Building the BITX20A

05 October 2010 @ 11:28 pm

I may have missed it but I can't see how to reverse the order of posts so that earliest is at the top so this is to say:

The post below this is the latest post, if you want to start at the beginning then please go to the first post on October 5th, 2007.

Charlie.

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31 December 2007 @ 08:30 pm

Putting It Together

I have started a page for the mechanical contruction of the 20A here. Hopefully I will get the PA wired up and running this coming weekend! I got diverted by Christmas and a new 2 metre rig (new to me, it's Kenwood TR-75E).

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06 December 2007 @ 09:17 pm

Rx Up and Running!

This receiver kicks! I have initially set the tuning range to be from 14,180 to 14,317 and have been pulling in stations from the US & Canada as well as the proverbially strong stations from Italy. Using an aerial switch I made comparisons with my Icom IC-703. Subjectively there is little difference in sensitivity and selectivity. The station manager declared that the same stations were much easier to understand on the BITX20A than on the 703. The Icom has a richer tone, but the BITX20A has greater clarity when listening through the local plasma TV QRM. I would imagine that the audio bandwidth of the Icom has more bottom and that the BITX20A has more top. There are a couple of pictures of the RX under test here

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25 October 2007 @ 09:22 pm

Stage by Stage Pictures

I have put up some pictures of the construction so far, see here.

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24 October 2007 @ 10:05 pm

The Kit Arrived

I picked up the kit from the local post depot last Saturday and spent most of Sunday working on it and I would estimate that I'm around three quarters of the way through. The instructions make construction plain sailing and, hopefuly, the stage by stage testing will ensure that any problems show up.

My plan is to build the BITX20A kit as supplied have it up and running on the bench before I start work on the case and extras. I will post pics of the construction in a few days time.

Tags: bitx20a, kit

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13 October 2007 @ 07:19 pm

LED Bargraph SWR/ Signal Strength Meter

I finished the LED Bargraph meter and after finding one bad joint it was working as expected. You can find details of the circuit and construction here.

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12 October 2007 @ 10:37 pm

An Aside

I've never really objected to Google's text ads and I would be a hypercrite if I did as I use them to help finance my own server costs. Google's method is pretty good at selecting adverts that relate to t he text. I'm nor sure that I would actually click on one myself but I guess that the nearer they get to stuff that interests me then the more likely it becomes that I will.

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12 October 2007 @ 09:08 pm

LED Bargraph Indicator

I've just started constructing an LED bargraph SWR and signal strength indicator for the BITX20A. The 5 LED bargraph is made by Kingbright, information here. It will be mounted vertically on the front panel to the left of the digital readout. The panel aperture for the LED array is only 2.1 mm wide so I will have some neat needle filing to do!

The bargraph is driven by a series of comparitors housed in a single inline (SIL) driver chip, BN1413N. The input sensitivity is around 100mv and input to the board is adjustable, inputs from the SWR bridge and RX signal strength will be ajustable. A SWR of 1:1 will extinguish the first LED and, on receive, an signal of around S9 will light all five LEDs. (That's the plan anyway!)

I hope to have the bargraph board finished and test tomorrow and will create a page for it on Radio Wymsey.

Tags: bargraph, led, swr

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07 October 2007 @ 08:47 pm

Progress

Yesterday I made the tone oscillator that will be used when adjusting the SWR. It's a simple twin-t sine wave oscillator using a BC109 running at around 900 Hz. It worked straight off but was distorted on the negative half cycle which was improved with a 100R in the emitter and completely cured by decreasing the collector resistor. The output will be adjusted so that the the output is two or three watts when tuning for minimum SWR. You can see the circuit here and the details here.

I have also created a project page for the BITX20A on my website. This will detail overall progress and details of the additions and mods that I make.

Tags: oscilator, swr

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06 October 2007 @ 12:19 am

Structure

This journal will contain detail and data regarding the construction and performance of the BITX20A and will be referenced from Radio Wymsey. Radio Wymsey will give a simplified account of the process and once on the air it will also include logbook details for the transceiver.

Tags: bitx20, bitx20a, radio, transceiver, wymsey

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05 October 2007 @ 11:07 pm

The Additions

First of all I intend to build the tranceiver in an extruded aluminum case measuring approximately 160 x 53 x 220 mm, see here. I am adding a frequency counter that can be offfset by the IF and therefore simply measures the VFO, the counter comes as a kit and costs almost as much as the radio kit! The counter can be seen here.

I will be adding a five segment LED bargraph that will be used to indicate SWR & signal strength, these will be simple relative indications. In order to measure SWR there will be a 1kHz audio signal that is injected into the microphone amplifier when tuning. I will most likely use some sort of audio AGC and this will be used to provide a relative signal strength indication.

The rear panel will include a fan (microprocessor cooling type) which will draw air over the PA heatsinks.

Tags: agc, counter, led, pa, swr, transceiver, vfo

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05 October 2007 @ 10:24 pm

The Project Background

The BITX20 is a homebrew 20m SSB transceiver designed by Ashar Farham, VU3ICQ. The philosophy behind the design is that it should be inexpensive to make from easily available components in the developing world and should perform well. The circuit is novel in that, except for the PA and one very common op amp, the transceiver makes use of NPN transistor pairs in bi-directional blocks. You can see the circuit and design details on Ashar's site here.

I came across the design in the October edition of Practical Wireless which carries an article by John Seagar, G0UCP, and was fired up. The simplicity of the design lends itself to expansion and variation and I have decided to go with the BITX20A variation. This uses a push-pull output stage to give 10 watts output plus numerous other enhancements and is available as a kit from Henricks QRP Kits for $85. The whole rig, using conventional components, fits on to a PCB measuring 5.5 x 3.5 inches (140 x 90 mm).

I have an order for one of the first run of 200 and hope to have the kit in the next couple of weeks. In the meantime I am working on a number of additions to the basic radio.

Tags: 20m, bitx20, bitx20a, qrp, ssb, vu3icq

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05 October 2007 @ 10:04 pm

The Project Starts

First a bit about me and then the background to the project:

In the autumn of 2006 I returned to amateur radio after a break of some 40 years, I purchased a RadioShack DX394 on eBay and strung a 40 foot wire around the picture rail of the living room, tuned it with a Mizuho Sky Coupler and was quickly re-hooked! Within a couple on months I decided that I wanted to became a licensed amateur.

Although I had taken my Radio Amateurs Examination in 1964 I had lost interest and never did get a licence, unable to locate the certificate and the City and Guilds Institute unable to find it I set about joining my local club and purchased the ICOM IC-703. At the end of March I, and two other club members, passed the the Foundation and Intermediate Examinations and within a couple of weeks I was 2E0WYM.

Over Easter during a clear out what did I find but my RAE certificate! So within a couple more weeks I became M0WYM. Why WYM? In honour of the on-line village of Wymsey at Wymsey which I created in 1998 and one of the first communities to be totally wired. Over the years we had the dubious pleasure of appearing in the Daily Mail, on Channel Four, Slashdot, The New York Times and others.

Currently I operate on all the HF bands but suffer awful QRM on 20, 17 & 15 m from a nearby plasma TV. I prefer operating on 6 and 10 when they are open but I monitor all the bands when I'm operating so maybe we'll work each other!

Charlie.

Tags: 2e0wym, dx394, ic-703, icom, mowym, wymsey

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