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Old 25th Nov 2017, 12:59 pm   #41
Andrew2
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Default Re: Another Pantry MW Thing

I'd guess the 4608 KHz one is right, as it gives 9 KHz when divided by 512. The 5120 Khz one will give 10 KHz for our cousins in the US.
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Old 25th Nov 2017, 6:42 pm   #42
Philips210
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Default Re: Another Pantry MW Thing

Thanks for explaining that Andy. There's also the option of using a 9.216MHz crystal by altering the the position of JP1. There seems to be more of those crystals available than the 4.608MHz type. CPC, my normal supplier, don't stock either type but ebay came up trumps again.

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Old 25th Nov 2017, 8:57 pm   #43
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Default Re: Another Pantry MW Thing

Those 9.216 ones seem to get everywhere. Every time I'm looking through my crystal drawer they spill out all over the place!
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Old 25th Nov 2017, 10:46 pm   #44
kalee20
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Default Re: Another Pantry MW Thing

Quote:
Originally Posted by Karen O View Post
Yes Dave, it is standard to express a network as A+jB where A is the ohmic resistance and B is the reactive (capacitive/inductive) component, i.e. as a series combination. Short aerials are almost all capacitive with a tiny bit of ohmic (the 'radiation resistance').
I'm definitely with this, yes! As a complex impedance, Z = R + jX is the simplest way to express it.

But - apart from that it may not be convention - it's equally correct to express a network as a complex admittance, Y = G + jB where G is the in-phase component (conductance), and B is the quadrature, reactive component (susceptance). As a parallel combination. And for a short-wire, G will be very low (equivalent to high resistance).

The big advantage of this is that because the components are in parallel, it is possible to measure the voltage applied to G, whereas the series case, you can't.
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Old 26th Nov 2017, 2:42 pm   #45
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Default Re: Another Pantry MW Thing

4608 kHz gives 9kHz steps for the Medium Wave in Europe (and most of the world) 5120 kHz is used for the Americas, where they have 10 kHz channel spacing up the Medium Wave Band.

Hope this makes things clear.
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Old 26th Nov 2017, 10:50 pm   #46
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Default Re: Another Pantry MW Thing

Quote:
Originally Posted by G0HZU_JMR View Post
Can you give details about the 5 turn loop antenna. eg the wire diameter and the expected ohmic loss in the 5 turns and also what bandwidth you get from this antenna.
I'll attach the photos tonight and I'll do a bandwidth measurement. It uses 60 strand Litz. It has an input matching network built into its base to get the input impedance close to 50R. All of my pantry transmitters have 50R output impedance RF amplifiers.
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Old 27th Nov 2017, 2:07 am   #47
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Quote:
Originally Posted by G0HZU_JMR View Post
The only experience I have of this stuff is from a few quick tests of the single turn loop and it definitely wasn't 'hopeless' for pantry use.
Just to clarify on this point, I didn't actually say that the 1 turn loop was hopeless (I also know people have tried these and reported good results), just that the calculations (equations) for the radiation resistance of loops vs whips suggested a loop in general would be poor, but as I pointed out those don't take into account the much higher rms current in the conductors which comprise the loop at resonance, compared to the whip (unless the whip drive voltage is very high).

I also pointed out that the radiation resistance of a 5 turn loop is 25 times higher than a 1 turn one (as per the equations), which is obviously a helpful feature, but a 1 turn loop, if a good size could work surprisingly well too, under the right circumstances with good driver.
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Old 27th Nov 2017, 9:35 am   #48
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Default Re: Another Pantry MW Thing

Quote:
Originally Posted by G0HZU_JMR View Post
Can you give details about the 5 turn loop antenna.
Ok, I just measured it. I used a loop pickup coil connected directly into the input of a 2465B scope. I had the loop antenna on my pantry TX with a VFO. The center frequency, that the antenna is tuned to is 1.48MHz.

The transmitted carrier amplitude drops down to 70% when its driven at between 1.46 to 1.465MHz and also down to 70% at 1.495 to 1.50 MHz, you should be able to deduce the bandwidth from that.

I tried another rest. Using another transmitter, this version has a fixed crystal L/O at 1.48MHz, I looked at the modulated audio carrier with 50% modulation from an audio signal generator. By 13.95kHz the modulation had dropped to about 70% of what is was at 1kHz, the peak carrier dropped by only about 10% . Some of this test result relates to the audio filtering.

In any case I have not detected any audible high frequency attenuation at all via my hacker radio, mind you I can only hear up to about 10 or 11 kHz these days, but I think the IF bandwidth in my radio would be the limiting factor, not my pantry TX or its loop antenna.

I have attached pictures of the loop, it is actually 6 turns but one is the coupling turn.

The input transformer is made from two toroids in top of each other to create a bigger core, these are Jaycar parts with a 10mm ID, about 18.2 OD and each about 6mm tall and wound to be an auto-transformer. When the antenna is peaked at 1.48MHz, it represent very close to a 50R load to the pantry TX's output amplifier.

The turns of 60 strand litz were drawn around by pull wires inserted into the PVC pipe assembly before it was glued up. As you can see its electrician's conduit. I machined a PVC base for the bottom of the junction box where the auto-transformer is to carry a brass disc with the PL259 plug connector soldered into it. This whole loop is just under 1/2 meter x 1/2 meter (small really) and its performance is outstanding.

Although the input matching network looks elaborate to get the impedance of the loop antenna to 50R, its much more difficult to get the input impedance of a whip antenna this low. It required a pot core transformer, and by the time its done the voltage at the base of the 1.5m whip gets to over 140V, more than enough to light the neon in it as shown in the two extra photos.

But there is no comparison between the whip and the loop with the same applied power, the loop wins out for transmission range.
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Old 27th Nov 2017, 10:38 pm   #49
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Default Re: Another Pantry MW Thing

Thanks. It does look very well made. I'm not very good at making stuff that neatly so whatever I do construct it will just be a lashup I'm afraid. The 60 strand Litz wire may be hard to source cheaply but I'll have a go.
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Old 28th Nov 2017, 12:16 am   #50
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Default Re: Another Pantry MW Thing

I think this is similar to what I used:

https://www.tubesandmore.com/product...-minimum-order

You could always try it with something like 1 to 1.5mm diameter enameled copper, which would probably be reasonable for experimental purposes, but not as good as the litz. If the turns were tightly bunched the self capacitance might put it out of range with the tuning capacitor values I used, so you could spread the turns a little.

The 240pF coupling value is important so the reactance of the inductive coupling elements is cancelled at the operating frequency, so since your setup could be a bit different, make that a variable capacitor and peak it for the lowest input Z at the 1.48 MHz operating frequency, it should be close to 50R.
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Old 1st Dec 2017, 9:42 am   #51
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Default Re: Another Pantry MW Thing

Recently I found a picture of an interesting loop, it was for shortwave use probably, but I don't know the exact frequency, it would have been an aviation frequency. The designers didn't spare the size (attached)
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Old 9th Jan 2018, 10:41 pm   #52
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Default Re: Another Pantry MW Thing

Hi.

I have just completed building the MW pantry transmitter from mictesters design and can report success with it. I decided to design a PCB layout for the circuit rather than stripboard. It was made simply by applying the transfers to the copper clad board guided from my pencilled design on square grid matrix paper. I'll have to get back into doing the photo etching method again as it's a lot less time consuming. My old RS UV lightbox works well so I've no real excuses not to use this method of fabricating the PCBs in future.

The pantry transmitter is working generally well but I believe the aerial and matching circuit is important to get right in order to achieve a reasonable RF output power.

My unit was powered from either the bench power supply or a regulated wall wart. Supply voltage in both cases was exactly 12V and the current taken was about 15mA. Two audio sources were used for the tests, one being my old IBM T42 laptop computer, the other a Panasonic FM/DAB radio with line audio output.

When testing the unit, I got the impression the RF output power varied considerably over the MW band. This may have been due to mismatching of the aerial to the transmitter. I found that a wire aerial of 2m length worked quite well. With this type of transmitter, the output power level is important in that sufficient power will be needed to operate radios within the confines of your home but not too much power that's going to interfere with your neighbours' equipment. Personally, I prefer it to be at a minimum power level that's usable and this seems to be the case with my unit.
I found that bypassing the first three inductors gave the highest RF output but I also found I needed extra capacitance across the 120pF trimmer.

Experimenting with the DIP switches and setting them to MSB 01101100 LSB gives divide by 108. The carrier frequency is (108+1) x 9kHz = 981kHz or 305.6 metres. This seemed to fit into a quiet part of the band and reception was quite reasonable.
I also found the audio peak indicator circuit worked very well. Any slight overdriving of the modulator showed up with the LED flickering in sympathy with the modulating signal. Backing off the 47k pot to just extinguish the LED was the best setting.

I am very pleased so far and thanks to mictester for providing us with this interesting circuit. I am going to do more detailed tests on matching the aerial to the output stage so see what can be achieved. I wondered about using a coiled loop with low RF output for close range operation.
Attached is a couple of pics of my pantry transmitter.

Regards
Symon.
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Old 13th Jan 2018, 2:25 pm   #53
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Default Re: Another Pantry MW Thing

Hi.

I was wondering if anyone else has built mictester's circuit and had any success with it? It would be good to see a few more proven examples of his circuit.

I hope to mount my completed PCB in a painted diecast aluminium box and try to get the best RF output by experimenting with the aerial and matching circuit. I am impressed with the ability to readily alter the frequency in 9kHz intervals by changing the DIP switch settings. This makes things nice and simple.

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Old 13th Jan 2018, 2:55 pm   #54
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Default Re: Another Pantry MW Thing

I haven’t got round to trying it yet, but do intend to since the selection of stations on MW is so dire now. Mine will probably be built on vero board as I don’t have any way of making my own PCB’s. I did wonder about using a Pic to select the frequency with an LCD display to show what it was set to, extra complication I know, but I like adding daft things like that! Not that I have any idea how i’d Do it though...

Regards
Lloyd

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Old 13th Jan 2018, 7:29 pm   #55
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Default Re: Another Pantry MW Thing

Before I have a bash at building it, can anyone say if they have had any hum problems with this circuit? I've all but given up on the SSTRANS and never managed to totally clear it of hum.
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Old 13th Jan 2018, 10:10 pm   #56
Philips210
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Hi Dave.

I have found there is a degree of background hum present on mictester's circuit which I put down to the fact it was some mains induced interference due to it not being housed in a metal box. Hopefully mictester will read this thread and be able to offer some advice on this.
When I have housed the circuit in a diecast box then hopefully it will be eliminated or at least reduced, we shall see. It is clearly noticeable on quiet passages or where there are breaks between speech.
Seems strange why the SSTRANS circuit should also exhibit that hum problem as well.

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Symon.
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Old 14th Jan 2018, 9:57 am   #57
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Are you sure it's not 'modulation hum', Symon? Strong, local RF getting on the mains wiring and being rectified and re-radiated by the various gadgets plugged to the wall sockets can be a major problem. We have a couple of very strong AM transmissions here and they both have a hum. My own pantry TX also sets it off.
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Old 14th Jan 2018, 11:19 am   #58
Philips210
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Default Re: Another Pantry MW Thing

Hi Andy.

You may well be right, it's something I'm not too familiar with but found something similar regarding the SSTRAN pantry transmitter in this thread:
https://www.vintage-radio.net/forum/...=139596&page=2

I didn't realise that it can be quite a problem with wire aerial pantry transmitters. Member Argus25 points out the benefits of using a loop aerial as opposed to a wire. I hope to have a go at building the circuit he included in this thread.

As a next step I'll be using a battery supply for mictester's circuit and also use battery powered radios for the tests.

Regards
Symon
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Old 22nd Jan 2018, 12:56 am   #59
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Default Re: Another Pantry MW Thing

I found little or no hum when I kept the aerial wire away from the audio source (a cheap "Matsui" DAB tuner unit). The audio fidelity is remarkable given the simplicity of the circuit.

I mostly run mine from a 15V linear mains supply - toroidal mains transformer giving 15V AC, bridge rectifier (with 1n bypass capacitors across each diode), 1000F 35V electrolytic smoothing (probably overkill) and a 7815 regulator IC (with 100n SM capacitors soldered to the legs - input and output to ground), then a 100F 25V electrolytic across the output. The output 0V rail is coupled to mains earth and the whole PSU is in a small, earthed diecast box. I also have a "gel-cell" 15V rechargeable supply which I have used when away from home.

The transmitter itself lives in a diecast box, with LEDs for power, modulation peak and RF output. There are two controls on the outside of the box - mod level and power switch. The RF leaves through a BNC connector and the audio input is through a pair of "phono" sockets. Power enters through a 2.5mm power connector, allowing connection of the mains PSU or the battery pack.

I found that with a bit more wire attached to the aerial (about 8m more), an earth stake and fiddling with the match a bit, I could get quite wide coverage on a quiet medium wave frequency with the aerial wire thrown up into a tree! The little transmitter was used - with its battery pack - at a couple of open-air festivals last year. The best results were a perfectly usable signal at just about km! This might not be quite within the spirit of the "pantry" transmitter, but it was an interesting experiment nonetheless.

Incidentally - there's no hum whatsoever with the battery supply, as long as you keep the aerial wire away from the audio source.

I intend to put my simple audio processor details up here soon. It's just a quad op-amp and a few transistors, and provides audio bandwidth reduction and a measure of compression and limiting to the audio. In practice, it just makes the signal appear "louder" and has little effect on the treble content on most receivers.
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