matching very low z???

[ahmed osama]

hi all

now i want to make tx working at 1 Mhz & i begin to see the antenna design & i found if i used a small antenna (L<λ/600) , the internal Z will be very very low less than 0.1Ω ??

i want to know the matching tips for such very low Z & low Freq. antenna ??

also how such problems solved in the AM radio which have about 50 cm antenna or even without antenna??

thx all

 

[flatulent]

On receive at that frequency, you can have a high input impedance amplifier. There is so much atmospheric noise that the first stage noise figure is not important.

On transmitting things are different. You are going to have trouble with your matching because of the high reactive part of the antenna impedance. The bandwidth will be so low that it will cut off part of your modulation sidebands.

Try using higher power. Do not forget the legal limits on radiation in broadcast bands.

 

[tketke2002]

hello Flatulent,


I am interested how you could connect the low Z (small cap) in case of transmit mode?


Tony

 

[flatulent]

The problem you will have is the losses in the matching network. Try a series inductor to cancel the capacitive reactance and then a transformer.

Also, try a loop antenna.

 

[biff44]

I am pretty sure that an electrically short antenna has a very HIGH impedance. Why do you think it will be 0.1Ω?

https://www.hard-core-dx.com/nordicdx/antenna/wire/shortdipole.html

 

[tketke2002]

Flatulent,

The problem you will have is the losses in the matching network. Try a series inductor to cancel the capacitive reactance and then a transformer.

Also, try a loop antenna.


Thanks for your quick response. In my case I want to drive a very short piece of wire, let say 2 cm that has to be used from 70 to 140 MHz without tuning. This rules out the loop antenna...
There should be some efficiency but it is less important.. I was thinking in a parallel tuned circuit with low Q but high impedance (high L value) between the short wire and ground. The input signal applied with a capacitive tap on the tuned circuit (inductor with parallel 2 capacitors in series, midpoint is the feeding point)
What do you think about this?

Tony