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'Knobless Wonder' single channel 7 MHz SSB transceiver

SSB transmitter SSB transmitter SSB transmitter

The 'Knobless Wonder' is a simple fixed frequency SSB voice transceiver for the 7 MHz amateur band. It is basically the IF and audio end of a BitX (featured elsewhere on this page). Unlike other filter type SSB rigs there are no RF mixers, VFOs or intermediate frequencies. Instead the SSB signal is generated and received directly on the operating frequency.

The project is made possible by the availability of cheap crystals in the 7 MHz band (eg 7.159 or 7.2 MHz) which form the carrier oscillator and ladder filter. The trade off is that you gain commercial-standard SSB transmit and receive performance but only on a single frequency. Whereas a ceramic-resonator DSB direct conversion rig like the 'Beach 40' is less selective on receive and less efficient on transmit (due to the two sidebands) but is frequency agile.

Frequency agility is so desirable for the QRPer that you'll make more contacts with an agile 2w DSB (1w per sideband) rig than with a fixed channel 2w SSB rig. In addition other stations are normally running high power so the broader unfiltered direct conversion receiver is seldom a problem if they can hear you. Best of all is you're not put off the air if there's others using the frequency, or, worse, one a kilohertz away.

On the other hand the novelty of using a simple radio lacking even tuning and volume controls sometimes appeals. Especially if it uses a handful of parts yet delivers high performance. Fixed channel operation may mean you're calling CQ longer before getting a contact (while, unbeknownst to you someone else is calling, unanswered, on a nearby frequency) but can encourage activity within clubs who make a fixed channel rig a group project. There is also the achievement in completing a rock-steady SSB transmitter (not just CW or DSB) with an impressively sharp receiver. The rig is also easily converted to a BitX later if you want full band coverage.

The unit pictured uses cheap 7.159 MHz frequency crystals to provide an output on exactly 7.160 MHz LSB. This is a suitable frequency for Australian builders. However, depending on band allocations and activity patterns, people in other countries may prefer 7.2 MHz crystals to get 7.201 MHz.

See the following videos and links to find out more

Rig description and circuit


On air from the beach


How it sounds on air (VK3XPT)


Publication

A detailed article on the Knobless Wonder appeared in April 2014 Amateur Radio, published by the Wireless Institute of Australia. Another article, by Peter VK2EVB, appeared in March 2016 Amateur Radio. This described construction on a printed circuit board.

Knobless Wonder links

Visit the following for others' experiences in constructing this rig:

* BD6CR (video)
* VK2TPM (write-up and circuit)
* VK6FH (write-up and circuit)

 

 

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Items were chosen for likely usefulness and a satisfaction rating of 4/5 or better.

 

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