Which shortwave transmitters are still available?

[BlackBeard]

Hi folks,

I just quickly wanted to ask which options there are still for SW-pirates (apart from used equipment).

What I found so far:
- Stretchyman's transmitters
- the stuff from dds.gr
- a HackRF one as an exciter (?), probably not clean enough though

And of course many chinese SW-amps & filters:
https://a.aliexpress.com/_mMoqEPq
https://a.aliexpress.com/_mMFpbTa
https://a.aliexpress.com/_mK4OiO4

Are those amps useful (of course in combination with a good filter)?

[Albert H]

Some of the Chinese PAs work pretty well, though many of the kit ones arrive without documentation.

The HackRF is NOT the way to go!

Rolling your own isn't too difficult. I did a design recently for 25W carrier / 100W PEP using PDM for the modulator. The PA runs in Class E, and the modulator and output FETs run almost cold because the efficiency is so high. All the coils are hand wound on toroid cores, and the results are pretty good. We built a batch (18 of them) that went to a couple of South American pirates and two went to Australia. They use a CMOS synthesiser and a cheap, standard crystal The reports from the users have been good. One of the Australian rigs puts a better than useable signal into Thailand!

As usual, the biggest problem with shortwave AM transmission is the aerial. Wire is getting expensive, and so the days of leaving aerials on field sites for future use are over!

[BlackBeard]

Some of the Chinese PAs work pretty well, though many of the kit ones arrive without documentation.

The HackRF is NOT the way to go!

Rolling your own isn't too difficult. I did a design recently for 25W carrier / 100W PEP using PDM for the modulator. The PA runs in Class E, and the modulator and output FETs run almost cold because the efficiency is so high. All the coils are hand wound on toroid cores, and the results are pretty good. We built a batch (18 of them) that went to a couple of South American pirates and two went to Australia. They use a CMOS synthesiser and a cheap, standard crystal The reports from the users have been good. One of the Australian rigs puts a better than useable signal into Thailand!

As usual, the biggest problem with shortwave AM transmission is the aerial. Wire is getting expensive, and so the days of leaving aerials on field sites for future use are over!

Thanks Albert

Where can I buy one of your transmitters or boards?

[Albert H]

We're not releasing the boards just yet - the first ones we had made in the Far East had two mistakes on them! I'll put all the details up here soon, and a suggested layout. Our board design is single-sided, and the kind of thing you could do with a steady hand and an etch-resist pen (I like the "Edding" permanent markers).

[shuffy]

Rolling your own isn't too difficult

Agreed. BlackBeard, you didn't specifically mention AM but like Albert I'm assuming that's what you meant. Don't go for a low level exciter followed by a linear amplifier. IMO the way to go is a class D/E "PA" and PWM. If you're on 49m or above, you'll need to pay more attention to some of the tech required especially the FETs in the RF switcher but the general topology is simple. You might have seen this already but one of my favourite resources for this is actually the Class E forum on AMFone.
https://amfone.net/Amforum/index.php?board=76.0

[Nige from Brum]

I'd be interested in any boards that were available.

I managed to build the RF strip from the LuLu transmitter and got it to work, being run by a AD9 something or other DDS board and Arduino.
I never got round to finishing it, it needed the suggested value capacitor switching out for a variable, and i needed to finish the modulator scheme.

https://hfunderground.com/blog/?p=72

[shuffy]

I'd be interested in any boards that were available.

Off the top of my head all I can think of in terms of people selling boards/kits is the FAT5 which I'm sure you will have seen, and also Stretchy who sells full TXs but is generally helpful. You could try dropping him a PM on here.
IIRC a couple of people (including Stretchy) have posted layouts for various boards on amfone / hfunderground, but there are also serviceable designs built "dead bug" or similar. There's a guy called Nigel on amfone who seems to have had success with this and his RF deck design looks good to me - he's posted plenty of photos of his stuff.

[eiradioguy]

Vintronics have 10-15 watt carrier PLL TXs, tunable from 5.9 to 7.2 MHz with frequency display on the front. The rigs are made in UK. Price is 230 sterling. https://www.sjvbroadcastservices.com/
* I've no connection to this company, just passing on a site I found

[BlackBeard]

Thank you very much guys A nice christmas time to everyone!

Albert, that sounds very interesting. I'll keep an eye on your boards as well.

Thank you shuffy for the hint about the AMF-forum. I actually didn't know about that. And yes, if I will start to broadcast on SW, it'll probably be AM.

[BlackBeard]

To follow up the question: There are plenty of sw amps on Aliexpress & Ebay. Now the question: Which of them are usable? I know they use class AB circuits which is not really the way to do it but if you add a LPF and if they do work?

Albert, as far as I can see, you built the so-called "MiniPA 70" which uses the IRF530N, the "MiniPA 100" uses the MRF9120 which is said to be quite unstable.

Short review: https://ea8arx.blogspot.com/2018/06/min ... p.html?m=1

I also attached some pictures of the models currently available: MiniPA 100, the XDT-PA70 and the RF_PA_250_3_HV V200 by ZGJ. On one picture, the MiniPA 100 comes with a MRF185. Are they any good (given you add a LPF)?

[Albert H]

The PAs we tried from China were all based on IRF-series FETs. Just for the sake of amusement, I tried a low-level modulator into one of these "linears" and the signal was horrible. It was virtually impossible to get anything approaching linearity! After several hours of fruitless messing around, I decided to lash up an envelope feedback circuit, and the mod was then acceptable, but at the cost of increased complexity. I had to use a lowpass filter in the feedback path (for stability) and the setting of the degree of negative feedback was very critical. The more I messed around with it, the more convinced I became that there was little point in persisting with these "cheap" PAs.

I've gone back to "Outphasing" with pairs of PAs, driven to 100% mod with just 2V p-p of audio! The only slightly exotic components are (so far) a couple of fast comparators, and a couple of little ferrite "pulse transformers" available through Newark (AFAIR), though it's easy enough to "roll-your-own" with type 43 ferrite cores. The PAs I'm using are Class D types (based on a design for a medical amplifier) and they seem to be burst-proof. I'm going to do some more work on these over Xmas - results in the New Year!

[Gigahertz]

Saw this on my trawl of ebay if its any good to you ?

https://www.ebay.co.uk/itm/204649001291

[yellowbeard]

The circuit for that transmitter is available here:
http://freeradiotx.blogspot.com/2009/06 ... artin.html

[BlackBeard]

Thanks yellowbeard and Gigahertz! Sounds good, I think I'll give it a try

[Albert H]

The problem with that one is that it uses a modulation transformer which is probably unobtainable these days. Unfortunately, you cant use a mains transformer for modulation (even if you get the turns ratio right) since the core will tend to saturate with the DC that necessarily has to pass through it. The mod transformer I used in a recent rig was physically bigger than the whole of the rest of the rig! (It was an old "Wotan" transformer, specially made for the purpose back in the 60s!).

It's well worth looking at the Class E website - http://www.classeradio.com/. This will point you towards various alternative ways of achieving modulation. My favourite simple approach these days is to use a PWM modulated power supply for either a Class C or Class E power amplifier stage. You can get reasonably high efficiency, reducing the need for huge heatsinks, and a clean and linear modulation.

Other approaches I've employed include using "Ampliphase" - phase modulating two carriers that are offset in phase by 135° in opposite directions, then summing the two carriers at the output network (before the lowpass filter) to achieve amplitude modulation with extremely high efficiency. The additional complexity makes moderately high powered FET rigs possible, but necessitates the use of pretty sophisticated test gear for correct alignment.

[BlackBeard]

The problem with that one is that it uses a modulation transformer which is probably unobtainable these days. Unfortunately, you cant use a mains transformer for modulation (even if you get the turns ratio right) since the core will tend to saturate with the DC that necessarily has to pass through it. The mod transformer I used in a recent rig was physically bigger than the whole of the rest of the rig! (It was an old "Wotan" transformer, specially made for the purpose back in the 60s!).

It's well worth looking at the Class E website - http://www.classeradio.com/. This will point you towards various alternative ways of achieving modulation. My favourite simple approach these days is to use a PWM modulated power supply for either a Class C or Class E power amplifier stage. You can get reasonably high efficiency, reducing the need for huge heatsinks, and a clean and linear modulation.

Other approaches I've employed include using "Ampliphase" - phase modulating two carriers that are offset in phase by 135° in opposite directions, then summing the two carriers at the output network (before the lowpass filter) to achieve amplitude modulation with extremely high efficiency. The additional complexity makes moderately high powered FET rigs possible, but necessitates the use of pretty sophisticated test gear for correct alignment.

If I'm not mistaken, Dave Martin proposes to use a bridge modulator circuit or a series modulator in the "Commando TX" (the latter only if mains AC is used of course). The series modulator uses a LM741, a TIP41 and a 2N3055. For the series modulator, a mod transformer is not necessary anymore, isn't it?


Otherwise, this (http://www.classeradio.com/pwm_generator_rev_f.pdf) would probably be a suitable PWM modulator, wouldn't it be?

[shuffy]

It's well worth looking at the Class E website - http://www.classeradio.com/. This will point you towards various alternative ways of achieving modulation. My favourite simple approach these days is to use a PWM modulated power supply for either a Class C or Class E power amplifier stage. You can get reasonably high efficiency, reducing the need for huge heatsinks, and a clean and linear modulation.

I know I keep saying this but for me, it's a no-brainer. The only way to do it for usable power "in the field", as the rigs are small and light - you're going to have enough of a challenge with your antenna so why complicate things lugging masses of weight around too? All things considered, the only advantage to using a mod tranny is the power supply, but there are switch mode inverters around which will give you what you need for the class D/E PWM combo with good efficiency.

Ampliphase

It seems to me that the only practical implementation of this would be in software. Your actual efficiency isn't going to exceed the Class E / PWM designs though - and once you've developed your mod chain, you still have the challenge of that high power combiner. Probably not a favoured approach of your average pirate constructor?

[Albert H]

The Cheirix / Ampliphase system can be made to work well (I generate the carrier at 8 X frequency, then use a ring counter, selecting the carriers 135° apart. These are then used as references for a pair of PLLs, which get modulated (deviated) in opposite directions by audio and inverted audio. You're right - the output bridge is tricky, but do-able. The results can be amazing!

The other "outphasing" method I've used - with close to 96% efficiency - is a digitally generated sinewave feeding a couple of very fast comparators, the other inputs of which are fed audio and inverted audio.... The pair of outphased squarewaves are then amplified in a couple of Class D PAs and combined simply with a pair of output inductors that meet at the input to the output lowpass filter. The mod quality is astonishingly good, and efficiency is spectacular. I add envelope feedback and bandwidth limiting, and we have a small, lightweight rig that develops a lot of power and very little heat!

[Gigahertz]

The problem with that one is that it uses a modulation transformer which is probably unobtainable these days.

Why are you rambling on about transformers being unobtainable? I've shared a auction where someone had a complete rig for sale and "Yellowbeard" posted the circuit diagram if someone wanted more details about it!

The Modulation transformers aren't as hard to come by as you make out. I tagged along to a radio rally not so long ago and a trader had 6 PYE transceivers with mic and lead for £10 each with the parts needed and people was walking past them.

[Zozo]

In all honesty you can still purchase modulation transformers “brand new” from Mouser or Digikey etc, admittedly for use in valve equipment ideally, and as an output to drive low impedance speakers, but you can use the primary like an autotransformer.

Another modulation approach is “Heising'' which requires only a simple Choke / Reactor; I plan to change my mini valve TX from “screen grid modulation” to "Heising" very soon. The large 5H chassis mount chokes are available on RS, Digikey and Mouser.

There’s plenty of information on “Heising” and can be used for either valve or solid state designs, Kyle Drake’s low power AM manual shows a simple solid state design, and the ARRL handbooks have plenty on the subject too.