Messages

736

The RF Workbench / Re: Engineering Paper in 1997 devotes discussion on class E amp 13 mhZ

« on: January 25, 2018, 1844 UTC »

Times and tech have changed, but the same basic rules still apply.  Mosfet technologies aside from GaN and SiC have not changed all the radically in the last 20 years.  Sine wave drive is an old way of doing things, which is why we see wide adoption of driver IC technology now, fed by TTL level control.  You cannot use sine wave drive with the newer SiC devices, due to their extremely limited negative gate swing.  Still, at 7 MHz, a 1000pf input cap mosfet with an IC driver, I'm still seeing several watts per device of gate drive input power.  The old rules still spply, high power requires high voltage.  Some number crunching on a 2.5-5KW solid state SW TX tells me around 550VDC is going to be required for modulation peaks, and that WILL HURT you.

With great power comes great responsibility....

+-RH

737

General Radio Discussion / Re: WWRB

« on: January 25, 2018, 0757 UTC »

The same could be said of pirate broadcasting.  To do it right, and with relative safety isn't cheap either.

+-RH

738

Equipment / Re: The best undercover fm antenna on the market

« on: January 25, 2018, 0751 UTC »

Pretty sure that most commercial FM broadcast in the US is vertically polarized anyway so conforming to that makes sense if you want to attract listeners.

Not true.  Most commercial FM stations use right hand circular polarization and have done so since the late 70's.  Early FM stations were almost always horizontally polarized, borrowing from the TV practice that most human made noise is vertically polarized.  When car radios became more popular, and FM became the dominant band in the 70's, broadcasters discovered that CP could make mobile listening better, as most receive antennas on cars were vertically polarized.  Since CP will excite both vertically and horizontally polarized antennas, and has the advantage of better building penetration and lower multipath distortion it its still used today.  The majority of stations that broadcast exclusively in the vertical mode in the US are those stations which were built on the low end of the band (88-92 MHz) and within a channel 6 (82-88 MHz) protection area, and utilized vertical polarization to protect a horizontally polarized television signal.

It's not all roses.  CP antennas require twice the input power than a conventionally polarized one to achieve the same level of ERP.  Early ring and stub antennas used for CP service were also very prone to icing events, as their design was pretty narrowband.

+-RH

739

General Radio Discussion / Re: Article; The FCC is still open for business

« on: January 23, 2018, 0908 UTC »

Ah yes, our gubermint at work...

+-RH

740

General Radio Discussion / Article; The FCC is still open for business

« on: January 22, 2018, 1540 UTC »

From an email newsletter;

Tom Taylor NOW

The FCC is still open for business
Open For Business Most of the “non-essential” parts of the federal government closed on Friday – but not the FCC.

Chairman Ajit Pai saw the potential shutdown coming and stashed away enough funds to keep his agency going for a whole week – at what sounds like full-tilt. Note that this isn’t the shutdown-contingency plan Pai shared last month, when he said they’d keep about 225 of the 1,492 staffers on the job. That’s still in abeyance and could be triggered if needed. Here’s what the FCC said Friday – “In the event of a partial government shutdown, because of available funding, the FCC plans to remain open and pay staff at least through the close of business on Friday, January 26.” There’s important work going on – especially next week, when Pai hopes to convene the scheduled January 30 meeting and start rolling back his Democratic predecessor’s Net Neutrality policy. Pai’s personally taking a lot of heat on that, from Congress and third-party protesters. He’d probably like to have that 3-to-2 party line vote taken on January 30 as scheduled, and get on with it. D.C.-based attorney David Oxenford says in a weekend post that “we have even heard that the FCC auctions may be funded differently from the rest of the agency, and may continue even if there is no resolution.” The Copyright Office is closed, though the Patent and Trademark Office says it’s good “for a few weeks.”

I guess tom foolery will have to wait until next weekend

+-RH

741

General Radio Discussion / Re: Pirate Radio Podcasts?

« on: January 19, 2018, 0556 UTC »

@Redhat  >   I guess this is the one http://mwfreeradio.blogspot.nl/

Yup I think that's the guy.

Thanks Cool AM!

+-RH

742

General Radio Discussion / Re: Pirate Radio Podcasts?

« on: January 18, 2018, 1931 UTC »

Several years back there was a guy out of the UK that had a weekly podcast about MW pirates, mostly Dutch.  I haven't seen any of his shows in a while, not sure if he's still doing them.

Domestically, we had Ragnar's Pirates Week but he retired the show years back.  There was also Pirating with Cumbre, but this show has also concluded.

+-RH

743

The RF Workbench / Re: A 500 watt Class E amp

« on: January 18, 2018, 0020 UTC »

Just read thru this post again...

The Drivers don't need a heatsink as the Qg of any 'Modern' FET is <10nC.

Forget those silly bolt on things as they're way too slow!

Your gate protection sounds interesting but a resistor is all that's needed!

Str.

Apparently we're back to our breakdown in communication.  A resistor is not going to keep the fets alive when the driver has latched them ON because of a drive signal loss to the input of the splitter arrangement, which is based on the circuitry from a Nautel NX series AM transmitter.  The drive splitter produces push-pull drive required by the fets in the bridge.

BTW I've tried chip type drivers before and all of them burn up at 7 MHz, the power dissipation is just too high when driving 1000pF gates, and please spare me the rhetoric about the merits of GaN as they are cost prohibitive and not suitable for the power levels I'm heading towards.  My only complaint about the SiC devices outside of their cost is that they want +20V/-5V drive for best switching perfomance.

Gaga, the 40% figure come from the speed limitations of the driver/fet combination.  The 40% allows the fet to switch off before the other device begins to conduct, preventing a condition known as 'shoot through' in which for a short time there is a dead short across the power supply rail and the devices can be destroyed from excessive current flow.

+-RH

744

General Radio Discussion / Re: Has anyone seen a decrease in US Comm. SWBC since Bro. Stair has gone?

« on: January 15, 2018, 2219 UTC »

I wish the commission would allow US broadcasters to do what the Canadians do; relay some of their programming on SW for domestic purposes.  1-5 KW would be enough to blanket the US in most cases on a reliable basis.

This whole business of 50KW MINIMUM beamed to places afar seems disingenuous to me.  Give the folks here something to listen to as well, and at a low cost!

+-RH

745

The RF Workbench / Re: A 500 watt Class E amp

« on: January 15, 2018, 0530 UTC »

Just look at the pictures.  No pride in the workmanship.  Sloppy may work fine for 160 and 80, but it won't work on 40.

+-RH

746

Equipment / Re: Wow, here is a 250Watt Less Money than than 150watt

« on: January 15, 2018, 0529 UTC »

Bulk electronic devices are often sent in esd plastic tubes.

+-RH

747

Equipment / Re: Wow, here is a 250Watt Less Money than than 150watt

« on: January 15, 2018, 0247 UTC »

Be sure to buy a tube of fets when you buy it.  I've heard those things are not very reliable.  The thermal management wasn't well thought out, if at all.

+-RH

748

The RF Workbench / Re: A 500 watt Class E amp

« on: January 15, 2018, 0212 UTC »

When the input to the phase splitter loses clock, one side of the bridge will stay on (as designed) until the current through it destroys the device.  I later fixed this with a retriggerable one-shot set for 1.5 times the normal rf drive period.  The outputs from the drive splitter with duty cycle control are AND'ed with the drive status line.  If drive is lost, the fets are tuned off.  I haven't lost a fet yet since this was implemented.

Stupid stuff happens no matter how good the protection system is, but this should take care of the majority of failures.

As for the drivers you mentioned, I'll take a look.  For long term reliability I prefer something I can tie straight to a heatsink, tough to do with SMD power devices without exotic board materials.

+-RH

749

Equipment / Re: Procaster AM Transmitter Review

« on: January 14, 2018, 2015 UTC »

Once I have three built for myself, I'll consider it 

Although, $1200 isn't a bad price considering someone had to design and build it....of course it is illegal to sell such things in this country...perhaps a trade is in order

+-RH

750

The RF Workbench / Re: A 500 watt Class E amp

« on: January 14, 2018, 1928 UTC »

His website hasn't been updated now for nearly a decade.  I get it, people want to stick with what they know works.  Change and the learning curve of new technology are things we all have to deal with.  That said, I'm still working on SiC stuff for the moment.  I just havent had the time to move the design past the first two prototypes.  Here is an early build from mid last year.  I had it running on 75 meters, but the efficiency was not so good as rf drive duty cycle control had yet to be implemented.  I was getting 500W carrier out of it fully modulated.  The deck is currently tuned for the high end of the MW band for testing.  The deck blew up when rf drive was interrupted, something that was fixed in version 2.0 with a drive loss protection circuit.

https://drive.google.com/open?id=1lbmY5WaVqBtR1BtVfAC54SDbAbE0HhyT

+-RH