Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: Novice Q5er -- Coil Help Sought  (Read 7926 times)
KB1WSY
Member

Posts: 724




Ignore
« on: July 22, 2012, 01:36:03 PM »

It's me again. This is very long. If anyone thinks the sheer number/length of my posts is getting abusive, please speak up! I have a relatively thick skin!

The receiver section of my Novice station, circa 1950s/1960s, will be a beat-up ARC-453 surplus receiver which I will be renovating, coupled to the mid-1950s era W6TNS Novice Q5er ham bands converter (80m and 40m). Here's the circuit diagram:

http://tinyurl.com/cpz4lmr

I already have nearly all of the parts for this project apart from the famously "unobtainium" Miller Antenna and RF shielded coils specified in the original design.

This shouldn't be a big deal. The homebrewer deals with that by ... homebrewing. I have spent several hours over the past few weeks reading up on tuned circuits, specifically the transformer stages used in old receivers. The two Miller components are both "shortwave coils" with an overall range stretching from 2.1 MHz to 6.3MHz. Although the upper bounds are lower than the 40M band, it seemed to have worked fine anyway.

My understanding of the theory is that these circuits typically have an untuned primary coupled to a tuned secondary. The secondary is typically in parallel with a variable capacitor, in this case a "broadcast type" with a max value of 365pf. In the original W6TNS circuit the Antenna and RF caps are ganged together, but I will be using two separate capacitors to eliminate tracking problems (and increase the number of knobs to twiddle!).

Some basic bits of theory that I've picked up:

--The self-resonance of the coils should be above the maximum frequency that you are planning to tune (because, above that frequency, the coil behaves like a capacitor rather than an inductor).

--A relatively wide passband is desired in the front end of the converter, to avoid having constantly to retune the converter's capacitors (the main tuning control is the BC-453 used as a variable IF). Since I am only interested in the CW portions of the 80/40 bands, I'm thinking that a passband of about 150 KHz would be about right. To achieve the wide passband, the coupling between the coils in the front-end shielded cans should be relatively tight i.e. you don't want too much selectivity.

--The combination of two tuned circuits in the two stages creates a "broad shouldered" selectivity curve with an almost flat top over the range of the passband. That is exactly what you want.

--I can fairly easily figure out what inductance the *secondaries* of these coils should present because they are basic tuned circuits in parallel with the variable capacitor, and I know that we want to "hit" at least 3.5-3.6 MHz and 7.0-7.1 MHz i.e. a range of 3.5-7.1 MHz within the span of the variable capacitors.

--To me, the primaries are a bit of a mystery. In the very basic circuit diagram from my Miller catalogue, the primary is shown with fewer turns than the secondary but that's about the only information that Miller provides, apart from specifying that the Antenna coil's primary is *low-impedance* and the RF coil's primary is *high-impedance* (therefore the two transformers are not the same, except that they share the same working frequencies). Here's the Miller catalog listing:

http://tinyurl.com/blml967

I doesn't matter very much how I *physically* construct the coils, but I thought it would be fun to put them together using my Harnett IF coil kit from the 1950s. I have the wherewithal to homebrew as many as 33 shielded coils, so I might as well make use of it! The basic pieces look like this:

http://tinyurl.com/cd2nx5a

Any help people can provide to help me solve the "puzzle of the primaries" would be greatly appreciated. I'm not looking to replicate the Miller coils exactly, all that matters is that my homebrewed equivalents behave electrically as needed. For what it's worth, most of my knowledge on this subject comes from vintage ARRL publications and specifically, several chapters in the 1963 Radio Amateur's Handbook, in particular the section on "Radio-Frequency Circuits" and "Coupled Circuits" plus repeated readings of "Understanding Amateur Radio." After several weeks scratching my head over this one, it is however time to ask for help from my betters.

73 de Martin (KB1WSY) and TNX for your patience!!!
Logged
G3RZP
Member

Posts: 4492




Ignore
« Reply #1 on: July 23, 2012, 12:51:57 AM »

The dynamic resistance of the tuned circuit at resonance, Rd, is QXL. If you want a 50 ohm input impedance, then you need a turns ratio of Square root (Rd/2 divided by 50) or Rd/100.

This is because the 50 ohms transferred up to Rd gives RD across the circuit and halves the Q.

The coupling you need is either such kQ = 1 or a bit over, where k is the coupling factor. If you use bottom inductive coupling, k is sq rt of Lc/(L1L2) where L1 L2 are the tuning inductors and Lc is the coupling inductor. Don't use a small capacitor between the hot ends  of the variable caps because this gives you a coupling factor that changes as tuning. Where tuning is by variable capacity, use inductive coupling and vice versa.

The working Q needed for the bandwidth you want at 7 MHz is about 50.

You will find a copy of Terman's Radio Engineering useful: another good book is Langford-Smith's 'Radiotron Designers Handbook'.
Logged
KB1WSY
Member

Posts: 724




Ignore
« Reply #2 on: July 23, 2012, 03:55:06 AM »

Peter, that is exactly the sort of information I was after. Many thanks.

First of all, I will definitely get hold of one of the books you recommend. Second, I do have a few follow-up questions you may be able to help with.

(1) Your information about impedance will help a lot with the calculations for L1 (the antenna coils) since I should now be able to calculate the number of turns, etc., to get the right impedance for the primary. But for L2, whose primary is connected to the plate of V1, initially the only data I knew (from the Miller catalog) is that the primary is "high impedance." Today I looked up the data sheet for the 12BA6 at http://tinyurl.com/c7vlswu which lists the "plate resistance" as 1 megohm. Is that the impedance I should be aiming for on the primary of L2?

(2) You mention Lc, the "coupling inductor" but I find no reference to this parameter in the ARRL chapter on coupled circuits although I do find calculations that include M (mutual inductance). In practice what does Lc model? The powdered-iron tuning slug, or something more theoretical? I don't think a tuning slug has an inductance all on its own ... but then I'm a newbie with no background in EE. The two obvious ways of varying the coupling between the coils is (a) to vary the distance between the coils and (b) to move the tuning slug.

(3) What is "bottom inductive coupling"? In this case I will be winding classic stacked transformers, primary at bottom and secondary above it, with the bottom of the primaries and secondaries connected to ground, both coils wound in the same direction on a slug tuned form. (If the number of turns needs to be large, I will also attempt to use my "Universal" winder for a "basket weave" effect and lower capacitance.)

I wish I didn't have to ask these follow-ups but I seem to have run into the limitations of the otherwise excellent 116 pages of radio theory at the front of the vintage ARRL manual. Alternatively, and perhaps more likely, I could perhaps reach the appropriate mathematical conclusions by working with M (mutual inductance), which is sort of tossed out in the caption to a graph on page 48 -- without further definition.

FB on not using fixed caps across the variable caps.

One more, interesting issue. In the original text by W6TNS, and in the original schematic, there is a 50 pF coupling capacitor connecting the "hot" ends of the primary and secondary of L2. The author states that "the signal emerges in the plate circuit of the 12BA6 greatly amplified and is coupled to the 12BE6 though a 50 uuF capacitor." But in the reprint that I have, someone has emphatically written that this capacitor is "NOT NEEDED: Do not install" (and it has been whited-out in the schematic I posted online). Indeed, according to the radio theory in the ARRL handbook, you don't need the capacitor if you are using transformer coupling between the stages; it is needed only if you are using resistance coupling or single-coil impedance coupling. Right?

This is interesting because I did manage to find what seems to be a Merit Corp. equivalent of the vintage Miller RF coil. When I took the can off the Merit coil, I found that the primary had been replaced by a resistor, and that there was a 50 pF capacitor connecting the live end of the resistor to the live end of the slug-tuned secondary coil. If my reading of the theory is correct, this would work fine, even though it is a major modification of the original W6TNS circuit (replacing transformer coupling with resistance coupling). I have uploaded a photo of the Merit coil and its data sheet:

http://tinyurl.com/bw5bsgq

In any case I would like to try winding my own RF coil although just for fun, I might play around with the Merit RF coil too, just to see which works better -- the Merit, or my homebrewed equivalent of the original Miller. I'll have to wind the Antenna coil from scratch in any case, since it really seems unobtainium (it, and its Merit and Meissner equivalents).

73 de Martin, KB1WSY

Logged
G3RZP
Member

Posts: 4492




Ignore
« Reply #3 on: July 23, 2012, 06:46:53 AM »

Hi Martin

When coupling from a plate to the tuned circuit, the gain of the stage will be determined (in a pentode) by gm times the load resistance RL. Strictly, you need to take into account the plate resistance of the tube, but that's going to be a lot higher than the load resistance. The load resistance is given by the square root of the turns ratio between the coupling winding and the tuned winding. When covering a wide range - say 2:1 in frequency, the load impedance varies and thus the gain does. One way round this is to use  a large coupling inductance self resonant just below the bottom of the band: this is not necessary for just the ham bands.

So the bigger the coupling winding, the bigger the load impedance, BUT.......

you need to make sure that gm times 2 pi times f times Cpg times Rd1 times Rd2 is less than 2.

gm is tube mutual conductance, f is the frequency, Cpg is the tube plate - grid capacity, Rd1 is the dynamic resistance of the grid tuned circuit and Rd2 is the resistance  of the plate load. This is called the 'stability criteria'. Incidentally, the coupling winding are assumed tightly couple i.e. wound over the cold end of the tuning coil.

Antenna coupling windings are sometimes made fairly large and loose coupled to accomadate a wide range of antenna impedances without detuning.

Coupling inductor.

You can couple tuned circuits in several ways. One is putting the coils close together. the otehr ways are

               Coupling cap

       tuned cct           tuned cct

     (top coupling)

       Inductor to ground , shunted by tuning cap in series with a larger 'coupling cap' to ground (bottom capacity coupling). From the junction of the tuning and coupling cap, another tuning cap to the top of the second inductor.

         Tuning cap, one end grounded, one end to inductor, other end of inductor through a coupling inductance to ground (bottom inductive coupling). From the junction of the two inductors, a third tuning inductor with its other end connected to a tuning capacitor to ground.

        Or as in top capacity coupling, use an inductor instead. Not usually very practical becasue it needs a big inductor which has too much self capacity.

Critical coupling is when the coupling factor multiplied by the square root of the product of the two
tuned circuit working Qs is equal to 1.

See if you can get 'Radio Receiver Design' Volume 1  by K.R. Sturley, published Chapman and Hall, London, 1945. Also 'The Technique of Radio Design' by  E. E. Zepler, Chapman and Hall, London, 1951. I got mine quite cheaply from the Abe books site. Incidentally, Zepler was an interesting man. He worked for Telefunken in Germany and designed some stuff used by the Wehrmacht in WW2. As a Jew, he left Germany and came to the UK, where he designed radios used by the RAF in WW2! He was the first Professor of Electronic Engineering at Southampton University, and one of the principal lecture buildings there is the Zepler Building. He was also a well known expert setter of chess problems......

Those two books will give you a good insight into all these things, although you may want to skip some of the maths. And get a copy of Langford-Smith, from where you can learn why certain types of multigrid mixer can have a negative input resistance above 20 MHz!

The problems of doing diagrams may make it easier for me to email you direct as I can scan diagrams then....

73

Peter G3RZP

Logged
KB1WSY
Member

Posts: 724




Ignore
« Reply #4 on: July 23, 2012, 08:27:31 AM »

When coupling from a plate to the tuned circuit, the gain of the stage will be determined (in a pentode) by gm times the load resistance RL. Strictly, you need to take into account the plate resistance of the tube, but that's going to be a lot higher than the load resistance. The load resistance is given by the square root of the turns ratio between the coupling winding and the tuned winding. When covering a wide range - say 2:1 in frequency, the load impedance varies and thus the gain does. One way round this is to use a large coupling inductance self resonant just below the bottom of the band: this is not necessary for just the ham bands.

Forgive me for the dumb question, but shouldn't it be self-resonant just *above* the *top* of the band?

Another thing that occurred to me in terms of reverse-engineering the W6TNS design (or rather, figuring out the parameters of the original Miller coils) is that by definition, both of the transformer circuit secondaries for the Antenna coil and the RF coil are in parallel, at any one time, with exactly the same capacitance (because, in his design, C1a/b is a two-gang variable type, 365pF per section). Ergo, because they are resonant at the same tracked frequency, the secondaries of the two transformers have the same inductance -- I hope I haven't made a stupid mistake in drawing that conclusion. If I am right, it means that the physical difference in the designs of the two transformers affects only the primaries -- L1 with a low-impedance primary of about 50 ohms and L2 with a higher impedance, to be determined by the mathematical methods you have laid out.

The problems of doing diagrams may make it easier for me to email you direct as I can scan diagrams then....

That would be cool. Even better would posting links to an online picture host (such as Picasa) so that other eHam readers could follow the rest of this rather theoretical discussion through to its practical conclusion.

I see a process that will involve, in part, some mathematics and then, some coil-building followed by tests using my trusty Eico RF signal generator, grid-dip meter and VTVM. The building of the Q5er converter itself will follow a bit later, but I would love to get a head start on the coils. In fact I think you've already given me enough information to build the Antenna coil or at least to start experimenting; the RF coil can follow as soon as the optimum Z for the primary has been calculated.

I really appreciate the help!

73 de Martin, KB1WSY
Logged
G3RZP
Member

Posts: 4492




Ignore
« Reply #5 on: July 23, 2012, 08:57:44 AM »

Martin,

As Rd = QXL, XL goes up with frequency: assuming Q stays the same, then the load impedance increases with frequency and so the gain goes up. By having the coupling coil resonant below the LF end, you raise the impedance at the LF end and so tends to equalise the gain.

If you go to http://n4trb/amateurradio/national/hro.htm and read about the famous National pre war HRO receievr, it goes into how they equalised gain over 2;1 frequency ranges by this method. Incidentally, it is this variation in dynamic resistance which can make oscillators touchy when tuning a wide range. Output  goes up at the HF end, and you can get 'squegging' while at the LF end, there's not enough signal to let it oscillate.

Yes, L1 and L2 tuning coils are the same inductance: to cover two bands, a 365pF variable is over kill. You only need a bit over a 4:1 capacity range. The 365pF is likely to have a fairly high minimum capacity, but 10 microhenries would do fine for the tuning coils. As you are not ganging the tuning capacitors, matching doesn't matter too much: if you do gang them, then you need a slug or other system to adjust the inductances to get ganging. (I use the term 'ganging' for the RF circuits which are on the signal frequency and 'tracking' for oscillator which is separated by the IF from the signal frequency - at least when talking of superhets with tuneable oscillators.)

The circuits in W6TNS design are not coupled, so the dissertation on coupling may be disregarded if you are following that design. But look at the ARRL handbooks in the mid to late 1950's: they ahd a double superhet design for 80 and 40 with a bandpass coupled pair in front of the mixer (a 6AC7), although the tube line up changed a bit over the years.

You will find a frequency counter useful - don't believe everything the frequency dial of a signal generator of that ilk tells you.

73

Peter G3RZP
Logged
K9MOV
Member

Posts: 42




Ignore
« Reply #6 on: July 25, 2012, 08:46:46 PM »

Great post, Martin, your projects are very interesting.
Peter, if I may speak for all of us homebrewers, Thanks for your expert advice and HELP.
73 and look forward to more of these kind of posts,
Lane--K9MOV
Logged
G3RZP
Member

Posts: 4492




Ignore
« Reply #7 on: July 26, 2012, 03:36:24 AM »

Lane,

Just ask!

At college, back in 1964, we had an Indian lecturer, Mr. Anand. We had three 2 hour sessions a week with him, and 4 weeks on tuned circuits, couple tuned circuits, triple coupled, damping and so on. Some of it has actually stuck, and doing RF design work for living up to a few yaers ago has meant I haven't forgotten it. These days, though, professionally, I'm a 'meetings engineer'.
Logged
KB1WSY
Member

Posts: 724




Ignore
« Reply #8 on: July 26, 2012, 07:07:02 AM »

Peter,

Here's a summary of what I'm thinking:

--As you said, the coils in the W6TNS design apparently aren't coupled, therefore we can drop your excellent dissertation about the coupling constant k. Unfortunately there are two versions of the W6TNS schematic. In the original published in CQ Magazine in January 1956, in the RF circuit there is a 50 pF capacitor across the live ends of the two coils but in another copy of the schematic floating around, someone has written, "NOT NEEDED. DO NOT INSTALL." Anyway for the moment I will omit the capacitor, it greatly simplifies the theory.... if the build doesn't work, I can always add the capacitor and see if it improves things....

--I have now calculated a notional desired inductance for the *secondaries* for both coils using the classic formulae for tuned circuits:

http://tinyurl.com/cuym5ed

Since I am using a 365 pF variable, I think I'll aim for about 7 uH, which yields C = 258 pF at 3.5 MHz and C = 73 pF at 7 MHz. (You had suggested roughly 10 uH which would also work, yielding C = 207 pF and 51 pF respectively; but at 7 pF I can spread it out a bit more, producing finer mechanical tuning control, not that it matters very much given the need for a barn-door 150 KHz passband.)

--For these tuned secondaries (Antenna and RF) you've indicated a desired Q of about 50, in order to get the bandwidth of 150 KHz at 7 MHz. This requires figuring out the optimum combination of turns/form diameter/iron slug position. Rather than messing around with some rather complex mathematics, I will experiment with the slug-tuned secondary and my Eico 710 GDO. To get the barn-door selectivity of 150 KHz I need to screw the slug down to somewhere in the middle of the coil right? Should the slug overlap the primary and secondaries? (Yes this is a really basic question, but my various books don't really address that issue.)

--My design reference is the 1950s/60s with test equipment available/affordable for a ham. In my case, Eico 710 GDO, Eico 232 VTVM and Eico 315 RF signal generator. To calibrate this equipment, I also have a digital frequency checking system consisting of a small Sony ICF-SW1 general-coverage receiver with LCD readout. I use this to make sure my vintage equipment is "in the ballpark" then I banish it to the other end of the house!

--First, experimentally measure the resonant frequency of the secondary when used in a tuned circuit: connect a known, low tolerance fixed capacitor (say, a 100 pF silver mica) across the coil and "dip" it with my Eico 710 GDO.

--Then use the following formula for calculating the inductance (I've already been doing this in my transmitter-building work and it seems to work very well):

http://tinyurl.com/cfxx42w

--Calculating the Q, using the Eico 710 GDO and Eico 232 VTVM. I have not done this yet because I haven't yet acquired (or built) an RF probe for the VTVM, but will do so shortly:

http://tinyurl.com/d4nxmzh

Obviously a fair amount of playing around will be required, considering the number of variables: diameter of form (I have 2 to choose from), style of winding, position of slug ... that's all part of the fun.

Concerning the *primaries* I also need a way of experimentally determining whether I've hit the correct *impedance*. Short of actually building (or acquiring) an RC bridge meter, can this be done by "solving" for Z, once I know L and Q? Or is there more than one unknown? (Apparently I also need to know reactance, X?)

For L1, the Antenna coil, I can shoot for a primary impedance of 50 to 100 ohms. For L2, I'm still very confused except that I know that the plate resistance of V1 is 1 megohm and that the primary of L2 is in series with R3, a 2.2K resistor to ground. So the DC resistance of that circuit is something like 3.2 megohms not counting the coil itself. J.W. Miller's specs vaguely describe the original B320-RF coil as having a "high impedance" primary. I'm wondering whether I can get away with just using a ballpark of, say, Z = 500 ohms? It probably doesn't matter too much, as long as it is relatively high impedance?

Finally, dear reader, please remember I have no formal EE training and forgive the inevitable theoretical "clangers" in my thinking.

Peter, many thanks for sharing your expertise and transmitting the wisdom of Mr. Anand.

73 de Martin, KB1WSY
Logged
KB1WSY
Member

Posts: 724




Ignore
« Reply #9 on: July 26, 2012, 07:48:04 AM »

Quote
For L1, the Antenna coil, I can shoot for a primary impedance of 50 to 100 ohms. For L2, I'm still very confused except that I know that the plate resistance of V1 is 1 megohm and that the primary of L2 is in series with R3, a 2.2K resistor to ground. So the DC resistance of that circuit is something like ****3.2 megohms**** not counting the coil itself.

Sorry, I was even more confused than I let on. 1 megohm + 2.2 Kohms = slightly more than 1 megohm total DC resistance (I originally confused 2.2K with 2.2meg).

73 de Martin, KB1WSY
Logged
G3RZP
Member

Posts: 4492




Ignore
« Reply #10 on: July 26, 2012, 09:28:31 AM »

Martin,

I'm not sure exactly what you mean by 'In the original published in CQ Magazine in January 1956, in the RF circuit there is a 50 pF capacitor across the live ends of the two coils but in another copy of the schematic floating around, someone has written, "NOT NEEDED. DO NOT INSTALL."'

Do you mean 50pF across the coil, or 50 pF from the grid of the 12BA6 to the grid of the 12BE6? If the latter, DEFINITELY DO NOT INSTALL!! It will oscillate.

The plate coupling coil of the 12BA6 is, as far as RF is concerned, straight from plate to ground because of the bypass capacitor C5. So it is now a case of figuring how much gain is needed in the 12BA6 stage, and in fact, not very much at all. At these frequencies, the noise is dominated by the noise coming in from the antenna, so the relatively noisy 12BE6 mixer would probably be OK on its own. However, there is image interference to consider, so you need two tuned circuits. Incidentally, from the viewpoint of image, you want the BC453 operating at the top end of its frequency range. Another matter is that the more gain ahead of the mixer, the more prone the mixer is to overload, cross and intermodulation. The gm of the 12BA6 is 4.4mA/V or 4400 micromhos in American terms. So a 1 kohm load would give a gain of 4.4, 2k a gain of 8.8 and so on. A voltage gain of 8.8  is around 19 dB or so, and will vary by 6dB from 3.5 to 7 MHz.

As gain isn't too important at these frequencies something like a gain of 5 seems a good start, so a plate load of 1.1k seems about right.

The reactance of the 7 microhenries will go from about 154 ohms at 3.5 MHz to 308 ohms at 7 MHz, and so if Q is constant at 50 - a not unlikely value - the dynamic resistance will go from around 7.7k ohm to 15.4 kohm.

Now at these frequencies we can ignore the effect of input impedance of the 12BE6, which eases matters - you cannot necessarily do that at 30 MHz, though. So if we go for an average value of around 11k, then the turns ratio primary to secondary will be 3.2:1. That will increase the nominal gain of 5 to 15 because the voltage is stepped up by transformer action in L2.

If we were doing something fancy like trying to equalise gain, we might have to rethink, but lets keep things simple. This means that the coupling winding should be wound on top of the tuned winding, at the end of the tuned winding that is grounded. For insulation purposes, put a layer of Scotch tape between the two windings.

The plate resistance is in parallel with the 1kohm, so can be ignored - in this case. T'aint always so.

The antenna coil is best done as between 1/10 and 1/15 as many turns as the tuned coil, again wound over the coil at the grounded end.

Don't worry about the Q too much. You would like the coil to be about 1.5 to 2 diameters long, but it is not that critical, and the usual approach is to wind for the inductance. In the older days, manufacturers of coil forms and the ferrite or dust iron parts to go with them would give a 'K' value, where the number of turns could be calculated. You don't have that option, though.  So on your chosen former, wind 20 turns, close wound, and find with the GDO where it tunes with 100pF across  it. Run the slug in and out to get the range of inductance. Now the inductance is proportional (nearly enough) to the square of the number of turns, so you can now figure out the number of turns you need - and on a slug tuned former, you've got some leeway anyway.

You can get a feel for the Q using the GDO. Measure the grid current change as you go through resonance, using a loose coupling between the GDO coil and coil under test. Now find the two frequencies where the change is half as much as in the dip, and calculate the difference. The resonant frequency divided by the difference is, very approximately, the Q.

Where you have two tuned circuits on the same former mutually coupled, as in an IF transformer, it makes a difference if the slugs are close together  - the 'inside' tuning position - or apart, which is the 'outside tuning' position.  Doesn't matter here.

None of this should be too critical, though, except getting the crystal frequency right. That means two crystals, one about 4MHz - they are common and cheap - and one about 7.5MHz. It might be easier to make the first oscillator variable too. On 3.5 MHz, you will tune up to 3.8MHz: to go up to 4MHz, you need a crystal around 4.2 MHz. You would probably be better using a tuneable oscillator and keeping the BC453 up around 450 to 500 kHz and use it for bandspread.

Hope this helps

73

Peter G3RZP
Logged
KB1WSY
Member

Posts: 724




Ignore
« Reply #11 on: July 26, 2012, 10:09:29 AM »

Do you mean 50pF across the coil, or 50 pF from the grid of the 12BA6 to the grid of the 12BE6? If the latter, DEFINITELY DO NOT INSTALL!! It will oscillate.

Partly because of the confusion over this 50 pF capacitor, I subscribed to the official CQ magazine archive yesterday and downloaded a copy of the original article published in January 1956. Here 's part of the schematic:

http://tinyurl.com/c5h8ytp

As you can see, it shows the 50 pF capacitor C6 connecting the plate of V1 to the grid of V2. However I also have a later samizdat "reprint" of the article, on which someone has written, "DO NOT INSTALL" C6. And I won't....

Your practical advice about winding these coils is priceless! I feel so much more confident about it now, and will learn some valuable radio theory and practice along the way.

None of this should be too critical, though, except getting the crystal frequency right. That means two crystals, one about 4MHz - they are common and cheap - and one about 7.5MHz. It might be easier to make the first oscillator variable too. On 3.5 MHz, you will tune up to 3.8MHz: to go up to 4MHz, you need a crystal around 4.2 MHz. You would probably be better using a tuneable oscillator and keeping the BC453 up around 450 to 500 kHz and use it for bandspread.

I already have two crystals but at the other end: 3300 KHz and 6800 KHz. Since acquiring them, I've been given advice by several people (including you I think, at one point) that images are less serious if you heterodyne with crystals whose frequencies are above, not below, the tuned band. I think I'll use the crystals I have ... and see whether images are a serious problem.

For the moment I want to keep things simple, so would rather not use a tuneable oscillator. But again, it's an option if I find that the performance just doesn't cut it.

Thank you so much, once more.

73 de Martin, KB1WSY
Logged
KB1WSY
Member

Posts: 724




Ignore
« Reply #12 on: July 26, 2012, 10:40:14 AM »

None of this should be too critical, though, except getting the crystal frequency right. That means two crystals, one about 4MHz - they are common and cheap - and one about 7.5MHz. It might be easier to make the first oscillator variable too. On 3.5 MHz, you will tune up to 3.8MHz: to go up to 4MHz, you need a crystal around 4.2 MHz. You would probably be better using a tuneable oscillator and keeping the BC453 up around 450 to 500 kHz and use it for bandspread.

To make absolutely sure that I understand the image issue. If I tune at the top end of the BC-453's range (say, 450 KHz) this would mean that images would be separated by at least 450 KHz. If I stick with the crystals I already have, the images could be as little as 200 KHz apart and therefore there could be a lot more of them. Correct?

73 de Martin, expert in dumb questions, KB1WSY
Logged
G3RZP
Member

Posts: 4492




Ignore
« Reply #13 on: July 26, 2012, 02:00:25 PM »

Martin,

yes , that's right about the images. In general, high side injection (xtal high side) produces fewer spurious responses than low side injection. 200kHz means an image 400 kHz away and you won't get a lot of rejection. With high Q coils, it could be possible to produce some image reject circuitry, but it is rather complex and I wouldn't even consider it without a decent set of lab equipment and fair bit of experience.

The 50pF capacitor will make the gain difference between 80 and 40 even greater. Leave it out.

With those crystals the images are in the marine and aeronautical bands, so hopefully won't be too much trouble. A good rule of thumb is that the IF should be at least 5% of the signal frequency.

Although the exams here for a ham licence have a practical test, I doubt even the full licence exams go into the practicalities of actually designing a radio and thus really understanding how it works. Maybe if more amateurs went down your route of building, asking and learning, there would not be quite so many dumb questions asked here as there are - by Extras! More power to you....

73

Peter G3RZP



Logged
KB1WSY
Member

Posts: 724




Ignore
« Reply #14 on: July 26, 2012, 02:43:24 PM »

Although the exams here for a ham licence have a practical test, I doubt even the full licence exams go into the practicalities of actually designing a radio and thus really understanding how it works. Maybe if more amateurs went down your route of building, asking and learning, there would not be quite so many dumb questions asked here as there are - by Extras! More power to you....

It is quite common in the U.S. to hear that the exam has been "dumbed down." After a 35-year professional life in non-scientific pursuits (international journalism, followed by musically oriented graphic design) I passed the Technician and General tests six months ago and have to say that I gently disagree with that assessment. Among other things, the General test has some questions that require you to work out resonant frequencies, including square roots and pi symbols and what have you.

I studied for the test in full readiness for answering such a question and brought along a nice scientific calculator to ease the task. In the end however, the few math questions on the test were so easy that I could do them in my head; along the lines of "what is the resistance of these two identical resistors if wired in parallel?" But I *could* have been faced with a harder question, if you look at the question pool. I studied for the test by intensively reading the exam manuals, which are full of radio theory. The problem is not the content of the exam per se, but the multiple-choice format coupled with the publication of the entire question pool in advance. This makes it possible for anyone with a reasonably good memory to just memorize the answers and not really grasp what they are learning. I didn't do it that way, but I could have.

I took, and passed, the British City and Guilds ham test in 1971 (I got a "1" in the "radio regulations" and a "2" in the "radio theory"). IIRC it was quite hard, with plenty of mathematical questions and circuit knowledge beyond simple "what is this component?" questions. Also, but here my memory fails me, I think it wasn't multiple choice. You had to calculate the answers and stick your neck out and give your answer without selecting from a list. I think you had to draw simple circuit diagrams. You are of about the same generation, can you remember whether I am right about this?

Within the next few months I hope to take the U.S. Amateur Extra test which is billed as having a lot more mathematics and theory. I'm looking forward to that a lot. I learned (or, re-learned) a lot of stuff from the Technician/General test manuals from ARRL and it will be good to learn more, and revise what I supposedly know already. In terms of maths, my British level is "add maths" (the old "A/O level") so I went as far as elementary calculus and so on. This is easily enough for most of the problems I am coming across in ham radio.

Radio theory is actually intricate and fun. Beyond the simple Ohm's Law stuff, I must admit that I find the AC/RF-related properties (such as impedance and reactance) very confusing but haven't lost hope of fully grasping them some day. Those of you who have formally studied EE have an enormous advantage in having learned that stuff from first principles, building everything up step by step. The others, like me, get plunged into it at the end of the process rather than the beginning....

73 de Martin, KB1WSY
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!