generator without the attenuators: It is 50ohms output impedance until you connect it anywhere on the radio to do measurements.
@@electronicsoldandnew Fair enough Manuel. Thanks for the quick response and stopping me from scratching the little away hair I have remaining.
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I have an idea for a video. I think you will like it is it ok to contact you ?
Sure, but I only do videos that log my hobby projects.
Hi I note that you described yourself as an [ AMATEUR ] well if so I must be a 747 CAPTAIN and person in the WHITE HOUSE is ?
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There is nothing wrong with your dummy antenna. You make a mistake. The dummy antenna has a output impedance
True. I wasn’t thinking straight when I did the test.