Electronics > Beginners

Effect of aerial on resonance?

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aneevuser:
I'm trying to make an AM radio.

I have a LC tank circuit (currently based on a a couple of 1N5817s being used as varactors) which I can tune from 910 kHz to 1480 kHz when it is not connected to an aerial. I have L = 150 \$ \micro \$ H nominal and I think the varactors vary from about 70 pF to 180 pF roughly.

I have 4 m of 0.6 mm single core wire which I'm using as (half of a) dipole aerial. When I connect it to the tank circuit, the resonant range is now 830 kHz to 1210 kHz.

1) Is this magnitude of change expected?

2) What exactly is causing this? Apparently an aerial with these dimensions should have an inductance of about 6 \$\micro\$ H so the effect on L should be small. Also if I assume that L is constant, then we should have:

\$\frac{C_\text{low}}{C_\text{high}}=(\frac{f_\text{high}}{f_\text{low}})^2\$

but that ratio varies from \$(930/810)^2 \approx 1.2\$ to \$(1480/1210)^2 \approx 1.5\$ over the resonant range, so it's not even constant.

Can someone tell me what's going on?

[apologies for dodgy latex - no idea what i'm screwing up]

vk6zgo:

--- Quote from: aneevuser on March 28, 2019, 10:09:40 am ---I'm trying to make an AM radio.

I have a LC tank circuit (currently based on a a couple of 1N5817s being used as varactors) which I can tune from 910 kHz to 1480 kHz when it is not connected to an aerial. I have L = 150 \$ \micro \$ H nominal and I think the varactors vary from about 70 pF to 180 pF roughly.

I have 4 m of 0.6 mm single core wire which I'm using as (half of a) dipole aerial. When I connect it to the tank circuit, the resonant range is now 830 kHz to 1210 kHz.

1) Is this magnitude of change expected?

2) What exactly is causing this? Apparently an aerial with these dimensions should have an inductance of about 6 \$\micro\$ H so the effect on L should be small. Also if I assume that L is constant, then we should have:

\$\frac{C_\text{low}}{C_\text{high}}=(\frac{f_\text{high}}{f_\text{low}})^2\$

but that ratio varies from \$(930/810)^2 \approx 1.2\$ to \$(1480/1210)^2 \approx 1.5\$ over the resonant range, so it's not even constant.

Can someone tell me what's going on?

[apologies for dodgy latex - no idea what i'm screwing up]


--- End quote ---

It is not so much inductance, as capacitance between the antenna wire & the common side of the LC network.

Traditionally, antennas are not connected straight across an LC circuit, but either a small coupling winding is used, or the inductor is tapped, with the antenna connected to one of the taps closest to the "cold"end of the coil.
Real MF radio external antennas are many times larger than 4m, & yet radios work.

"Re inventing the wheel" is not as rewarding as many would think----It might be better to build a circuit that is known to work, then analyse that!

aneevuser:

--- Quote from: vk6zgo on March 28, 2019, 12:54:18 pm ---
It is not so much inductance, as capacitance between the antenna wire & the common side of the LC network.

--- End quote ---
Could you explain what you mean by the common side. It's not clear to me.


--- Quote ---Traditionally, antennas are not connected straight across an LC circuit, but either a small coupling winding is used, or the inductor is tapped, with the antenna connected to one of the taps closest to the "cold"end of the coil.
--- End quote ---
I see. Is this some kind of impedance matching technique?


--- Quote ---Real MF radio external antennas are many times larger than 4m, & yet radios work.

--- End quote ---
Yes. I'm doing something wrong.


--- Quote ---"Re inventing the wheel" is not as rewarding as many would think----It might be better to build a circuit that is known to work, then analyse that!

--- End quote ---
I don't understand this. I'm definitely not reinventing any wheel. It's simply a piece of wire attached to the top of a tank circuit, pretty much like the crystal set radios that I used to make when I was a kid. I'll put up a diagram of the whole circuit later, when I get some time.

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