In my quest for an inductor with a an inductance that can be varied with the magnetic field of a relay coil. What criteria would I look for to find a relay with the maximum magnetic field? I'm open to AC relays also, I just need to limit the dc current I use.
Wouldn't it be even better to use the field inside the coil? That is, put the inductor he wishes to modify inside another coil which creates the magn etic field to adjust the inductance of the first? Does it make sense to us e two windings of a transformer, one to create the modification field and t he other to be the inductor getting adjusted?
Or would appropriately shaped pole pieces create an even stronger field tha n inside the coil?
I suppose the inductor being adjusted should have it's axis perpendicular t o the adjustment coil in order to avoid significant interaction of the indu ctor field with the adjustment coil.
+ Get 1,000 miles of free Supercharging
I have tried Iron Ecores, ferrite E cores, stacked flat washers, modified toroids, relay coils, solenoid coils. They all work to some extent. the stacked washer had a remanent magnetism, the relay coils I have didn't quite have enough field strength to do the job, I have a large solenoid that I'm sure with a little work on the magnetic path would work, unobtainable and there is a better way, the Ferrite Ecore is great, but I put 700 turns on two legs of the E core, it is expensive and I have to have it gapped (large), The iron E cores may be a good solution, but I didn't have the right size and I'd still need to modify the center leg of each lamination. The electromagnet at $3.15 is a reasonable price, I'm expecting to use nine. I'm hopeful about the electromagnet, it is 3 watts vs less then
1 watt for the relay coils I have, I guess it comes down to how the field is concentrated. I still have the problem of requiring high RF isolation on to my DC control so as to not make those control wires part of the antenna. The inductance runs into resonance before reaching the needed reactance. I'm thinking 3 in series might do it. I also realize there may be a problem with non-linearity induced mixing products, but I'm buoyed by this article.
I appreciate any help provided, I'm outside my knowledge base. Mikek
A common mode choke should do that and if it has an undesired resonance at an inconvenient frequency use another of different make in series with it.
If you need really good isolation also against high voltage spikes use hand-wound toroid transformers with a (thick) PTFE-coated primary. Just a few turns, send power up on 6.78MHz or another ISM frequency, rectify on the antenna side.
If you need really, really good isolation use a series resonant transfer like is often used for electric toothbrushes and the like. That allows for 1/4" or more of gap.
Oh yeah ...
I only see the last page but magnetically pushed cores can create what an engineer at my first employer called "crosstalk city". Lots of intermodulation, including harmonics mixing with each other.
Now for the difficult part.
There is one other method but it requires stepper motors or something similar and mechanical design. Old AM car radios (the ones that still worked well) had variable inductors instead of capacitors, in order to have a somewhat linear range on the AM dial. They were a piece of art with a ganged set of 3-4 cores moving in and out of coils via a worm gear. This avoids the saturation effects of magnetically controlled inductors.
If you can find such an old car radio that would make life a lot easier but I doubt that pick&pull places still have such old vehicles coming in. We are talking 50's and 60's here.
The electromagnets only use 3 watts if you apply the full voltage. You can adjust that to the current needed to do the job. You can even adjust that by using a pulsed control to the current with a diode to bypass the voltag e source.
-- Get 1,000 miles of free Supercharging
Please understand that the transductor effect is ill-suited for adjusting tuned RF circuits, due to the strong non-linearity needed for effective inductance change.
The LF and MF radio bands contain so many signals that it is quite sure that any non-linearity will create undesired cross-modulation products.
If I were attempting to create the tuneable resonance circuits, I'd use either a fixed inductor and a binary-weighted capacitor set with suitable analog switches, or a fixed capacitor and a binary-weighted inductor set with analog switches. It seems that 3 x 500 pF air variable capacitors do not exist anymore (if you do not succeed in cannibalizing an old tube radio).
Yup, big time. It can become nasty if a strong AM or coast guard local station is in the neighborhood. Or another ham radio operator using high power.
EBay can get you one:
If using high power it can pay to gently sand the edges of each plate. BTDT, a lot. This reduces the chance of corona discharges
Another concern with capacitors high up there are nearby thunderstorms which are very common in Mike's area. That can cause such capacitors to "weld shut" in their bearings or between plates, where the shaft won't turn anymore.
I don't think a pulsed current is wise, sounds like modulation to me. The plan is a variable current source form zero to full saturation. And then if I find problems close to saturation I can back off. If I can really make it work 500kHz to 4MHz, I think I have some help in that The ham bands run 1.8MHz and then 4MMHz, so complete saturation at 4MHz and not close at 1.8MHz. In the end, I'd me happy with tunability over the AM band.
I have been told that and do have enough understanding to know what that means. I was putting my hopes on the article below that says the products were down considerably from, in there article, a varactor. I don't know if the numbers in the article are as low as one would need for good operation in the AM band. And, I'm not sure, but putting 9 of them in the antenna make it 9 times as bad.
I have added your other options to my notes, but what I was hoping for was one knob in the shack that would tune the antenna. I also based my nine inductors on what I know think is a wrong analysis on my part in that I thought a Beverage on the Ground was like a beverage at 10ft. And 9 inductors because of this article about loaded beverages, >
Later reading says a BOG is a resonant antenna where a Beverage is not. The line was "Yes, this is a resonant antenna, nothing to do with Beverages!" And then measurements showing that.
can adjust that to the current needed to do the job. You can even adjust that by using a pulsed control to the current with a diode to bypass the vo ltage source.
The current in the coil would not be pulsed, only the source. The inductor would continue flowing the current at a level set by the duty cycle. Puls e it quickly enough and there would be vanishingly small variation in curre nt.
The point is the coil only needs to pass enough current to set your inducta nce.
But as others have indicated, if you are trying to pass low level RF you ma y find non-linearity effects mucking your signal.
To be honest, I don't get what you are trying to do. What are these tunabl e inductors doing for you? Is this part of a tuned circuit? Why five of t hem in series in what appears to be a distributed string?
-+ Get 1,000 miles of free Supercharging
Split your coil in two identical coils in serial. L = L/2 + L/2 On each coil add a secondary and connect them in serial but in opposition. Feed the DC in the secondaries that are then free from RF voltage. HTH
The understanding I have is that BOG antennas are a tuned antenna.
The suggested length for a 1.8MHz antenna is 200 ft, but it is highly dependent on the ground conductivity. One more reason tuning would be desirable, season and weather changes in soil. My limited understanding is that an inductor in the antenna wire slows the VF which has the affect of making the antenna tune to a lower frequency. I want a BOG that works 500kHz to 1700 kHz or even 4MHz. I hope to make this possible by adding inductance to the wire, and ideally a variable inductance that is controlled in the shack. I went with multiple inductors, actually 9, with info from this article.
I had a BOG up until a years ago (destroyed during hurricane cleanup) and it was the best receive antenna of my 3, a miniwhip, 150ft longwire and the BOG. Here's a comparison of the 3 antennas.
The problem of resonating an LC circuit over a large frequency range is that if only one element is tuned (either L or C) the needed reactance range is the frequency range squared. Thus for 500 to 1700 kHz range the reactance range must be nearly 12:1 (and that includes strays). Tuning from 500 kHz to 4 MHz requires a 64:1 reactance range.
However, if you tune both L and C, the broadcast band would only require 3.5:1 tuning range on both L and C. A multitap inductor selected by reed relays and a motor driven variable capacitor should do the trick. This could handle strong RF fields easily. An AM varicap with a small fixed capacitor in series (to limit the capacitance range) might be used instead of a mechanical variable capacitor.