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Author Topic: Novice Q5er -- Coil Help Sought  (Read 7886 times)
K3ANG
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Posts: 177




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« Reply #15 on: July 26, 2012, 10:56:23 PM »

PDF copies of the Langford-Smith book Radiotron Designer's Handbook, Fourth Edition,
'Radio Receiver Design' Volume 1  by K.R. Sturley and
The Technique of Radio Design, by E. E. Zepler

are found here

http://www.tubebooks.org/technical_books_online.htm

Some of these PDFs are huge.
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G3RZP
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Posts: 4465




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« Reply #16 on: July 27, 2012, 12:04:01 AM »

Langford-Smith is a HUGE book - over 2 inches thick, and small type, too. I'm amazed anyone took the trouble and time to scan it.

Martin, the Radio Amateur's Examination you took was like the one I did - an old fashioned exam with no multiple choice. Multiple choice exams are often referred to here as 'Vote for Joe' exams.

One problem I have seen with the US Extra exam is that some questions do not actually offer the correct answer. An example comes with resonance. We happily use resonance as being where XL = XC, and for most practical purposes, that is so. Not low Q networks, though, such as the pi network on the input of a grounded grid power amplifier. It is true for series networks, bu not parallel ones, although the difference is usually negligible. Alternatives for parallel tuned circuits are maximum volts across the circuit, maximum circulating current and maximum impedance, all of which if you do the analysis give marginally different answers. The only definition that holds for both series and paralle is that the applied voltage and current are in phase, i.e. the power factor is 1 and the circuit looks like a resistance. So XL=XC is an approximation, but it doesn't hurt to teach the  V and I in phase.

Like here, in the intermediate exam, the feed impedance of a dipole is 50 ohms - because, it is argued, most dipoles are low enough that that is true. In the Advanced exam, it  becomes 75 ohms. Strictly, even that is wrong - a true half wave dipole is 73 ohms with a small series inductance.

So the simplification doesn't matter and it's only a hobby etc.......until you hit an area where it does matter, and having been taught the correct answer in the first place and how and when to use the approximation is much easier.

BTW, as far as math is concerned.......I did Laplace transforms in 1969. Had to look it up to use it the only time I ever needed it, which was 1985. Never have had to dig deep with Fourier, not used much calculus and yet designed radio and radio systems pretty successfully over the years.....

Good luck with the Extra class exam - in some cases, you'll have a better chance with a good memory of the answer  they want than anything else!

Peter G3RZP
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N2EY
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Posts: 3877




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« Reply #17 on: July 31, 2012, 02:38:11 PM »

Been out of the loop a while. Couple of comments:

On the coils:

All you really need are two coils of about 10 uH with a link of about 1/10th the turns. Plastic pill bottles make good forms, and enameled wire of about #24 close-wound will do the job. Exact inductance, coupling and such aren't all that critical; just keep the coils away from each other.

Remember that the converter uses a 12BA6 amp and 12BE6 converter. You don't need a perfect match and the last dB of noise figure unless you have a VERY quiet location for 80 and 40 meters. The goal is to get the thing built and working; you can always try different coils later. Since you are not ganging the caps, they don't need to be perfectly identical, either.

Do NOT worry about bandpass and such! You will have to adjust the converter tuned circuits as you tune across the band; this is normal and actually a good thing because you want as much selectivity before conversion as possible, for image-rejection purposes. Remember that this is a simple, basic setup to get you started; improvements can always be made later.

As for image rejection, it all depends what you want the rx to do most of the time.

Say you want to cover 80 and 40 meters,primarily for CW, but with as much 'phone band coverage as one xtal will allow.

 
Suppose you choose a 3300 kc. xtal. 3500 kc will then fall at 200 kc on the BC-453, 3600 at 300 kc, 3700 at 400 kc, etc.

But where will the images be?

With a 3300 xtal and the BC-453 tuned to 200 kc, the image will be at 3100 kc. IOW, the tuned circuits in the converter have to accept 3500 but reject 3100.

Now suppose you choose a 4050 kc xtal. 3500 kc will then fall at 550 kc on the BC-453, 3600 at 450 kc, 3700 at 350 kc, etc.

And the images will be farther away: With a 4050 xtal and the BC-453 tuned to 550 kc, the image will be at 4600 kc. IOW, the tuned circuits in the converter have to accept 3500 but reject 4600.

Since tuned circuit response is a matter of percentages, rather than absolute number of kc., you can see that the image rejection is better if you choose an xtal that has the BC-453 tuning higher. The effect is even more pronounced on 40 meters.

This does not mean xtals at 3300 and 6800 won't work. It simply means that they don't give the best image rejection on the low end, which is where the CW is.

Since the converter changes bands by swapping crystals, and there are no tuned circuits in the oscillator, you can try all sorts of xtals in it with no problem. If a suitable xtal shows up, (say, 7500 kc.) just give it a try.


The mental math required in using the BC-453 dial soon becomes second nature, btw.

73 es GL de Jim, N2EY
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KB1WSY
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Posts: 715




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« Reply #18 on: July 31, 2012, 05:22:52 PM »

All you really need are two coils of about 10 uH with a link of about 1/10th the turns. Plastic pill bottles make good forms, and enameled wire of about #24 close-wound will do the job. Exact inductance, coupling and such aren't all that critical; just keep the coils away from each other. ...(snip)

Do NOT worry about bandpass and such! You will have to adjust the converter tuned circuits as you tune across the band; this is normal and actually a good thing because you want as much selectivity before conversion as possible, for image-rejection purposes. Remember that this is a simple, basic setup to get you started; improvements can always be made later.


Apart from the 3300 and 6800 xtals that I already have, I've now ordered 4050 and 7550 xtals so I can compare/contrast the two options.

Concerning the coils, I will do it in such a way that experimentation/adjustment is easy, even after the project is completed. I have lots of plugin coil forms as well as coil form bases on which I can mount shielded coils ... it would be fun to play around with them. I take your point about selectivity. In any case, as you said, the main thing is just to get it up and running and the finer details are not a big deal.

It looks like I may be concentrating on 40m to start with because the plot on which I live is perfect for a 40m dipole but there is nothing like enough room for an 80m dipole. (I am thinking of eventually putting up a separate "compromise" sloper for 80m, tuned to a low SWR at about 3.525 MHz). The aim being to operate without an antenna tuner. A physical orientation on 40m is possible such that the main lobe would be at 60 degrees from north, which on an azimuthal map from my location is prime for U.S. East Coast and U.S. South, and European DX. The dipole would be at about 10 or 15 meters above ground depending on how high my landlord (a former ham) allows me to go (there are some nice trees in prospect). I believe that means it would be quite directional. While I'm about it, I will probably make it a fan dipole (parallel dipole) by adding a 20-meter band dipole underneath, even though it may be a while before I have a suitable 20m receiver.

I too will be somewhat out of the loop, for the next month or so, as my youngest daughter is preparing to go to college in Europe plus we have lots of work in our graphic design business. (Good news for us, but temporarily bad news for my hobby.) Will try to get building in earnest in September. Will try to do a bit of coil experimentation meanwhile, using my trusty vintage test gear!

73 de Martin, KB1WSY
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G3RZP
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Posts: 4465




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« Reply #19 on: August 01, 2012, 02:13:30 AM »

Martin,

Send me your email address: I have some published material which may help.
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KB1WSY
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Posts: 715




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« Reply #20 on: August 01, 2012, 01:02:39 PM »

I just thought of a potential problem with the new xtals I ordered. With 4050 and 7550 a portion of the tuning range of the BC-453 will coincide with commercial AM (medium wave) radio here in the U.S. and I just found that there are strong AM signals on 530 and 550 in my area (they just sound like a loud hum, curiously, without any voice modulation). Even though the BC-453 is being used as a tunable IF and not a front end, I can imagine there could be some breakthrough. So I am attempting to change my xtal order so that the xtals will be 50 KHz lower.

73 de Martin, KB1WSY
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KB1WSY
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Posts: 715




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« Reply #21 on: August 01, 2012, 04:00:32 PM »

OK it's done, xtal order changed to 4000 and 7500.

73 de Martin, KB1WSY
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KB1WSY
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Posts: 715




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« Reply #22 on: August 02, 2012, 06:45:32 AM »

Looking forward a bit. My pending BC-453 converter should be reasonably image free on 80m and 40m because the BC-453, used as an IF strip, is more than 5 percent of the tunable frequencies. On 20m serious image problems can be expected though: the BC-453 even at the top of its range is about 500 KHz which is only 3.5 percent of 14 MHz.

So I could use the existing 80m converter itself as an IF, right? If I leave it set up for 80m and then put an additional, crystal-controlled converter in front of it, injecting a signal of 10.5 MHz or 17.5 MHz, I would end up with a triple-conversion, reasonably OK receiver for 20m CW don't you think?

73 de Martin, KB1WSY
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G3RZP
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Posts: 4465




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« Reply #23 on: August 02, 2012, 10:12:32 AM »

The selectivity is a bit late in the day, so you would need to keep the gain of both the 80m and 20m converters as low as possible consistent without losing sensitivity.
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