Simulate an AM antenna in Multisim

[BkraM]

Hi,

I'm trying to build a regen AM radio in multisim ( http://www.mtmscientific.com/swradio.html ).
How can I best simulate the antenna so i can check how the circuit design works?
I currently use a AM_VOLTAGE block to do this, is there a better way to simulate it?

Thanks,

 

[ronsimpson]

I don't use multisim but I use other.... I don't know of a "antenna" in sim.
I would use "am_voltage " block with the out put of 100uV. (some thing small)
I would also do a frequency sweep. 5-15mhz to see how the radio rejects unwanted signals.

 

[BkraM]

To see how good the selectivity is you'd just add a couple of Am-block to simulate different signals?

 

[ronsimpson]

The whole point of looking at the frequency response of the receiver is to see how selective it is. This will test all frequencies not just a couple.

 

[RCinFLA]

You need to measure the inductance and unloaded Q of the loop antenna.

Assuming you have little equipment you need a voltage indicator (AC voltmeter, or voltage sensing) capable of 0.5 to 1.6 MHz and a signal generator covering the same range.

Assuming you have a typical AM loop, start with a parallel resonating capacitor of about 250 pF for parallel resonating cap across loop antenna. Make sure you do not place any metal object near loop antenna that may de-Q it. Lightly couple the generator and signal voltage indicator to the loop by small value (<5 pF) capacitor. Sweep RF freq to find resonate peak. Calculate inductance of loop by knowing resonate frequency and paralleled capacitance value.

Your voltage reading must be accurate enough to measure -3 db down points (freq above and below resonance). Calculate unloaded Q from resonate freq / (F3db highside - F3db lowside). Calculate Rp of loop by knowing XL from first step divided into Q measurement. The reason for small coupling caps is to minimized loading on parallel tank circuit, yielding effectively the unloaded Q of the loop antenna itself.

Model will be inductor of measured L and parallel shunt resistance from Q measurement. You can change known cap value to get resonance measurement at various frequency across band. Inductance will not change much, Q will vary more across band.

The circuit you show is a regenerative receiver. It can be very frustrating circuit to get working correctly. It also can wipe out other folks AM reception due to interference creating by the regenerative first stage when it breaks into full oscillation (which it will at some point).
When adjusted just right, the regenerative circuit provide positive feedback, short of full oscillation, that actually increases the Q of the input tank circuit, yielding better selectivity.