Feeding The AM Radio - Signal Generator Connection

[AllTheGearNoIdea]

Hi I’m think of building some really cheap and Nasty AM kit radios over the Christmas holidays. I want to try and do some quantitative testing but don’t know how to feed a radio that has a ferrite rod antenna rather than a nice controlled 50 ohm antenna jack. I know I can just drape a wire over the radio and feed from my signal generator but that not exactly quantitative.  I’m kind of thinking I need some form reference antenna perhaps permanently mounted. Given the low frequency it doesn’t appear that feasible unless very heavily loaded etc or perhaps a mag loop. Any idea many thanks.

Regards Chris

[VK5RC]

If you want to quantify antenna input sensitivity you are getting into a 'whole area of hurt'. A really good source reference antenna, ie accurately defined rf field strength at a certain frequency and position /distance, is not cheap but the rf anechoic chamber you will need to put it in will be many times more. Also very difficult at such a low frequency (large wavelength) , near-field effects dominate within a handful of wavelengths from an antenna.
Perhaps signal to noise ratio may be a better parameter to measure - i recall seeing some nice techniques using a signal generator, and often rf attenuators on the internet.
Good rf receivers are in the 2-4uV sensitivity (50 ohm input) but AM broadcast receivers typically 50x less sensitive.
Selectivity - esp rejection of nearby, both in distance and frequency, strong sources is a very good feature for an AM radio.
Rob

[AllTheGearNoIdea]

Thanks what your saying is basically what I had assumed Regards it’s not easy and as you say probably not looking for that amount of hurt LOL. I guess it going to be a random length of wire permanently fixed to the back of the bench.


Regards Chris

[VK5RC]

I didn't want to be too much of a naysayer, the absolute sensitivity isn't always the best measurement.
I am sure even some simple measures will help.
The signal to noise is a pretty easily measured parameter - it will likely help your design even if pretty simple.
 https://www.electronics-notes.com/articles/radio/radio-receiver-sensitivity/signal-to-noise-ratio-s-n-snr-formula
FYI The amateur radio receivers have been tested by Sherwood Engineering and rank them by 3rd Order Intercept, a measure of non-linearity of the mixer/s and amps. http://www.sherweng.com/table.html
All the best for Xmas and 2019, Rob

[AllTheGearNoIdea]

I wasn’t sure if I could go for a GTEM arrangement but I’m guess this would be an electric field test and the ferrite antenna is mainly sensitive to the magnetic field. It would also be more work then I had considered for what is really just playing with old radios. I was guessing there was no obvious solution as from google and YouTube everyone just uses a leaky feeder arrangement.

Thanks for the consideration

Regards Chris

[tkamiya]

If it is an inexpensive type with tuning LC at very beginning of the circuit, you could place a small and high-Q (100pf or less) capacitor at "hot" end of the LC tuning circuit. 

It won't be quantitative as it is not reference to any known standard, but the reading will be constant and repeatable. 

Another will be a magnetic coupling on ferrite core but I recall it wasn't very stable.

[AllTheGearNoIdea]

Thanks  god yes they are £3 kit radios, but I’m trying to extract as much fun as possible. that is an option but I would really like to go in via the ferrite if at all possible.

[RoGeorge]

Unless you want to do precise measurements, a COM port and a piece of wire is all you need to Tx AM radio 
https://hackaday.io/project/162477-serial-port-sdr

[AllTheGearNoIdea]

Unless you want to do precise measurements, a COM port and a piece of wire is all you need to Tx AM radio 
https://hackaday.io/project/162477-serial-port-sdr

Thanks, but i am  looking for something better than a piece of wire across the bench. For what it’s worth i will probably use a ATU to match a random wire permanently installed in the workshop to the signal generator.  This should allow me to quantify the difference in levels between one radio and another if not the actual levels.



Thanks Regards Chris

[wolfp]

What about a Helmholtz coil? between the two coils you have a magnetic field that can be calculated. The ferrite antenna in your radio also uses the magnetic component of the EM-field.
Wolfgang