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6 meter mobile tranceiver-Class E?




 
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Author Topic: 6 meter mobile tranceiver-Class E?  (Read 1314 times)
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KI4YAN
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« on: December 11, 2016, 08:40:26 PM »

I am just finishing up a complete rebuild and repaint of my work pickup truck, and got the new stereo all sitting waiting to be installed once I finish the interior insulation and wiring-and I want to integrate a mobile VHF station into it. I've got a 2 meter HT that I use in the truck now, but after building my 6 meter tube receiver, I decided that I want to build a 6 meter, mobile-based, transceiver with a remote control head.

I've got an old motorola 3W VHF transmitter case that came as part of a package deal at the last hamfest-it was just the metal case but it's fairly nice. Two isolated sections, anodized aluminum, 8" long, 3" wide, and the case is 1"+ deep, per section, and the two sections are back-to back-so the total case is 8" long, 3" wide and 2 1/4" tall.

I've never done a solid state radio. Done a lot of digital microcontroller stuff over the years, but all the radio design has been tubes.

I've been reading up on class E, but it seems that there aren't many(any?) projects out there that are up in the 6 Meter band-and most of them are 300+ watt transmitters. I'm aiming for 30-40 watts output power. I have 13.8 volts DC available in the truck. Is this a good situation to employ a class E transmitter paired with a PDM/PWM modulator?

If it is, can/should the VHF BJT's that are all over Ebay right now (Old motorola stud-mount stuff, rated for 15W or 25W output each) be used, or (preferentially) can I press some salvaged mosfets into service?

I've got (8 ) IRF640, (2) IRF730, a single IRF840, and a dozen of HRF3205, plus a small stack of the UPS power supplies. Each UPS contains the above complement of mosfets.

I've got the matching heatsinks that held six each of the HRF3205 devices, as well.

What would YOU do to get 30-40 watts of 6 meter AM from a 13.8V supply, and how would you modulated it? I'm looking for ideas to start planning from, I'm putting together a block diagram and trying to flesh out sections before I start building.
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KD6VXI
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« Reply #1 on: December 11, 2016, 09:42:44 PM »

I've successfully gotten almost a hundred watts pep out of an irf640 at ten meters.....   I can vouch they will go that high.

It's not the rf FL device that is unobtanium....   It's gate drivers.  However,  sinewave drive does work,  but reduces efficiency.

IRF510/520s have been used on 6 meters as linear mosfet amps.

--Shane
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KI4YAN
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« Reply #2 on: December 11, 2016, 10:29:38 PM »

That's encouraging. I can probably get a discrete gate driver to deliver the power needed, if I get medieval enough on it. The gate driver might be a decent little transmitter on its own...

Next on the plate is the mixing scheme-how to get the transmit frequency and receive frequency synced up, with only using a single variable oscillator. The VFO can be a good sharp squarewave too, so that'll help with mixer noise.

So far, I've come up with the following:

VFO is 2-6Mhz, from the DDS, with a good hard low-pass filter in place.

For the transmit side:

Transmit LO is a crystal oscillator/tripler, I have a few 16.0000mhz crystals that will triple to 48Mhz.

Feed a diode ring mixer to produce the transmit frequency on 50-54Mhz.

For the receive side:

50-54Mhz bandpass filter, feeding the RF amplifier. (gain control and mixer loss)

RF amplifier feeds diode ring mixer with fixed 1st LO to produce 2.250 to 6.250 "receive band", with what amounts to an IF amp feeding a bandpass filter again. Again, the IF amp is mostly for gain control, to combat the mixer losses.

1st IF amp feeds diode ring mixer with the 2-6Mhz VFO to 250Khz 2nd IF. (I've got a few collins 250Khz mechanical filters with 6.7khz passbands) The low 2nd IF will provide good selectivity, even if I don't use the mechanical filter.

Detect the 250Khz 2nd IF as normal, and feed an audio preamp to get it up to line level to feed the truck stereo.

Not sure if I have too many filters, not enough filters, etc. Seems like a lot of filter and mixer losses to overcome, so I might need multiple IF amps in one or both IF stages. Not sure.

This way, I could program the VFO to display the transmit/receive frequency, and sample some of the AGC via an analog input and display received signal level.
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KI4YAN
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« Reply #3 on: December 14, 2016, 01:37:15 AM »

Worked on this a bit tonight. This is all just drafting-I have not built any of this yet. It's posted here for review in hopes of getting a good result.

Instead of diode mixers, which I don't really have experience with and have yet to get one to mix well, I went with what I know-cascode fet mixer. With a gain-controlled RF amp in front of it, it should be acceptable, I think.



The LO input to the mixer is 48Mhz, to mix the 50-54Mhz band down to 2-6Mhz. However, the 2nd LO/VFO is also tuning 2-6Mhz, so I guess I need to adjust my 1st LO some to get the mixer output somewhere usable-I've read direct conversion doesn't work for AM. If it does-and can be made to work *well*, then mixing directly with the 2-6Mhz VFO will be straightforward.

The reason for not changing the VFO is to make it such that the transmit VFO and the receive VFO are one and the same-this greatly simplifies the remote control tuning for me.
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KI4YAN
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« Reply #4 on: December 28, 2016, 03:45:11 AM »

So this got pretty complicated real quick, the breadboard *works* but it's not going to fit in the case I had planned for it.

Back to the drawing board, I'll figure something else out.
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WD5JKO
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WD5JKO


« Reply #5 on: December 28, 2016, 11:46:05 AM »


If you restrict your tuning range to 50 - 50.5, then the BPF following the 1st mixer gets a lot more simple. On 6m there isn't much above 50.5 unless you want to to receive FM. Could also do 50.05 to 50.55 to get in the occasional AM'er that runs a little higher.

Look at the qrp-labs 60m BPF (5.243 Mhz centered with ~ 500 Khz passband). Cost is $4.90.
http://qrp-labs.com/bpfkit.html

Perhaps stagger tuning, or increasing the coupling cap in the BPF will give you 750 Khz or more BW.

At the bottom of the page, it shows a 6m BPF made from a 12m kit. This might be useful to you.

Jim
Wd5JKO
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