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Messages - Albert H

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The RF Workbench / Re: Another helpfull site for making RF
« on: November 10, 2018, 1229 UTC »
I've experimented with a couple of the cheap Chinese DDS boards, and have had endless problems programming them - either with PICs or Atmel chips.  Most of the software that's around the 'net is either written for DDS ICs I've never seen, or just doesn't work!  I'd be very interested in your programmer software and details of the "cheap" DDS board you use.

The CARDINAL programmable crystal oscillators look like a good, cheap solution.  I wonder if I can get some "blanks" and programme them myself....?

I still favour my cheap 74HC CMOS approach, though.  The parts are dirt cheap, widely available, and if you're really anal about frequency accuracy, you can phase lock your PLL reference to a "standard" like the BBC 198kHz (accurate to 14 digits!) or other reference frequency station.  Incidentally, I use the 198kHz off-air to control the reference oscillator of my frequency counter, so when it reads 62XX.000000kHz, I know that the rig's bang on frequency!

Incidentally, I've been playing around with a Lulu of a rig on the bench, and 10 Watts carrier / 40 Watts peak is now pretty easy with a "car battery" supply.  On a quick test into an inverted-V dipole here in the southern Netherlands, I got reports from Switzerland, Italy, Germany, the UK, Ireland and Spain.  Signal strengths were described as "fading a bit" and pretty weak, but generally audible, and achieved S9 in Switzerland on peaks!  I'm quite astonished at the coverage attainable by such a simple (and cheap) set-up.  I used to just do Medium Wave (often through a -wave "tower-block sloper"), and got some great daytime results on pretty low power, but now I'm hooked on SW and will definitely put on something reasonably powerful over Christmas.  I had some big FETs arrive in the post this morning, and my target is 250W carrier / 1kW peak - just to prove that it can be done cheaply. 

I'm considering ways to modulate the 250W beast, and PWM looks favourite, but I'm tempted to go for Outphasing - I have an idea for a modulator that generates square waves at 135 to each other (derived from different outputs of a 8 CMOS IC).  These will provide the references for a couple of 4046 VCOs, which will be frequency modulated in phase opposition to each other by injecting the audio into the loop filters.....  This makes the PAs relatively simple - it's just the output bridge that gets tricky.  I've built outphased rigs before, but only ever with pairs of 807s or 813s in the outputs!

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The RF Workbench / Re: Another helpfull site for making RF
« on: November 10, 2018, 0117 UTC »
Hi Stretchyman

I've been using the '240 as a driver for IRFL FETs for years.  I usually use a CMOS PLL (74HC4046 for the phase comparator and the VCO, 74HC4060 for the reference, and diode-programmed 74HC4040s for the pre-set divider) and a standard crystal (usually 4 MHz because they're cheap) and can generate any frequency from <2MHz up to around 18 MHz in 5 or 10 kHz steps.  The synthesiser costs around a Fiver to make and avoids the need for expensively cut crystals (I got fed up with being asked what I wanted the "unusual" frequency crystals for!).  I've also found that it's a good idea to run the VCO at 2f, and use a 74HC74 to give an accurately 1 : 1 squarewave into the '240 buffer. 

I ran out of '240s and used a 4049 as the driver on medium wave.

I'm still messing around with various ways of modulating my rigs - for lower power (<10 Watts carrier), I've been using the TDA2040 as a "power op-amp" through a choke on to the drain of the output FETs.  I have a simple feedback compressor before the '2040, and have carefully chosen the feedback and input capacitors to shape the frequency response.  There's also a simple clipper circuit to handle the overshoots from the compressor to prevent splattery overmod.  There are even a couple of blinking LEDs to show you when the mod pot is about right - red for peaks, yellow for loud and green for OK.

The final issue is a simple and effective antenna for 6.2XX MHz and a simple matcher.  The rig itself is pretty much a "throwaway" item - if it gets removed by the authorities, there'll be another to replace it the next weekend!

I'll put the details of the synthesiser up here for everyone to use.  It saves a lot of headaches!

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The RF Workbench / Re: Another helpfull site for making RF
« on: November 07, 2018, 1213 UTC »
Cool, QRP!  That's a new site for me.  I see they have the Steve Quest transmitter there.  I feared that one was lost to the ages.  Thanks!

The site is (mostly) worthless.  It's just a collection of various diagrams from all over the 'net, many with errors so they won't work as drawn, and most of the higher frequency designs will be layout-critical, and there are no layouts provided.  Whoever put that waste of bits on the 'net doesn't have a clue about electronics - he's just collected stuff and put it on "his" blog.

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FM Free Radio / Re: Isn't it rather easy to get caught?
« on: August 30, 2013, 2035 UTC »
Given the way FM in the 88-108 Mhz band propagates, wouldn't it be very easy for the radio authorities to zero in on an FM pirate?

You'd be surprised how difficult it can be!  Over here in Europe, we NEVER broadcast from home - it's easy to put a transmitter, car battery, MP3 player and an aerial up a high tree on a hill and get plenty of coverage with just a few tens of Watts.  Many of the land-based FM pirates over here use UHF, microwave or even wireless ethernet links from studios to transmitters.  The transmitters are usually at the top of a housing "project" - easy access with standard Fire Brigade keys!  Some of the stations in major cities use hundreds or even thousands of Watts.  Many are technically indistinguishable from the "legal" stations.

Be bold!  Don't mess around with two or three Watts.  Get 40 or 80 going, and get some sensible coverage.  Get your modulation quality and deviation right - audio limiting is essential.  Make certain that your transmitter is "clean" - no spurs or harmonics - that precludes the use of ANY of the BA1404, BH1415 or BH1416-based junk.  Don't buy the Chinese crap off Ebay either - they just cause massive interference!  If you interfere with other services, you DESERVE to be caught and heavily prosecuted, so make sure you don't!

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