Understanding using an inductor to pass ac and block dc, at and above resononan - Page 1
Forum section: electronics / rf-microwave

author: Qmavam
author_rank: Regular Contributor
timestamp: August 28, 2019, 05:33:16 pm
content: I'm looking for an inductor that has a high impedance at 500kHz to 4MHz.It needs to pass at least 1 amp without saturating.I seem to find many that have SRF lower than 4 MHz.When an inductor goes capacitive, does it still block ac and is a low Q betterfor blocking especially at resonance?Do I lose effective blocking around that resonance point?I don't really now the reactance I need to block effectively so I'm going with 2000 or more.Not a 50 ohm system.Thanks, Mikek

author: bob91343
author_rank: Super Contributor
timestamp: August 28, 2019, 05:57:04 pm
content: You need to bone up on the fundamentals. Any component at a given frequency will have an impedance which can allow calculation of circuit conditions.In the case of an inductor, at self resonant frequency (SRF) its impedance is resistive, and a value that corresponds to the losses. Above that frequency it's capacitive. However, in the real world, there are many parasitic capacitances and inductances and resistances that complicate the situation.Slightly above SRF the capacitance is very minimal and thus the signal transferred is still small. As the frequency climbs, the capacitance, while not changing much, will pass more signal. The inductance of the leads begins to matter and limits how much signal will transfer, as it adds reactance in series.Designing a circuit that relies on SRF isn't wise, as that parameter is difficult to repeat. As for how much current, that depends on wire size. If it's air core, it won't saturate. A ferromagnetic core will have a maximum flux density before reaching saturation; that can be computed knowing the core characteristics and inductor configuration.Characterizing all of this is not a trivial exercise. The acid test is to build it and observe its performance.You say high impedance but don't specify how high. If you know the impedance you can compute the required inductance. If your circuit is at all critical, you probably need to build your own inductor. The Q varies with frequency and depends on losses versus impedance, as evinced by the formula.Study the literature on inductors and high frequencies. There's a lot to learn, and nobody knows it all.

author: Qmavam
author_rank: Regular Contributor
timestamp: August 28, 2019, 06:14:11 pm
content: I want to do this,but, I'm not using a pin diode, I hope to have a variable inductor where the pin diode is, and an antenna wire off each end of that inductor.Thanks, Mikek

author: radiolistener
author_rank: Super Contributor
timestamp: August 28, 2019, 07:07:41 pm
content: If you want to control directivity of the radiation, you can use phased antenna array:Beverage antenna (which you're trying to modify with inductors) has low efficiency. So, you can replace it with array of small resonant antennas and feed them with phase shifted signals to get desired directivity. If you want to change directivity, just change phase shifts

author: Qmavam
author_rank: Regular Contributor
timestamp: August 28, 2019, 07:35:21 pm
content: Yep, hundreds of other antennas, I have an area 10 ft wide by 260ft long. It worked great for my first BOG which got destroyed.I'm building a second one, and as long as I'm building it, I want to try to improve it.Here are two videos comparing my BOG to a long wire and a Mini-whip antenna.I think it is worth having.

author: fourfathom
author_rank: Super Contributor
timestamp: August 28, 2019, 07:46:37 pm
content: Since you only want to operate 4MHz and below, you might be able to build an active current source (two complementary sources) with transistors. This will provide high-impedance current source for your series element (diode or inductor). As the frequency increases the parasitic capacitance will become a problem, but below 10 MHz it seems pretty do-able with typical parts.Although in practice, using this current source on an antenna that is subject to high voltages (interference or transmit) might not be such a good idea.

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2019, 04:24:26 am
content: Using an inductor with two windings will isolate the RF path from the saturating current path. The turns ratio can also be changed to control the current or impedance required.Lookup how tank coils are constructed with multiple sections to maximize impedance and self resonate frequency. You might get away with pairs of inductors connected in series with the high frequency inductor closest to the saturable reactor.

author: fourfathom
author_rank: Super Contributor
timestamp: August 29, 2019, 05:10:58 am
content: Quote from: David Hess on August 29, 2019, 04:24:26 amAre you talking about a common-mode choke configuration? I almost suggested that, but quickly realized that when the common-mode choke is feeding the PIN diode or current-controlled inductor the mutual inductance of the two windings will couple any signal, effectively bypassing the PIN / etc. Since the far end of the choke windings are connected to ground at AC (or connected together at AC), the choke acts as a 1:1 transformer. Yes, the DC current will cancel which solves the core saturation problem, but the coupling will defeat the purpose of the series element.This is what I'm talking about:If this is something other than the common-mode choke, please tell me more. It sounds interesting!

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2019, 01:06:48 am
content: Quote from: fourfathom on August 29, 2019, 05:10:58 amNo, I mean the RF path through the inductor can be separate from the DC control current path if two separate windings are made on the same core.

author: Qmavam
author_rank: Regular Contributor
timestamp: August 30, 2019, 03:27:48 am
content: Still searching for a choke, Digikey shows a high of 4Mhz SRF for a choke of the right value. Yes, I will halve the value and see if I can find a higher SRF in a lower value choke and use two.Here's a drawing showing two ways to implement the saturating portion, with a bypass cap across the saturating coil.Feedback appreciated.Mikek

author: fourfathom
author_rank: Super Contributor
timestamp: August 30, 2019, 05:15:18 am
content: Quote from: Qmavam on August 30, 2019, 03:27:48 amThat bypass cap across the saturating toroid coil is going to make the signal coil look like a dead short at RF, killing any inductance you were trying to control.

author: Qmavam
author_rank: Regular Contributor
timestamp: August 30, 2019, 05:29:42 am
content: Quote from: fourfathom on August 30, 2019, 05:15:18 amYep! I keep seeing two cores used and I just read why! I came here to correct the idea. You beat me two it. So the electromagnet will work OK. Need to rethink the toroidimplementation.I read this,"Two separate, identical cores are used, each having a d.c. and an a.c. winding. Both a.c. windings are identical, so are both d.c. windings. However, the d.c. windings differ in physical characteristics from the a.c. windings. Since each d.c. coil is on a core with an a.c. winding there will be an a.c. voltage induced in the d.c. windings due to transformer action. Now, if the two d.c. coils are connected in series-opposing, the a.c. voltage induced in each will cancel, as a result of their being equal in magnitude but opposite in polarity. This is the only permissible connection for the d.c. coils. But the series-aiding connection for the a.c. coils, as shown in the diagram, is only one of the possible connections for them. The unit to be described here is based on this type of arrangement, as explained before. "From here,http://rfcafe.com/references/radio-news/saturable-reactors-radio-television-news-march-1952.htmThanks, Mikek

author: Qmavam
author_rank: Regular Contributor
timestamp: August 30, 2019, 05:43:42 am
content: It has been mentioned to use a constant current source to present a high impedance power supply for the current to control the inductor, so as to not affect the antenna.I see this LM317 circuit and wonder what the output impedance of this would be.Getting late, I'll try to expand on this tomorrow, it may or may not have enough context in the thread to make sense.Thanks, Mikek

author: fourfathom
author_rank: Super Contributor
timestamp: August 30, 2019, 03:43:03 pm
content: Quote from: Qmavam on August 30, 2019, 05:43:42 amThe LM317 can make a good constant-current source, but I don't think that it will present an adequately high impedance at RF frequencies. The device spec for "ripple rejection" may be indicative (this isn't in a C.C. configuration, but it is probably related). The graph in the Texas Instruments datasheet shows that ripple rejection is 60dB up yo 10 KHz, but then falls off at 20dB / decade. The rejection is only 20dB at 1MHz and it's still falling.Also, an antenna is a high-RFI environment, and exposing active devices to these random signals might cause problems such as damage or harmonics/intermod generation. You could put a filter between the CC source and the antenna, but then you are back to needing broadband high-current inductors -- if you can solve that you don't really need the CC sources!That 1952 paper on saturable reactors is pretty interesting! As long as the windings are well-balanced that method should work well.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2019, 04:45:07 pm
content: Quote from: fourfathom on August 30, 2019, 03:43:03 pmA cascode transistor (common base/gate) is added in series to raise the AC impedance of a current source and this applies to any current source.

author: fourfathom
author_rank: Super Contributor
timestamp: August 30, 2019, 06:44:45 pm
content: Quote from: David Hess on August 30, 2019, 04:45:07 pmI did a simulation with an reasonable 2A transistor as a current source (constant base voltage and emitter resistor to set the current). This was by no means a precision current source, but it provided a useful high-impedance up to 10MHz and was probably accurate enough for the OP's case. This was in a 500-Ohm system, since the OP wanted it in the middle of a "Beverage-style" antenna. With 50-Ohm impedance source/load it worked even better. This was without tweaking, or looking for the best transistor.But I still think that the potential RFI problems make this a less-attractive solution.

author: Qmavam
author_rank: Regular Contributor
timestamp: August 30, 2019, 07:11:12 pm
content: I just want to make it clearm the is a receive only antenna and a very low gain antenna (-40dbi).Does that makes any difference to the RFI or harmonics/intermod generation problem?Re: the 500 ohm system do you have any idea what the isolation impedance was?I could still add a choke, just smaller twith a higher SRF.Thanks, Mikek

author: fourfathom
author_rank: Super Contributor
timestamp: August 30, 2019, 07:21:17 pm
content: Quote from: Qmavam on August 30, 2019, 07:11:12 pmEven with receive-only, you still have to consider nearby lightning-strike (even if not enough to kill your radio, perhaps enough to fry an unprotected CC transistor). Also, any nearby broadcast radio stations can induce enough voltage to possibly cause the transistor to generate IMD (etc). You could possibly protect the CC circuit with common-mode chokes, assuming that the high impedance CC output doesn't allow coil-to-coil coupling. But that choke (or even dual chokes) impedance would have to be *very* high to actually limit the voltage seen by the high-impedance CC transistors. We're sort of chasing our tail here.I saw about 0.5dB loss at 10MHz in a 500 Ohm circuit, but that could no doubt be improved with better transistors or a more complicated CC circuit. I just did the sim as a proof of concept, no attempt to optimize or carefully measure the performance.I've never tried any of this in practice, so this is just speculation. Thanks for presenting an interesting problem!

author: radiolistener
author_rank: Super Contributor
timestamp: August 30, 2019, 07:25:51 pm
content: for receive, you can just connect a small (not-resonant) piece of wire to LNA with high gain and place that piece of wire with LNA far away from noise source, it will works great. The wire should be small enough to reduce it's reactive near field size. Low efficiency will be compensated with high LNA gain. The only one thing that you're needs to take care is to place it far away from noise source.This is how MiniWhip antenna works

author: Qmavam
author_rank: Regular Contributor
timestamp: August 30, 2019, 08:20:20 pm
content: Does a FET Constant Current Source have any advantage? Re: harmonics/intermod generation problem?https://www.allaboutcircuits.com/technical-articles/the-basic-mosfet-constant-current-source/If I used 50V as a supply, my Rset would need to be a 1000ohm pot and a power Pot. 25 watt!I'm sure there is a work around for that.Although it's possible 25V will be enough, if I can get enough saturation with 3V on each of 9 electromagnets.Thank you, Mikek

author: fourfathom
author_rank: Super Contributor
timestamp: August 30, 2019, 09:42:18 pm
content: Quote from: Qmavam on August 30, 2019, 08:20:20 pmI honestly don't know if the FET would be better or worse than the bipolar CC source, or if there is an actual problem or not. It's just that this is the kind of thing that can bite you, especially when you are trying to receive signals in the sub-microvolt range.That FET CC source (mirror) article shows how to make CC sources on a chip, but to build one like that from discrete transistors will require careful matching and realistically more parts to be reliable.There is probably a switching CC source you could use, but making that meet your requirements at RF would probably be a big chore. If you can use that two-transformer design from the 1952 article, then you could use any current source you wanted, with no RF requirements, and easy filtering of transients.