Re: Altoids tin? go small???

Arv Evans K7HKL <arvevans@...>
 

Miniature BITX builders:

Using a brass-screw type PTO also makes the VFO section much more compact, without compromising slow and precise tuning, but like Hans says, you will then need some sort of frequency counter to determine where you are in the band.

You can use very small low-voltage type capacitors (5 Volt rated units) for the emitter bypass caps because the operating voltage will not exceed that amount. Same applies to some other caps if you check the maximum voltage that they might be exposed to.

Chip type resistors and capacitors are actually not too bad to work with in ugly-construction method, and they will save additional space.

With GI (galvanized iron roof flashing) material or double-sided PCB as the sub-strata you can build on both sides of the board. This also saves significant space.

If you use earphones only, or an external AF amp & speaker (PC speakers work well) you can eliminate the space required for the LM386 or other high power AF amplifier circuit.

The whole microphone amplifier section (including VOX) might be built inside the microphone instead of inside your miniature BITX chassis.

An IC socket on the rear panel might provide all the external connections. Use a IC-type header plug to make connections to the rig.

Hans' G0UPL has a "FAST H&P Design" <http://www.hanssummers.com/radio/huffpuff/minimalist/fast/> that incorporates both VFO and stabilizer into a two-chip layout. This makes the VFO rather compact and still includes step-locked tuning for very good stability. You can further decrease the size of Hans design by making the VFO tuning a PTO where a brass screw tunes the VFO/PTO and eliminates the need for a large pot to adjust the frequency.

Relay based T/R switching can be replaced with solid-state switching methods that are probably more compact than even miniature relay units. This will also conserve power by elimination of current required by the relay coil.

Arv K7HKL
_._

On Mon, 12 Sep 2005 08:32:40 -0600, Hans Summers <Hans.Summers@...> wrote:


I like that kind of challenge too, though I have yet to even finish my
conventionally sized BITX :-(

If you look in your component catalogues you will find very small
alternatives to large conventional items such as potentiometers. One example
is a kind of preset trimmer which has an 11 x 11mm footprint nd is 10mm
high. See Farnell part number 452853 or this link:
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=452853 (I hope the
link works for you). Despite only being a trimmer potentiometer, is actually
has a tiny knob instead of just a screwdriver hole. The little knob can be
turned between thumb and finger, though obviously not as easily as a real
full size knob.

Other ideas:

The VFO course and fine tune could be replaced by a single multiturn
ultraminiature preset potentiometer, these come with a small protruding
metal shaft to take a flat-headed screwdriver. You can glue a small PCB disk
onto the shaft in order to turn it. This idea replaces the two separate
controls for course and fine tuning, but it does mean that you won't have
any idea where on the band you are tuned to. Unless you 1) count turns, 2)
miniature frequency counter such as
http://www.hanssummers.com/radio/sfreq/index.htm (could be made even smaller
using Surface Mount Devices) 3) meter the tuning voltage somehow.

You can get very tiny switches too, as well as other miniature components
for things like headphone sockets etc.

PCB: you can get board which has half the thickness of the usual stuff.
Might not sound like much difference, but volume consumed by the PCB is
wasted volume!

With "ugly" construction you can be VERY compact, provided you don't mind
double- or tripple-deckering your components!

73 Hans G0UPL

-----Original Message-----
From: Allison Parent [mailto:kb1gmx@...]
Sent: 11 September 2005 04:34
To: BITX20@...
Subject: [BITX20] Re: Altoids tin? go small???


Go for it, thats a real challenge.

I've put what amounts to the recieve side of a Bitx in a altoids tin. That
was easy, the knobs are always a problem. Seriously, the real problem was
keeping the BFO from swamping the RF amp from being so close. Shielding was
the required item to solve that. It's also
needed so that the Xtal filter can do it's work rather than blow by. I also
found SMT parts very handy.

The VFO is a problem. Heat, or rather heat transfer from other parts will
have to be attended to or the drift will be tough to manage. Any case flex
or the lid moving also caused frequency shifts. I
had enough heat from the audio too induce drift. You may wish to consider a
VXO at 24mhz (20m, IF at 10mhz and LO at 24mhz). A
higher freq VXO will tune more.

You can use FT25-43 toroids or BN43-2402 binocular cores for the
transformers as they are small. The turns ratio is all you need for those as
8-10 turns will be enough. Consider using TUF1 DBMs as they are very small
and would save space over a core and diodes for the modulator and mixer.
You'll have to use shielded cans (maybe IF transformers rewound) for the
bandpass filter. The crystals used for the filter and BFO consider the hc49
size or smaller.

The transmit chain will have to scale back. The size of the parts and the
current they need to handle presesnts limits for thermal and
RF isolation with conventionally available parts. Running without the
IRF510 can get you about 300MW with a good transistor and the right output
match.

You will have to use electronic TR switching, space for relays is scarce.
Antenna, Mic and earphone jacks must be small too.

It can be done but a lot of care will me needed to render a useful radio.

Oh, I do have a larger Altoids tin it's nearly 3x the size
of the usual one. That would be easy.

Currently I'm building a BITx for 6M in a computer AB switch box measuring
of 7.25"Dx5.25"Wx2"H and I'm finding that fairly roomy. the 5.25"x2" pannel
space is cramped for space to accomodate
tuning, audio gain, mic and headphone jacks and S meter.

Allison
KB1GMX

--- In BITX20@..., Jim Strohm <jstrohm@e...> wrote:

On Sep 10, 2005, at 1:37 PM, Rahul Srivastava wrote:

Unfortunately I do not get Altiods here in Lucknow so a rough
dimension will be helpful this looks like a neat challenge.

2.25 x 3.75 in.

Whoops, 5.7 x 9.5 cm

0.75 in deep (2 cm)





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