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?
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.
remove this resistor and put there 1MOhm and connect it to the loop
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).
I have seen both buffer amplifiers and toroidal transformers used for
loops. Be aware that loops have low signal output compared to other
antennas and the signal advantage is the noise pickup is lower (higher
S/N rather than higher output. So some gain is needed to connected it
to the Lfrx.
I had good sucess with a KAZ antenna, which is a triangle that is 40
feet long and 10 feet high. It uses a 9:1 toroid matching transformer.
It is meant for LW and MW but seems to be okay for low HF. I used my
usrp connected up to the 10.7 output of a receiver rather than connected
directly to the antenna.
73 Eric
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