The BITX20A Transceiver
History: The BITX20 was designed and developed by Ashar Farhan, VU3ICQ. His aim was to create a high quality single band transceiver that could be made using common components and therefore easy for amateurs in low-income countries to construct (see the links below). The original design produces six watts of SSB on twenty metres and is based on a common bi-directional amplifier block using cheap NPN transistors. The radio also uses a filter using cheap crystals and mixing is done with 1N4148 diodes.
The current project is based on the new version produced by Henricks QRP Kits of Dos Palos, California. Known as the BIT20A, the kit is based on a professional PCB, has push-pull output and a number of other enhancements. As of October 9th, 2007, the first batch of 200 kits are being dispatched around the world. These kits represent a real bargain at $85 shipped to the UK (with the dollar low against sterling it is somewhat amazing to realise that the frequency counter kit I plan to use is about the same price as the radio!
As mentioned I will be adding a frequency readout which means that there will no need for a calibrated tuning dial, in fact I will be using a varactor diode and ten turn pot as the tuning control. Other additions include a built in SWR bridge for matching and relative SWR will be indicated by a five segment LED bar graph. The latter will also be used to indicate signal strength on receive (see menu, left).
Pictures of the construction can be found below (click on the images for a larger version). An account of the construction and testing can be found here, the menu on the left contains links to my additions and modifications and below you will find links relating to the BITX20.
For me, this is the first big project since returning to amateur radio after a very long break and, hopefully, the end result will be a home made radio on the air and an encouragement to other amateurs.
BITX20A Construction Stage by Stage
Ready to go!
(Click on any image to enlarge it.)
The initial components in place, electrolytics, crystals, relay and trimmers.
A small push button switch on the top side of the PCB allows switching from RX to TX.
The Audio Amplifier:
Testing the audio amp. The volume control is added to the top of the board, I would advise adding terminal pins before solder the 10k pot as it is easy to damage the track - as I found out! Also If you have a fluorescent bench lamp the AF amplifier will pick any radiation from it - mine was about 30 KHz.
The 2nd Mixer:
I hope that transformer's OK!
2nd IF Amp:
Winding that coil has to be the least enjoyable part of this process!
The First Mixer:
The RF Amplifier:
The Front End Filter:
The PA driver:
The RX on test:
This receiver kicks! I have initially set the tuning range to be from 14,180 to 14,317 and checking during the afternoon I 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.
I have not made any objective tests of the VFO stability but considering the PCB is sitting out in the open with the tuning capacitor hanging off by it's legs this is a very stable beast! It's certainly good for 15 minute QSOs and should be pretty much rock solid when the radio is mechanically tied down.
The PA stage assembled and ready for setting up which has to be done with the dummy load attached.