Re: [Discuss-gnuradio] lfrx+loop antenna

[John Ackermann N8UR]

Matt Ettus wrote:
> konvak wrote:
> > hello all,               I am thinking about building loop antenna for 
> > 3-6 MHz to
> > receive some DRM stations. As I understand it the loop antenna is 
> > parallel
> > resonant circuit so without any amplifier the parallel res. circuit 
> > should
> > look into big impedance(~1 MOhm) to get high Q. Lfrx has 50 Ohm input 
> > due to
> > the resistor at diff. amplifier. My question is, would it be possible to
> > remove this resistor and put there 1MOhm and connect it to the loop 
> > antenna
> > with coax. Would it work?
> >
> >   
>
> You can remove that resistor, but then the input impedance will be 348 
> ohms.  You can increase that up to maybe 2K by changing out all of the 
> 348 Ohm resistors surrounding the diff amp with 2K ones.  However, if 
> you really need a 1 MOhm input resistance you are better off using a 
> buffer amplifier.  Usually loop antennas like this for receiving (like 
> you would get at Radio Shack) will have their own buffer for just this 
> reason.  Besides, you will want to have some additional gain in front of 
> the LFRX board.
> I think Chuck Swiger did something with a loop antenna and a USRP a 
> while ago.  You might check out his web pages.

Just a thought -- a high-impedance loop can be coupled to a low 
impedance input by making it into a transformer.  For example, a loop I 
built for WWVB at 60kHz had a 100 foot coil of RG-59 coax as the primary 
loop, with a capacitor across the ends for tuning.  The impedance across 
the cap was very high.  To couple to the receiver, I just threaded one 
turn of wire into the coil and put my coax connector across its ends; 
that acted as the secondary of a transformer and brought the impedance 
down to a useable value (I never did any calculations or measurements on 
exactly what the input and output impedances were; it just worked).

John