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|homebrew||Posted - 6 February 2014 14:58 |
Lower inductance coils, 160 UH or so, and high capacitance variables 400 to 500 PF seem to bring in the lower part of the band a lot better.
Most military and telephone sound powered elements are 300 ohm each, 600 in a series headphone set up. If you are not using a matching xformer and Benny both should improve volume and tuning sharpness quite a bit.
|raypat||Posted - 6 February 2014 20:15 |
Thanks Homebrew and Garry. There is something I don't quite understand with the coupling. If I move the coils apart so that they don't interact, i.e. turning the cap on the aerial circuit has no effect. Then what is the point of having it? I emailed Scott about the headphones and if mine are not very good for crystal radios I am going to buy a pair from him.
|gzimmer||Posted - 6 February 2014 20:30 |
If neither ATU cap affects the tuning, then the Detector coil must be picking up the signal directly (by induction or stray capacitance).
Try sitting the Detector coil vertically.
Edited by - gzimmer on 2/6/2014 9:16:42 PM
|raypat||Posted - 6 February 2014 21:56 |
Hi Zimmer. we were talking about loose coupling. The main tuning cap still tunes fine. Garry is suggesting I move the coils apart until they don't interact. I know I am missing something. Just don't know what.
|gzimmer||Posted - 7 February 2014 0:11 |
Critical coupling requires (amongst other things) that both tuned circuits have the same Q.
I'm guessing, but at very wide spacing the loading on the ATU (from the Detector) will be negligible, so the antenna coupling need only be very light.
To put it another way, because the Antenna coil is unloaded, its Q is very high (likewise its impedance) so the antenna capacitance has little effect as long as it's above some minimum value.
However, once you do load the ATU with the Detector, everything changes.
Edited by - gzimmer on 2/7/2014 3:26:42 AM
|Garry Nichols||Posted - 7 February 2014 7:18 |
By not interacting, I meant that tuning one or the other circuit does not alter the tuning of the other one. My rig used to "pull" the tuned point of the other circuit if the coupling was tighter than (what I thought was) critical. I used to set them apart such that I could peak either one on a station without needing to alter the tuning of the other stage from its peak point.
This is a little like looking through two pinholes at the world. Unless both pinholes are aligned you won't see anything. Same with the two stages. You won't hear anything, unless there is a very powerful station on the band. Tuning can be tricky.
That is why I suggested at least one calibrated dial. You can then set to a known station and sweep with the other variable (slowly) until you hear the station.
I used to calibrate both dials. I could retune fairly close to a known frequency quickly and easily. When tuning the band I would increment both dials in the same tuning direction a little at a time (with calibrations, to the next 10 kHz set point -- after a while I had most of them on the dial), then touch up the tuning(s) very slightly.
Something to watch out for with this is that you place everything in exactly the same place each time you set up to listen. Use the same length and location for antenna and ground wires. Otherwise the tuning will be altered at least a little from previous calibrations.
Did this info help at all?
Also, I was rushed in my last post and forgot to mention a few things in the headphones area.
Are you using both the mic and ear elements of your handset? Wired in series or parallel? Usually the mic elements are very low impedance and the ear elements are higher (typically 300-600 as homebrew has said). The mic elements that I use in my SPP measure only about 10 ohms DC and I guesstimate 50-80 ohms impedance. So I use a transformer to boost the impedance up closer to what the detector would like to see. More on this later, if needed.
Can you measure your two elements with an ohm meter? (It sounds like you have what we call "a sound powered phone" or SPP. These can be very sensitive, but usually need to be impedance matched to a crystal set with an audio transformer.
But, I'm guessing that you have elements of different impedance and so they cannot be used together. I always find that weak stations are much better heard with both ears being used . . . .
You may want to air your thoughts on a headphone purchase if you go that route. We may be able to provide some perspective on the wide array of phones available.
And, I forgot to mention that because the QC on the crystal/ceramic earplugs is low, if you decide to try some I advise ordering at least 3, if not 4 or 5, so you can take "the pick of the litter". Some folks have reported that some do not work at all, even when tapped sharply on the desk . . . .
But I do enjoy mine and did the entire recent nontest with them. They had to be tapped twice. Sometimes one or the other just cuts out. Other times known strong stations become weak. So it's phone tapping time!
I'm still wondering about your lack of night time DX . . . .
Edited by - Garry Nichols on 2/7/2014 7:24:41 AM
|raypat||Posted - 7 February 2014 19:22 |
Thanks Gzimmer and Garry. I think I get what you are talking about now.I will give the coupling another shot as soon as time permits. My earpieces were in fact one from the ear and one from the mic elements of that old phone but they appear to be identical in number and appearance. Anyway I have contacted Scott and on his advice I am purchasing a pair of Western Electric 509-W phones and a few diodes that I saw on his site just to play around with. So in a few weeks I should have the headphone issue sorted. I will post any results from coupling experiments when I get time to have a muck around in the shed.
|Garry Nichols||Posted - 7 February 2014 20:20 |
OK, Ray. If the elements are identical, then they may (or may not) make up a good set of SPP. But you would likely need a matching transformer, which is no big deal.
Was Scott able to give you any info on the elements? I'm curious to know if he had even heard of that maker.
The 509W is one of the best antique phones in the 2k DC range, according to a lot of folks. I have never used a pair. They are likely not as good as a good pair of SPP (which can vary according to model and condition).
I'm still wondering about your lack of DX. If you care to, very approximately, where are you located?
|raypat||Posted - 7 February 2014 23:31 |
Scott was unable to help with my phones but he did suggest I can check the impedance with a multimeter (at least I think that was what he said). I wouldn't worry about my lack of tuning DX stations as yet. I haven't really tried yet preferring to focus on improving the tuning and learning about coupling etc. I am located in Sydney Australia and I live quite close to the city proper so I have a lot of powerful locals to get past.Also I don't spend much time in the shed of a night time but that might change when I get better phones and get the tuning sorted.
|Garry Nichols||Posted - 8 February 2014 13:6 |
OK, Ray. I suspected a possible down under location when you used the expression "muck around in the shed"!
Let me clarify something so that we are all on the same wavelength (pun intended). When we measure the coil resistance of a headphone element with an ohm meter we are measuring DC resistance. The AC impedance is a different value and can be thought of (approximately) as the resistance presented to an AC signal. For any magnetic type headphones (as opposed to the crystal/ceramic earplugs I have mentioned) it will always be higher than the DC resistance.
In many cases (but not all) its value can be guesstimated by multiplying the DC resistance by 5 or 6. As an example, my antique phones measure about 2000 ohms DC and the usual impedance value for these is 10,000 to 12000 ohms.
Also, impedance is often abbreviated as "Z".
If you have an ohm meter you can measure the DC resistance of your SPP elements and guesstimate the Z. If they are about equal, wiring them in series works and yields their sum as Z. If they are about equal in value, wiring them in parallel will be about half the Z of one of them.
If they each are in the typical 300-600 Z range then the most Z you will get will be 600-1200 ohms (Z). That is way too low for a good match to the detector of a crystal set.
That little red transformer that you may have seen on Scott's site transforms 1000 ohms Z to 50,000 ohms Z (or any 1:50 ratio, provided the starting value is not wildly different than the transformer's 1000 ohms Z). That would be an appropriate match to a decent 1n34 diode and some other diode types also.
Edited by - Garry Nichols on 2/8/2014 1:17:53 PM
|raypat||Posted - 19 February 2014 3:20 |
I got my headphones from America today. Haven't had much time to use them but so far they seem to work just fine. A little uncomfortable to wear but nice and crisp and they seem sensitive so maybe I will hear my first DX station soon.
|Garry Nichols||Posted - 19 February 2014 10:14 |
OK, Ray, very good! You might eventually acquire a few diodes to compare. They can vary in performance even amongst the same type (e.g., 1N34A). I use a 1N34 that I selected from a few that I compared. Sometimes it is hard to hear a difference, but some diodes will be noticeably less sensitive.
|giftsocool2||Posted - 19 February 2014 20:33 |
hi, raypat. Congratulations
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|davidwendel21||Posted - 11 May 2014 15:51 |
Hi Folks. Just back to Crystal Radios since I retired. The last time I worked with Crystal Sets was in the late 50's when I was in Boy Scouts. Things have really changed since then!! I am really enjoying brushing up on my radio theory and looking at the websites that cater to the hobby. One quick question - I saw a page on a website where Mike Tuggle talked about his 4 wire Flat Top antenna a few weeks ago and have lost the reference. Does anyone have an idea where it might be found?? I have six pages that come up on my browser homepage that are all devoted to Crystal Radio. I think I have checked most of the links, but can't seem to find the page. Tried to Google it, but no luck. Any help would be deeply appreciated!! Thanks!!
Edited by - davidwendel21 on 5/11/2014 4:27:32 PM
Edited by - davidwendel21 on 5/11/2014 4:29:21 PM
|sky_99||Posted - 12 May 2014 20:22 |
Just a quick note to say hello to my old friends at Rap 'n Tap. I hope you are all well.
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