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Ok, I’m a hobbyist/tinkerer and not an expert of any sort, so some of these ideas may be very naive but i figured I’d toss them out and see where the holes are in them.
baranger1 mentioned in the “Talking House” thread about the tuner for the Talking House rigs being ferrite cores with some sort of motor array to move them in and out of a coil or coils.
Ok, I’m a hobbyist/tinkerer and not an expert of any sort, so some of these ideas may be very naive but i figured I’d toss them out and see where the holes are in them.
baranger1 mentioned in the “Talking House” thread about the tuner for the Talking House rigs being ferrite cores with some sort of motor array to move them in and out of a coil or coils.
I remember wondering when I first started looking into the hows and whys of part15 AM, why the loading coils are air core instead of using ferrite. Air core only takes wire and a form, so it has simplicity going for it, but is there some inherent factor of the air core that makes it better for part15 AM transmitting?
Back when I was a kid in the 70s, I made quite a few junkbox crystal radio sets out of the ferrite rod and tuning capacitor out of any old AM radio (from a car, pocket transistor radio or whatever) and a germanium diode. Logically, since crystal radios have to be fairly efficient to work much at all (at least if you live in out the country like I did back then), the ferrite rod and coil in them can’t be all that inefficient for AM BCB?
Another thought comes from hearing about the difficulties in tuning the part15 base loaded AM antennas because of the problems caused from being physically so close to the antenna that it affects the resonance. I was thinking a stepper motor or small low-speed motor would eliminate that problem (and also make it relatively convenient to touch up the tuning as weather changes and etc put it a little out from “perfect”). Whatever system one is using to tune the antenna, be it capacitor or moving ferrite or a variometer coil, a small motor would eliminate the need for the person to be close enough to throw off the tuning. If the antenna is high in the air (or one just has an old toy around), many sorts of small radio remote control toys would have the parts and assemblies necessary to do adjustments from a little distance away.
Yet another thing I’ve wondered about is the occassional question I’ve seen come of as to whether the wire of the loading coil could possibly count towards the 3 meter antenna length limit. My thought (which would pretty much apply only to homebrew, since such a modification would probably void the type acceptance on a device like a Rangemaster) would be that if the loading/tuning coil/assembly were inside the chassis of the transmitter (which would obviously require a somewhat large chassis unless ferrite core loading coils are a feasible idea) that it wouldn’t be part of the antenna/feedline/ground wire in any usual sense, at least if the chassis/case is grounded metal, since it wouldn’t be radiating much (if at all). Okay, so it might make the transmitter the size of say a 5 gallon drum.. But if one is talking about things like digging up the yard to bury radials then the transmitter being a little bigger isn’t such an outlandish concept.
Just some questions and thoughts, probably there are good reasons to avoid all of them, but I figured I’d toss out the concepts and see what anyone has to say about them.
Daniel
Total posts : 45366My first set-up used one, and it worked great. They are a little hard to find these days and need to be ordered on-line or by mail, so in general it’s easier to grab a scrap piece of PVC and start winding.
Strangely enough, the Rangemaster uses an internal matching toroid and is FCC certifed, so you just might be more wise than you give yourself credit for 🙂
My old set-up, including a photo of the toroid, is here:
http://www.geocities.com/scwis/scwis.html Warning – Geocities site with pop-ups galore.
Here’s the inductor itself:
Inductive Coupler - SO-259 Input.
- Banana Plugs Out.
- 50 Ohms.
- ~200 uH.
- Improved Antenna Match.
- Protects Transmitter.
Experimental broadcasting for a better tomorrow!
Total posts : 45366I havent had much luck with the PVC type loading coil,anyone have the details for making a toroidal matching network for medium wave use???? Regards,Lee
Total posts : 45366A reminder of the obstructions we are working against..
http://www.ee.surrey.ac.uk/Personal/D.Jefferies/radimp.html
“Re-radiation from near-field objects and ground ”
Things that make you go.. Hmmm.. 🙂
Total posts : 45366Here is the pictoral drawing I used to build the inductor.
Click for a larger image.The formula for calculating the number of turns on a toroid is here, at
http://www.amidoncorp.com/aai_ironpowdercores.htm
Order the toroids here: http://www.amidoncorp.com
Experimental broadcasting for a better tomorrow!
Total posts : 45366SCWIS,
Thanks for the info. on your torroid. This is interesting since it appears that it not only cancels the Xc of the antenna but also provides for an impedance transformation unlike a simple base loading coil.
I do wonder, though, if the numbers cited in other posts for a typical 3 meter antenna with a ground are representative (20 to 30 ohms Rr and Rg) with the Xc cancelled, if this is what we want to do since this impedance will be stepped down as viewed by the transmitter. Did you happen to experiment with just using the coil as a series inductor only?
Also, is there anything special about the coil form material? I have several here salvaged from PC power supplies that I could use but I don’t know about the core material or the core losses at AM frequencies.
Neil
Total posts : 45366SCWIS,
…since this impedance will be stepped down as viewed by the transmitter.To the best of my knowledge this is why the reactance and 50 Ohm tap point are calculated, to counteract that effect. Only guessing though.
Also, is there anything special about the coil form material?
Yes. In an ideal situation you would use a material formulated to be most effective in the AM BCB, which is Material 15, IIRC. As explained elsewhere on the Amidon site, the other numbers in the toroid SKU are for the physical size.Amidon has a great data set and a good selection of products especially formulated for this use. The PC source toroids tend to be formulated for the higher frequencies found in the PC.
The iron power cores best for AM BCB are here
http://www.amidoncorp.com/aai_ironpowdercores.htm
Experimental broadcasting for a better tomorrow!
Total posts : 45366SCWIS,
Your link to the Amidon site was useful, I was just looking for a junk box solution. A quick glance at their data sheets gives me the impression that any of their toroid cores will work at AM frequencies. The one you suggested may be the best choice.
Let me explain what I meant by impedance transformation. I realize that your sketch may not be a literal description of what you built, but I will use it as an example. The primary turns in your sketch are 4, and the secondary turns are 23.
If the resistance between the antenna output and ground terminals is 30 ohms, then the resistance seen by the transmitter will be:
(4/23)^2 x 30 ohms = .9 ohms. (impedance transforms according to the square of the turns ratio)
Again, I just used your sketch as an example, but my calculation indicates that an autotransformer constructed this way will present a very low load resistance to the transmitter. Perhaps the transmitter will function properly with this load but I do not see how the transmitter load could be 50 ohms unless the antenna impedance is really high.
This is why I was interested in any experiments you might have done with the toroid. Many times theory guides us but experimentation leads to workable solutions.
Neil
Total posts : 45366Yes, the diagram is not an actual illustration of the tap points. The calculations I was referring to were here
http://www.part15.us/node/1161#comment-2870
rather than in this thread, sorry.
Anyway, my understanding of the approach laid out by the original author of the work the formulas were taken from was that the process of calculating inductive reactance, then the impedance ratio, then the turns ratio which would give the number of turns from the grounded end of the coil to connect the antenna terminal to, would result in tapping the coil where the impedance (not resistance) presented to the XMTR would approximate 50 Ohms.
I had no way of verifying that, other than it worked really well 🙂
“well” meaning a really good solid signal for several blocks with the toroid in place versus a few feet without.
Experimental broadcasting for a better tomorrow!
Total posts : 45366SCWIS,
In my posts here I have assumed that the capacitive reactance of the antenna was cancelled by the inductance of the coil above the tap, therefore my reference to a resistive load to the transmitter rather than an impedance (which is a resistance and a reactance) was appropriate.
Unless the antenna resistance or impedance is known it is not possible to predict the tap point for a 50 ohm load as seen by the transmitter. My point in my previous post was that with a typical 3 meter antenna and ground presenting a 30 ohm resistance with the antenna capacitive reactance negated by the toroid inductance will not give a 50 ohm load to the transmitter regardless of where the taps are placed. Tapping the coil in this circumstance will always result in a load resistance of less than 30 ohms as seen by the transmitter.
There is nothing magic about a 50 ohm load except when the transmitter is optimized for this. I know of no part 15 AM transmitter manufacturer who specifies the optimum load for their unit. For example, I have data which indicate that the maximum power output of the Ramsey AM-25 occurs with a resistive load of 23 ohms. It may well be that your transmitter operates best when presented with another resistive load through your toroid.
This is not meant to detract from your experimental experience, but rather to encourage experimentation. This appears to be needed because, though we know from other posts what the approximate antenna and ground characteristics are, we really have very little idea of the optimum load for AM transmitters. I have presented my data regarding the Ramsey unit, but I have no idea about other transmitters.
Neil
Total posts : 45366Ok, so what’s stopping us from using a coil with a sliding metal loading bar for the tuning. Or just a coil formed around a metal bar with taps to set the value. Wouldn’t this be more efficient than a larger air wound coil?
Metal bar is just an expression for the proper magnetic material for the center of a coil.
Total posts : 45366That’s pretty much what I was thinking, Greg.
With a crystal radio for AM BCB, if you make it open air you can use an old-fashioned round oatmeal box for a form (which is pretty close to the size for the open air coils in the sstran type antenna’s loading coil). But using a ferrite rod, it’s considerably smaller physically.
At first, I assumed that the ferrite rod option might be too high-q (not allowing enough bandwidth for reasonable voice/music) or too lossy for some reason for it to be effective for transmitting. But then hearing that the talking house transmitters use it, I got to thinking it obviously must be at least reasonably practical.
I note that “hot” 160 M antenna coils like the “Texas Bugcatcher” are open air, but as scwis mentioned in another thread, they’re also different from the sstran style coils in regards to dimensions, wire guage, spacing and etc.
I figure there must be reasons why the air-core is chosen for the sstran style antenna, and reasons why the talking house uses sliding ferrite, which is what I am mostly wondering about. I was thinking that if the TH (talking house) can use ferrite effectively and even goes so far as to incorporate an auto-tuner, then logically it might be possible to use a smaller ferrite core coil and use a small motor like the TH does to tune it, maybe not automatically, but remotely would be useful enough.
I haven’t taken the plunge to AM yet, so I’m still pondering over the assorted options. The TH sounds good to me at this point if I can find one at a good price off ebay, since it’s frequency agile, and it’s FCC type approved or certified (I don’t recall which offhand, but in either case it has a sticker on it where the manufacturer claims it’s legal which could demonstrate that so far as I knew it was legal provided I haven’t messed with it). A Hamilton would be way beyond my budget at this time, I’d rather avoid kits since they seem to fall in a nebulous area so far as I could tell reading part15, and I’m not sure as I want to start with homebrewing without running some sort of a rig first so I have some idea if my homebrew is putting out as well as a commercial part15 rig. The sstran is also tempting even though it’s a kit, but though I’ve built some kits I haven’t tackled one that compact and complex in a long time. My concern there is that I could spend more time debugging it and hunting for solder bridges and overcooked parts than I spend running it.
Also I have an attic, and having read an instruction book for the TH, it indicated that trying higher floors was a good idea. I figure the added elevation of 3rd floor above the aluminum siding would get the transmitter above many of the trees in the area while staying inside the manufacturer’s instructions and recommendations.
But if I do end up going with homebrew, then ground level mounting with a loaded vertical within the 3M rule would be the obvious choice and I’m thinking ahead to tuning. Even if I go the TH route, if there’s some obvious (to people who know more of such things) reason why the ferrite core should be avoided, it’d be good to know it before buying one.
Basically trying to figure out enough of the how and why to make a somewhat educated choice. This forum has been excellent for ideas and information. The amount of technical and innovative expertise here is very impressive. Kudos to all, by the way.
Daniel
Total posts : 45366
Used to use these on crystal radio sets.
An inductor core is inside the cardboard tube, and the shaft is threaded so that turning the shaft moves the slug in and out of the coil.
Might be interesting.
Images taken from this ebay page: http://search.ebay.com/_W0QQsassZhummerdudesQQhtZ-1
Pretty cool stuff…
Experimental broadcasting for a better tomorrow!
Total posts : 45366[quote=scwis]Anyway, my understanding of the approach laid out by the original author of the work the formulas were taken from was that the process of calculating inductive reactance, then the impedance ratio, then the turns ratio which would give the number of turns from the grounded end of the coil to connect the antenna terminal to, would result in tapping the coil where the impedance (not resistance) presented to the XMTR would approximate 50 Ohms.[/quote]
Maybe there is an expectation that the tapped loading coil behaves similarly to a tapped resistor used in a circuit carrying only direct current. But that isn’t the case in this situation, because we are dealing with alternating (r-f) current.The DC resistance of a coil is a linear function of the wire characteristics used to wind the coil. The value of inductive reactance of the coil is not linear with length, but even when a point is chosen where the inductive reactance is 50 ohms with respect to the coil end, that does not mean that the tx will see a good match across those terminals. Here’s why.
Disregarding the rest of the coil and the antenna, the load termination seen by the tx will be a small amount (maybe a few hundred milliohms) of the total DC resistance of the coil, along with the ~50 ohms of inductive reactance. This condition presents a huge VSWR mismatch to a transmitter designed to work into a load of say, 25 to 50 ohms non-reactive, because most of this load impedance is reactive, and only the pure resistance of the load (the few hundred milliohms in the coil to the “50 ohm” tap) is capable of dissipating power from the tx.
A NEC model of this circuit using enough inductance to produce 50 ohms of reactance, and assuming 0.2 ohms of DC resistance to that coil tap showed that a VSWR greater than 300:1 will be seen by the tx. In the NEC model the tx was connected across the “50 ohm” tap and the bottom of the coil, and the bottom of the coil was connected to an r-f ground of 25 +j 0 ohms.
When adding the 3-m antenna and the inductance to resonate it (1,600 kHz), NEC shows that antenna system efficiency would be about 10X worse than when using a conventional, ground-mounted, base-loaded, 3-m vertical with no “50 ohm” coil tap, and where the bottom of the coil connected to an r-f ground.
//
Total posts : 45366A NEC model of this circuit…
Important to note that I can’t take credit for, or defend, the tapped inductor concept.
The approach outlined in the two threads noted was an adaptation of some ARRL materials for the AM BCB by radio engineer Ernie Wilson of Pan Axis, taken from his practical guide to short AM antennas called ANTSAM, still available here:
http://www.panaxis.com/p-ants.html
Secifically,
ANTSAM AM/MW ANTENNAS ($7.50)
Iincludes general antenna theory, vertical half-sloper, multi-element, shunt-fed, and helical wound antennas, PLUS increasing efficiency by good ground conductance, radials, loading coils, and capacitive hats. Vertical antennas from 10 to 50 feet are covered. Includes excerpt of “Part 15” FCC Rules which limit antenna height of 100mW “broadcasters”.Excellent reading, highly recommended.
While the ordering info states a $10.00 minimum is required, if you send a money order and a large SASE you can order for less.
Reminds me of Don Lancaster’s addage about “an hour in the library is better than a month in the lab” 🙂
Experimental broadcasting for a better tomorrow!
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