Re: mono band 2 meters rig based on the micro BITx circuit #ubitx #2meters

Ashhar Farhan
 

I would add that Si5351 could be directly to drive a harmonic mixer. A harmonic diode mixer is singly balanced mixer (like the one in the bitx transceivers) except that there is an additional pair of diodes strapped across each of the two diodes. So, the mixer turns on on the high positive peak of the oscillator as well as the lowest negative peak of the oscillator, in effect it turns on twice for every oscillator cycle. this would reduce the requirement to just your regular radiuno driving 1N5711 or 1N4148 diodes in the mixer. The big question is, what core do we use for these mixers? Should be mangle an ADE-1 to get at its transformers? Allison, can you help?

- f

On Saturday 28 April 2018 10:43 AM, Ashhar Farhan wrote:
there are just a few challenges and many ways to meet them in transiting to 435 MHz, VHF is just a milestone along the way. The idea of making something on VHF/UHF is no longer economically justified if you are doing it just to save money. You can buy a baofeng for twenty dollars. Probably, it is a good idea to do that in anycase, you will have a ready rig to test your homebrew with.

The local oscillator:
The first challenge is to get a local oscillator going at around 400 MHz. There are three ways to do it:
- A crystal oscillator with a multiplier chain. This needs you to have a good way to sniff RF frequencies. A wavemeter of a lecher line would do the trick.
- Just triple the Si5351. This means, you need to tune just one bandpass filter at 435 MHz
- Use an Si589 or Si570, LVDS version. This costs as much as a Baofeng, you can order it from Mouser.

Architecture: The options are:
- Two diode mixers that directly mix down to audio, to make a phasing, direct conversion transceiver. This is simple, it involves cutting a phase line down by millimeters until you get the phase angle right. But you don't dabble with all the fun (really??) of a superhet.
- Single conversion to 20-25 MHz IF with stripline filters to keep the image rejection high, it is a bitx from the other side
- double conversion to 45 MHz, this throws the image to around 350 MHz, easily suppressed by LC bandpass filters.

Modulation/Demodulation:
There are many ways to achieve it. You can build analog mod/demod with conventional technology as done in the bitx transceivers. Or...
add an SDR back-end (if you are lazy).

What do you guys think?

- f




On Saturday 28 April 2018 08:32 AM, Tim Gorman wrote:
Respectfully, as Allison points out, it's not just the PA active element
that will be the problem. When your frequency width is an order of
magnitude, e.g. 14Mhz to 144Mhz, you begin to run into all kinds of
issues with components. Lead lengths and circuit trace lengths/widths
at 2m cause many more problems than at 14Mhz. It gets even worse at
432Mhz.

I agree with others on here. It would be a lot more feasible to do one
band modules, e.g. one for 2m and another one for 432Mhz, that are
small enough they could be placed in one case along with a ubitx
being used as an IF amplifier.

tim ab0wr

On Sat, 28 Apr 2018 01:12:16 +0000
"Ashhar Farhan" <farhanbox@...> wrote:

Allison,
For a power chain of about 5 watts that goes from 50 mhz to 500 mhz,
what would be your recommendations? The RD15HVF1 seems to be used
frequently at 435 Mhz. Are there any broadband alternatives? What
kind of cores can we use at UHF?
- f

On Sat, 28 Apr 2018, 06:37 ajparent1/KB1GMX, <kb1gmx@...> wrote:

Several things, like others have repeatedly pointed out the uBITX
is very unsuited for FM or AM.
FM needs wider filter and far more gain to get the needed limiting
action.  Its not drop in its full
replacement.  A Baofeng UV5R goes for 30 bucks and does 2M and
70CM.    AM there are
issues with drive level and sustained power out, that and its
sparsely used.  My opinion is
that if you want all that get a FT817, its a do all and has a noise
blanker.

To get the front end to cover VHF the LO system deliver a VHF LO
(95mhz for 6M, about 99 or 189mhz for 2m)
and the input filter needs to pass the VHF band and not a low
pass.  IF memory sers the 5351 can go to 220Mhz
A LNA before the mixer would be required for reasonable performance.
For UHF if you had a version of the
5351 or maybe used a si570 flavor to generate the LO and an
improved mixer 432 is possible but its more
effort and would likely need a board redesign.

UHF is better done with a competent converter/transverter with a
404mhz LO and run that into a uBitx at 28mhz.

In all cases 6 though 70cm the tx power chain would need work as the
drivers are all 300mhz FT devices
just will not do it.   The IRF510 I've used at 50mhz as a monoband
linear and its respectable
(40+W for push pull @ 28V properly done for 6M only) but I think
maybe 70mhz is a stretch without
first trying.  In all cases the TX chain is mono band only as VHF
impedance matching from stage
to stage is required. Just dropping in higher FT devices will not
help its a across the board redesign
for a specific band.

Receiving is easy enough, transmit above 50mhz is going to be
harder.


Allison





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