The other day I was asked;"@John Brock hi there.. are you able to recommend any simple receiver circuit kits to build ?Just to help me understand some of the fundamentals I'm learning.. some of it is unintuitive."
Pause for thought, yes! But this is not an easy question to answer.
If this were a first world country, I would recommend a 'local' shop to go into and buy a kit.
There are lots of them both kits and shops...
I know because while staying with my daughter in Bristol UK, I ordered stuff and it was delivered to my (sorry her) door the next day!
There were (once upon a time) local shops here on the West Rand and in Johannesburg that sold components. Where hobbyists and Radio Amateurs could buy components or even test gear. Sadly they are long gone since last century.
You can buy kits and components - for a price - and have them delivered by courier. Definitely not the Post Office! But for anyone who is not wanting to spend serious amounts of money, this is not really an option.
The club does have a certain amount of components. Most left to the club after hams going 'silent key'. Some members and other Radio Amateurs sell them off at our regular 'flea markets' / 'boot sales'. I prefer the term 'boot sale' as the other makes you want to bring your own fleas!
Some hams have 'junk boxes' with components and old radios that can be used for new construction projects. I do.
I was going to put a cartoon in here but I did a search for the one sent to me recently. It is from CartoonStock thus has copyright! So no cartoon of an old guy with his son outside the garage saying;"One day son all this will be yours."
To answer the Question
The 'simplest' type of radio useful to a Radio Amateur is a "Direct Conversion" type. In the past I have recommended several designs. All needing components that are getting scarcer by the week.
Recently (in the last few years) I have been component 'scavenging'. This produces all sorts of components. Some basic like transistors and others like PWM generator integrated circuits. Lots of electrolytic capacitors of low and high voltage. These are from 'blown' ATX power supplies used in personal computers. One good Highveldt storm and the power supply is 'dead'! As it is always connected to the mains supply.
So here is a circuit from last century. In fact QST 1968. It has been built by many Radio Amateurs and it does not require 'fancy' components.
If you have been following the RAE course, then some of the blocks should be familiar to you. The 'local oscillator' is a 'Hartley Oscillator'. The mixer is a 'double-balanced' mixer using diodes. Which in those days would have been germanium types. Giving a low level requirement on the 'local oscillator'.
These days with the interference levels being so high, you should consider a 'high Q' tuned antenna. To reduce the 'broadband' noise from the front end.
This is a discussion piece maybe it will trigger some construction enthusiasts. I hope so.
73 John Brock ZS6WL
The double-balanced diode mixer can easily be replaced by another transistor mixer. The FET in the oscillator as well. Most of the 'scavenged' transistors have FT's of above 200 MHz so will oscillate up to 150 MHz.
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