RadioBanter - View Single Post

April 4th 04, 10:09 AM

On 04 Apr 2004 01:16:22 GMT, oUsama (Yuri Blanarovich)
wrote:

Not really. What you say is the general "wisdom".
I want low noise, high (adjustable) gain preselector. Something with below 0.1
uV. There are situations when that is needed (small loops receiving antennas,
Beverages on low bands, etc.) The gain should be adjusable so it is (band
noise) just above the noise threshold of the mixer for particular antenna/band
situation.

While you definitely need the lowest noise figure and a high gain when
using small magnetic loops, I very much doubt that you are going to
need the strong signal handling capabilities for these antennas. Why
not use a dedicated preamplifier constantly attached to these
antennas, possibly with some protection circuitry to avoid damage when
transmitting through your nearby transmitter antenna ?

With modern transistors you can get quite noise figures, however, some
transistors may obtain those low noise figures only at UHF and above,
since the 1/f may start already in the MF or HF range.

One other thing to watch when using devices with gain up to the
microwave bands is the direct pick-up of local FM and TV signals,
unless the circuitry is well shielded and the input filters contains
capacitors with low parasitic inductance.

Just switch the receiver input between the magnetic loop preamplifier
output and the full sized antenna and avoid the hassle with variable
gain stages.

At the same time have high dynamic range to handle strong signals S9+60dB
without overload.

You are going to encounter such signals from a full sized antenna, but
even at 14 MHz, the band noise is still quite high, so the ultimate
noise figure is not so important.

By the way, did you check the "A High-Dynamic-Range MF/HF Receiver
front end " by Jacob Makhinson N6NWP (QST February 1993 p. 23-28, with
corrections in June 1993 p. 73) ?

It contains push-pull preamplifier designs with feedback with nearly
+33 dBm input IP3 and gain compression starting around S9+80 dB.

Paul OH3LWR