The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> Antenna matching at 2 MHz - urgent!
https://designers-guide.org/forum/YaBB.pl?num=1371217922

Message started by vitto on Jun 14^th, 2013, 6:52am

Title: Antenna matching at 2 MHz - urgent!
Post by vitto on Jun 14^th, 2013, 6:52am

Hi all,

I am designing a transmitter-receiver system working at 2 MHz. This is not for radio communication, but for environment sensing. There is no bandwidth to worry about and the power is low (<1W). My antennas are small loops (approx. 4 inch) diameter and are each connected to a tiny tx and rx boards respectively.

I was given the schematics of these boards, which are very simple. The transmitter consists of a simple matching network, to match the impedance of the antenna to the 50 ohm feed line and to bring resonance at the desired frequency (2 MHz). The surprise was when I looked at the receiver: there is no matching network whatsoever. The antenna is fed directly into a differential amplifier and that's it. When I asked them why they told me "we tune the transmitter, then the receiver just picks up whatever".

Now they are going to send these boards to production and I am not feeling comfortable with that!

Does anybody have suggestions on how to design a simple matching network to match my receiving antenna to the receiver amplifier? I don't have much experience working in this frequency band (I normally work in the microwave region). Any example circuit that could help get me get started?

Thanks!

Title: Re: Antenna matching at 2 MHz - urgent!
Post by Frank Wiedmann on Jun 14^th, 2013, 7:26am

The free program presented at http://www.iowahills.com/9SmithChartPage.html looks useful (but I have not tried it).

Title: Re: Antenna matching at 2 MHz - urgent!
Post by wave on Jun 17^th, 2013, 1:57pm

Is this a near field system?

Have you calculated the wavelength at 2 MHz ?

Title: Re: Antenna matching at 2 MHz - urgent!
Post by vitto on Jun 18^th, 2013, 1:12pm

Since asking the question I have been able to figure out some answer.

Yes, my system is near field, my antennas are a meter or so apart and the wavelength is about 40 meters. This does not necessarily mean that matching is not important, since the power transfer theorem apply also in DC regime.

However, on the receiver side impedance matching is not important because the antenna is connected to an amplifier with a very high impedance (ideally infinity). Therefore the goal is not to transfer RF power but simply to pick up a voltage and sample it.

I hope this makes sense.

Title: Re: Antenna matching at 2 MHz - urgent!
Post by RFICDUDE on Jun 19^th, 2013, 6:39pm

Hi Vitto,

It depends on what the impedance of the receive antenna is as to weather or not there needs to be impedance matching.

If the antenna impedance is high at the operating frequency then it may be ok to directly couple it to a high impedance amplifier. But if the impedance is low then adding something to resonate out the impedance may be beneficial for improving receiver sensitivity.

What type of antenna is used for the receiver?
Straight wire, coil, something else?

I have built "crystal" radios and antennas that operate between 500kHz and 1.7MHz. The antennas I use are essentially big coils around 250uH that are resonated with an air variable capacitor. The antenna is also impedance matched to the detector using additional LC components.