10/17/2020 11:05

Twinplex

 

A regenerative receiver satisfying both the least components count and the maximum performance is the 'Twinplex'. A regenerative detector stage plus an audio amplifier, which makes a '0V1' configuration. To decouple the antenna from the tuning circuit, a grounded grid amplifier has been added, thus turning the '0V1' into a '1V1'. The frontend is a 6AS6/5725 (but any triode or pentode is OK), and the '0V1' section is made by a '5814', or a special 12AU7/ECC82 (any double triode with comparable voltage amplification would do fine equally).

One of the crucial aspects with the regeneratives to achieve the best performance, is a relatively high amplification level, which in the original concept of the 20's shall be partially achieved by an inter-stage audio transformer, between the detector and the audio amplifier. This component is almost vital to get a strong audio with normal amplification level achieveable by only one 'low mu' triode (second half of the 5814. In my receiver it is connected as voltage elevator providing an audio gain of 4 (i.e. about +12 dB).

The second crucial aspect is a 'high Q' coil in the detector: the use of 'Litz' wire is the only way to get the lowest losses at RF, which raises the intrinsic selectivity of the tuned circuit at the onset of oscillation in the detector.

In my prototype (... nowadays, for me any homemade device is a 'prototype' due to lack of time which forbids to buld something showing proper look, stability and performance ...) an audio filter has been added, ahead to the audio amplifier section: featured by a low loss inductor, offers a high selectivity around 800 Hz, which allows at some extents to pick up signals even in 'pile-ups' on the radio amateurs bands.

The knobs on the front panel are: main and fine tuning, regeneration control and volume control.

On the back panel, RF attenuator and the filter switch + power supply, antenna and headset sockets.

 

 

Both top and bottom layout pictures are self explanatory. Everything has been taken from the 'radio-depot', the metal pieces for the chassis included !

 

 

 

The RF attenuator is very useful when full size antennas are used, but the receiver shows an impressive sensitivity even with a piece of wire thrown on the bench. Besides, the grounded-grid front-end offers such a separation from the antenna, that the audio pitch when listening to CW stations is little affected by the usual oscillations of the same antenna in the wind. The front-end offers only a limited voltage gain, but is essential to preserve the tuned circuit 'Q', that would otherwise be damped by a straight connection to the antenna, even with very light capacitive direct coupling, as is usually seen in the schematics.

As it is, the listening to CW stations on low HF bands is very comfortable, even though the overall selectivity against strong near signals is poor. Experiments made on another regenerative have shown that the use of a small tuned 'magnetic' loop improves dramatically the lack of strong interfering signals rejection. Others have tried the use of tuned passive loops with regeneratives, reporting very interesting results.

High impedance headsets are used with this receiver and normally I have to keep the volume at 1/4 .. 1/3 due to the very loud sound achieveable by this configuration.

A side to side comparison with a professional, surplus, receiver, shows no relevant differences in sensitivity. The AGC absence in the Twinplex makes sometimes the reception more comfortable in presence of natural atmosferic noise.

What else ? The next step will be to couple the Twinplex to the Hartley transmitter: the circle will be closed. The 'evolution' will be complete: a series of progressive steps back in the radio technology, to arrive at the point where it started. Because nothing more than a handful of triodes is what's needed ... to surf the Shortwaves ...

 

Radioman, Oct 2020


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08/03/2019 16:36

Regen in a BC221

 

In the group of friends with whom I share my radio-experiences and knowledge, there has been a recent stir in the pot with Regenerative receivers.

Mr. RegenMan threw the stone in the lake insisting on making a receiver with only 12 V on the plates. On the web there are many examples. I made mine and he made his version, but inside a recovered old BC221 Frequency Meter.

 

 

 

 

 

 

The schematic is rather self explaining, other that Mr. RegenMan used a 12AU7 pre-RF/Detector + 12BH7 audio section. The adopted tubes are not 'space charge' type, but rather meant to be used with 100 ... 200 Volts on the plates. We wanted to see how they perform at much lover voltages.

The optimization has lead to a device that can drive a normal High-Impedance headset without need of further amplification, even though the audio level would actually need some help ...

The operating point at the RF preamplifier is set by the 'rf set' potentiometer. One should find the proper setting noticing the noise raising, but not too much. The preamplifier has a chocke at the grid, and feeds the main tuning tank: this configuration, if the amplification is too much, would generate self oscillations and mixing cross products with the incoming signals.

Mr RegenMan uses only a 4 mt whip on his window, and by the whole setup we can easily carry out radio-traffic down to 3.8 MHz, being our preferential frequency around 5 Mhz though ...

Along the audio path, a parallel resonant circuit has been added, exploiting a 220/6 V transformer as inductance, to narrow a bit the very wide frequency response typical of these receivers: the Radiotelegraphy reception is better.

On some evenings, the overall performance of this receiver has comparable (if not better) reception quality of a modern 'rice box'. Again: the key of success is the lack of the Automatic Gain Control and a very minimal number of active stages, resulting in a high signal-to-noise ratio.

I can confirm the performances as I made a similar receiver, with 12 V on the plates, but with different tubes, i.e. pentodes instead of triodes on the RF path: EF805S at the preamplifier and the detector, and an ECC802 double triode on the audio section, special version of the 12AU7 (... first to come in my hand when digging in the boxes ...). My prototype is still a bit far from being properly optimized, and as I prefer loudspeaker listening, I added an external audio amp.

On these nights we use our respective '12V B+ regeneratives' to make some good QSOs ... on the Shortwaves.

Radioman, Aug. 2019


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02/22/2019 16:16

Regenerative Receiver

 

What about a '1-V-2' regenerative receiver ? '1-V-2', according to the long time tradition, means: 1 Tube RF preamplifier, a regenerative detector ('V', for Valve), and 2 Tubes in the audio chain.

In the prototype below, the RF amplifier is an EF805S, the detector is an ECC81/12AU7, with both sections in parallel (I hate to have unused tubes in a device ...), and the audio amplifier section is made by a triode-pentode, ECL84/6DX8.

Single band, from 3.5 to 4.0 MHz, the RF amplifier uses a tuned circuit, separately from the main tuning. The detector and the front end coils are not mutually shielded, but the RF tube is: it was necessary, otherwise any tuning variation in the RF section around the frequency tuned by the detector, makes the tuning setpoint of this whole thing unstable.

Soon it has been discovered that feeding a preamplified signal to a regen detector might be an overkill often, especially if long antennas are used. The receiver as it is allows to listen easily on 80..90% of the stations on the 80 mt band, by using only 5 ft piece of wire as antenna.

The detector is fed by a VR90 gas voltage stabilizer and the regeneration is set by tapping this voltage with a potentiometer. With 470 kOhm as bias resistor on the 12AU7 grid, from 1 to 2 Volts are sufficient (!) to enter the oscillation and therefore to copy CW and SSB. The greater this resitor, the greater the anode supply voltage to enter the regeneration. And less distorted tends to be the SSB reception.

Overall, the same sensitivity, stability and excellent signal to noise ratio have been possibile by this radio receiver, as compared to those achieved with 'Polyphemus'. The only substantial difference is regarding the dial resettability, which in this case is moderate, being this essentially the consequence of a regenerative detector, which is a free running analog oscillator and therefore the stability entirely depends on the mechanical rigidity of the all assembly.

In any case, it is again very pleasant and satisfying to tune in radio stations on the Shortwaves, by simple means.

 

 

 

Radioman, Feb. 2019


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11/09/2018 21:41

Improvements


The time to make some further improvements to the Hartley transmitter has come. I wanted a more stable tone when the wind blows. It's Fall at the time of writing this post ... so having a warm tube on the desk, to make some good QSO with my friends, is just perfect on this part of the year.

The antenna link made by rigid Copper wire has been replaced by two turns of coaxial cable, wound right upon the main coil. The braid is left open on one side, and grounded on the other. The central conductor is the usual link to the antenna.

It's important with a Hartley having a center tapped coil, to place the link right in this position: the ground connection of the tap brings the electric potential of the turns, here, the minimum, therefore making also minimum any stray capacitive coupling to anything around. 

By placing the antenna link here, plus electrostatically shielding it, by the grounded coax cable braid, the tone quality is now much better and the frequency sensitivity to a windy weather, reduced.


And here, the result: the transmitter, loaded into the antenna, normally waving in the air ...

Radioman, Nov. 2018

 


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07/31/2018 21:31

Ham Radio ... where are you going ?

 

Below, a very quick promenade along the 'ham radio' way in the years. From the 'I need to get on the air at all costs', of the beginnings, to the 'world is mine by just one mouse-click'.

I haven't to tell you what I consider as true Radio ... and it's pretty obvious it's not that interesting playing with a PC for controlling a 'rice box'. I did this for living enough hours each day (i.e. sitting and using a PC), and I already have someone else shouting on my back 'run to reach the goal!!!'.

Anyway, the short collection of pictures illustrate a variant among many, on the direction of the humankind.

I don't feel ashamed at all in saying ... let's stop and revert a bit our way. We are loosing too much fun ... believe me.

 

 

Radioman, July 2018


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01/27/2018 23:23

Why 'Full Spectrum' ?

 

By 'Full Spectrum' is meant a kind of radio-activity managed throughout the HF, using very low power, improvised radios & antennas, on any or on a casual frequency.

If the web is surfed looking, for example, for 'shortwaves beacon', one can find a lot of sites on this topic. Many are experimenting with very low (and sometimes even not low at all) powers, in the range of tens or hundreds of milliwatts, by running beacons on almost any frequency on the Shortwaves.

A cathegory of these experimenters rely on those portions allocated to Short Range Device applications, industrial heating, medical applications, so to reduce the impact on the spectrum and therefore reducing possible disturbancies to some other radio stations.

This kind of activity is a new frontier. By suitable programs it is possible to filter the output from a receiver so that morse code sent by a few milliwatts can be perfectly decoded across the oceans. If the power is very low, the signal would not be noticed by a professional user of a given frequency, so everything would not be harmful. It is a frontier in the sense it is being experimented and investigated the real limit of the informaiton delivery by the ionosphere, with ultra-small power.

On top of this activity, a few more points should be added. Operating on non-conventional frequencies has always been a source of attraction by many. By the same many that, at a certain point, became licenced radio amateurs.

The bands today are often overcrowded and the users are just limiting themselves to push the buttons or to play with the menus of devices looking more domestic appliances than radios.

By operating improvised 'wireless sets', like, for example, a couple of transistors, crystal controlled, as TX, and further two or three transitor as Direct Conversion RX, with a power in the range of a few hundreds of milliwatt, anywhere in the HF away from the crowd, from the QRM and the infinitely boring guys using domestic appliances, is fun ! 

The fun is increased if these operations are run during outdoor trips, and agreed with the correspondent operating in the same conditions on a distant place, hundreds of miles away. Each time from a different location and on different frequencies. Low power, with the trusty piece of wire hanging between trees or hidden in a wooden roof.

Electronic components dealers offer plenty of crystals on frequencies in the good part of the Shortwaves, from 2 Mhz up to 12 MHz, where the limit of the experiments is only the imagination. These frequencies are often 'standard', so if more experimenters of this kind will be active, there would be chances to meet on the air other radio-fellows and establish contacts.

If receivers capable of demodulating a phone signal, typically AM, are needed, a NE602 mixer + a MC1350 IF amplifier would make a reasonable device, if the homemade way is preferred. Many 'world receivers' today available on Ebay, can be purchased by a few tens of $ and have more than sufficient performance to run a reliable radio-traffic.

Schematics with 3, 2 or even 1 solid state component transmitters can be found anywhere on the Web. The components are cheap, ready to get and perfect for cut-and-try optimizations, in which they might fail in search for the best performance. No worry, they are cheap and therefore the game will still run with no dramatic financial impact.

Sadly the Shortwaves Spectrum is day after day emptier. On the other side, those empty spaces might be used, occasionally, for unconventional radio experiments for very low profile radio traffic.

It can be often noted, besides, that on the same Forums where beacons-hunters and beacons-runners exchange their info ... there is apparently no discretion, and often people tell the time, frequency and power they will run with on a certain experiment. And they sign the posts by nicknames, but sometimes even not personal names (or ham calls !).

I have always thought that this is the prelude of a revolution regarding the HF Spectrum use, having its main origin in the frustration coming from the mess currently found on the 'official' bands. It is certainly generated by the curiosity of experimenting new things, and by the courage of operating out of band.

If this practice is not abused and managed with care (i.e. listening a lot, before sending a signal on a frequency, and, again, at extremely low power levels), for sure it will bring a breath of fresh air in the world of the radio enthusiasts!

 


Radioman, Jan 2018


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07/02/2017 13:16

Another Old Picture

 


I have nothing against modern radio-stuff, apart that it doesn't appeal neither interest me. A matter of choices.

It would be too long for me and perhaps too boring for you to explain my view.

I limit myself publishing another illustration from the old days, recently discovered on the Web (thanks to whom that has published it).

Stylized. Essential.

Neverhteless deeply evocative and explanatory about what I mean by 'Radio' and what I see in it.


Radioman, July 2017


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05/21/2017 17:40

3579 kHz

 

A few months ago I was listening on the 80 mt band in search for a free tiny piece of spectrum, available for a QSO on radiotelegraphy.

The 3.579 kHz frequency is one of the preferred by Radio homebrewers because it corresponds to a common television crystal. Common means: plenty, cheap, easy to use ... And also, as a consequence, it allows easily to many to find on the air other fellows experimenting with the Radio, on that frequency.

Hoping to find a radio-fellow, that night, to share my same interest on home-making simple radios, I sent my 'CQ ...'. A gentleman from a 1000 km away, replied. And immediately after a digital signal also appeared, making the whole radio contact quite hard. But we managed to make it.

That OM told me, along that QSO, he has been often around 3.579 to resist against 'the aggressive behaviour of the digital guys', as he stated.

Along with that, I'm forced to recall that also the frequencies commonly allocated to QRP traffic are often disturbed or occupied by QRO stations, so disrupting the peace of those tiny spectrum portions, where more than anywhere else ... Radio homebrewers gather to test their creatures and/or to talk about Radio, on CW.

That OM ... I found him on many other evenings, insisting to defend the 3.579 kHz channel.

Well done Mate. You are an example to all that claim the right to have a few kHz where enjoying their radios, and that often are unable due to the poor conduct of the majority.

As a matter of fact he defended the right to operate with very minimal and low performance devices.


Radioman, May 2017


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05/21/2017 17:01

Comparisons

 

On a noisy evening, while listening to a CW Service station on HF, I made these videos.

An EK07/D2 with AGC and set bandwidth 4.0 kHz


Polyphemus 2.0 ...

 

... and again the EK07/D2 without AGC (same bandwidth as above):

>

 

Interesting. Polyphemus does not have any filter: nor in 'IF' circuit (impossible because the Intermediate Frequency is variable between 1.7 up to 3.5 MHz), nor the Audio stage. Even though, its audio response is a bit narrow due to the frequency cut of the loudspeaker.

Its overall performance looks really good (perhaps better in this particular case ...) on a noisy day as compared to an old, but professional and still well performing receiver.


Radioman, May 2017

 

 


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04/08/2017 12:21

Polyphemus 2.0


After some funny time with the first edition of a mixed regenerative/conversion receiver, where a poor ECH83 did the Local Oscillator, the RF Amplifier and the Mixer functions together, all these tasks have now been split among separate stages.

Now a '6AK5' UHF pentode is the RF amplifier, in a Tuned Plate - Tuned Grid configuration; an ECH83/6DS8 still works as Mixer + Local Oscillator (Pierce configuration, at the triode section); a '9002' triode now is the regenerative Detector and last an ECL84/6DX8 as the usual pre+final audio Amplifier.

After some rummage in my junk-box, a nice Yaesu FT-something transceiver Preselector was detected: perfect for a broadband tuneable front-end Amplifier, that allows to tune from about 3.0 up to 30.0 MHz. The Local Oscillator is rock bound on 5 bands, and there is provision for an external Oscillator also. The regenerative Detector section is tuneable from 1.7 up to 3.5 Mhz.

 

 

In comparison to the first edition of this receiver, the RF Preamplifier allows a more comfortable listening and the audio level is now much stronger. It has been observed, besides, that the same signals are now receiveable as what could be received with the former prototype: before, the ECH83 was amplifying a bit the incoming signal, while now there is a separate stage for this function. In the former prototype, then, the audio throttle was most of the time at full steam.

RF Amplifier, Mixer & Local Oscillator are all run at 90VDC, provided by a 'VR90' gas tube stabilizer (the big ceramic resistor on its side becomes pretty hot, as it manages the voltage drop from about 320 VDC from the supply). The audio Amplifier is run at 320 VDC.

The use of the BC221 variable capacitor resulted in a very smooth tuning and practically linear. Some 35 kc/s per turn are a very comfortable way of zero-beating any station!

 

 

The preselector has not yet being optimized, but the unhearting of a 18 MHz crystal in a remote region of my depot, allowed also to listen to many stations on 21 MHz band. At higher frequencies, the L.O. needs some help by additional tuning circuit, to raise the L.O. injection level at more than 10 V peak. Up to 14 Mhz band, the peformance of this receiver resulted more than satisfying and such that it allows to make also DX traffic when needed.

Still, the 'Tuning Eye' tube, configured as audio 'VU Meter', is helpful for tuning purposes. Fluctuations of the green light depends on the Volume regulation, and for peaking the RF Preselector it is just needed to reach the greatest closure of the 'green wings', driven by the background HF noise ...

The interesting thing of the Regenerodyne concept is that you have all the controls brought to the front panel and there is no need to align any fixed-frequency circuit inside, like MF transformers or whatever. Besides, the sensitivity is equal to at least a decent modern transceiver.

I have also been given an electronic audio Filter and a strong selectivity can be added if traffic is to be handled in one of the usually crowded Amateur Radio bands. If Full Spectrum operation is preferred ... Polyphoemus 2.0 is more than an excellent performer.

What else do you need ?


Radioman, April 2017


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