Lash up Direct Conversion Receiver

[Skywave]

O.K. Lawrence: read and understood. I'll take a temporary diversion from the main topic of this thread to talk about and enquire of your aerial, if that's O.K.

Considering what you have reported to have received on your test-bed receiver, I think that what you have heard with that aerial is quite remarkable. So this is where I get a bit technical. We both know that in order to hear weak, distant stations on the short-waves, propagation permitting, you need more than just a crystal set    : there is a 'minimum spec.' that is required. Having said that of course, if the signal isn't arriving at the set from the aerial with enough sheer µV, and also at a low-noise site, it wont be heard. Now I'm not decrying your test-bed radio: a quick look and think about it clearly shows that is meets that 'minimum spec.' So I reckon that a significant contributor to your success in hearing those weak and distant stations is your location: close to the sea and probably soil that is a relatively good conductor.

And all that leads me to enquire:
(a) how close are you to the shore-line?
(b) what is your height a.s.l.?
© what are the brief details of the earthing system for your aerial?
(d) do you use an A.T.U.?
(e) Using that aerial, what is your typical reception with your other radios - like the SX28?

Just curious: aerials/ earthing and consequent s.w. reception is a real interest of mine.

Al.

[pwdrive]

Hi Al, I will start this in the Military and Amature Radio section if that's ok.

Lawrence.

[Mark R]

Lawrence,
This thread has been an eye opener. I really wish I had the confidence and time to do something like this.
Between you and Al, I am lost for words!

thank you for sharing it.

cheers Mark

[Skywave]

Hi Al, I will start this in the Military and Amateur Radio section if that's ok.
Lawrence.

Fine; excellent idea: keeps this thread on-topic. I'm going to the Military and Amateur Radio section now . . . . see you there . . . .

Al.

[pwdrive]

Lawrence,
This thread has been an eye opener. I really wish I had the confidence and time to do something like this.
Between you and Al, I am lost for words!

thank you for sharing it.

cheers Mark

Hi Mark, no confidence required, you just have to be nuts...

Lawrence

[Mark R]

I must admit Lawrence I am a bit nuts, but would never have thought to attempt this!

cheers Mark

[pwdrive]

I must admit Lawrence I am a bit nuts, but would never have thought to attempt this!

cheers Mark

Hi Mark, it's nothing complicated, it is basically a mixer as you would find in any valve/transistor superhet followed by a broadly tunable IF (there is no IF amp as such) in the form of a regenerative detector which tunes a much narrower band of frequencies than the traditional regen receiver would tune, this makes the designing and building the regen detector which is always the critical bit a lot easier and you can then end up with a constant tuning rate and good regen frequency stability irespective of the band selected to tune.
If using an unganged oscillator you also have the option of the usual Flo = Fs + IF or the less popular Flo = Fs - IF (down to a limiting frequency) which can sometimes be a help on strong unwanted images, choose the IF to suit, at present the test receiver is running on an IF center frequency of 2.25 Mhz, the regen detector tunes plus or minus 250 Khz either side of the center frequency ie: 2 to 2.5 Mhz, this means that you can tune the whole of the SW spectrum in nice easy to manage 500 Khz chunks with a stable regen det. the tuning rate and stability of the regen detector remain unchanged which ever frequency range you want to tune.
Ie: to tune the 40 meter amature band (7 - 7.5 Mhz) the Osc can be set to 9.5 Mhz or to 5 Mhz the regen detector tunes from 2 to 2.5 Mhz therefore the difference frequencies are 7 to 7.5 Mhz which is what we want to listen to.
Try it sometime, valves or FET/Transistor, makes no difference. You can get good results without an RF amp if you want a simple job.

Lawrence.

[pwdrive]

The band conditions on 40 were really bad this afternoon so I did a prelim. comparison test with the Regenerodyne as it is so far and a Murphy B40D, both aerial inputs were connected to my temp. wire aerial, not a lot on the band but those transmissions that were were very weak and well into the noise.
I tuned the band for about 3 hours, I have to say that whatever was resolvable on the B40D was also resolvable on the regenerodyne.
The received signals were a bit clearer on the B40D as I had the xtal filter in, without the filter I would say they were about the same bearing in mind that the B40D only has a tiny speaker which cuts out a lot of the mid to low range noise, the regenerodyne speaker is a standard 6" job.
I have noticed that whatever tunable IF frequency is chosen for the receiver there are good points and bad points,
The main bug at the moment is that any strong transmission within the spectrum of the IF's passband can sometimes appear right where you don't want them even with the aerial disconected and the aerial terminals shorted out, it is probably due to direct pickup by the regen det tuning coil itself, hopefully if this receiver gets built properly then screening will sort out a lot of the problems in that department.

Lawrence

[Skywave]

Quote, pw_drive: "it is probably due to direct pickup by the regen det tuning coil itself".

Certainly sounds like that's the reason. After all it only takes a few pF of 'hidden' coupling to produce that symptom.

Incidentally, have you retained that R.F. choke in the anode of the R.F.Amp., or did you implement my suggestion of replacing it with an R.F. transformer with a variable-capacity tuned-secondary?

Al.

[pwdrive]

Quote, pw_drive: "it is probably due to direct pickup by the regen det tuning coil itself".

Certainly sounds like that's the reason. After all it only takes a few pF of 'hidden' coupling to produce that symptom.

Incidentally, have you retained that R.F. choke in the anode of the R.F.Amp., or did you implement my suggestion of replacing it with an R.F. transformer with a variable-capacity tuned-secondary?

Al.

Hello Al, yes I'm pretty sure it's just stray pickup, it does not make to much of a problem, I feel sure when fully screened that it will be reduced by a lot or maybe even eliminated. I would be to embarrassed to post a picture of this mess but I gues you can imagine what form it takes.... coils hanging on wires everywhere...!
I still have the psuedo RF choke in the anode of the RF amp this is then capacitor coupled to the primary winding of the tuned circuit feeding G3 of the mixer valve.
The reason for this at the moment is that the coils that I found and that I am using all have a common pri/sec cold end ie: 3 connections therefore I cannot feed the primary with HT. I have a spare former for one of these coils so I might have a go at winding one with an isolated pri/sec ie: four connections and then connect the primary as the anode load in the RF amp. Eventually I will wind all new coils as the ones I am using (RF grid tuning & Mixer grid tuning differ a bit in their inductance values, I have managed to get them tracking "reasonable" but you have to remember that this whole test receiver is built from junk bits and I only have a limited number of resistor and capacitor values. I have the full coil winding details for all the B40 and also all the HRO coils which I can use as a guide when designing my own coil set as well as numerouse web based tuned circuit calculators (remember log/antilog tables?)
In all that I am pleased with the results so far as I am trying to design this thing so the controls do what they are supposed to do with the right operating feel to them which as you know is important for SWL especially on the Amature Bands.
Interestingly the Murphy B40D I have just found out uses a similar coupling arrangement from it's 2nd RF amp to the Mixer grid tuned circuit, they do not use a choke however but two carbon resistors in parallel, I think the parallel arrangement is just for dissipation purposes and I am guessing that a resistive load is better than a choke considering that the B40D has to tune from MW up to 30 Mhz. Interstingly the HRO uses high inductance wave wound primaries designed to self resonate just outside the low end of the band being tuned in order to provide gain compensation at that end of the selected band, later on Hallicrafters hit on the same idea, the primaries in the B40 are only a few turns though.

Lawrence.