Re: [loopantennas] 50MHz reception using a modified Wellbrook 1530 Loop
- Rick N6RK wrote on May 17.
Are you saying that the amplifier input impedance is a positive
reactance that tracks the positive reactance of the loop? Or
is it possibly a resistive input impedance that tracks the
magnitude of the loop reactance? Is the amplifier connected
directly to the loop feedpoint, or is there intervening coaxial
Rick, The amplifier input impedance is positive approx. +70 degrees over
most design bandwidth and this tracks the loops rising positive reactance
with frequency. The Amplifier connexion is directly to the loop.
If one looks the A F of the ALA1530 and a very low input z amplifier of
Wellbrook LFL1010 using basically the same amplifier. The LFL1010 has a
nearly constant A F of 15dB from 70kHz to 30MHz, whereas the A F of the
ALA1530 at 70kHz is 21.0dB gradually rising to 0.5dB at 25MHz
On average the ALA1530 impedance tracking yields a 10dB increase in
sensitivity from 500kHz to 30MHz.
Put it another way, very low input z amps. are okay from low sensitivity
loops for say EMC work.
A F is the Antenna Factor in dB
Antenna Factor to receiver indication converts dBuV to dBuV/m
Receiver indication assumes the antenna driving into a 50 Ohm system.
For example, if the receiver indication at 300kHz was 90dBuV, the Field
strength is 99dBuV/m. Hence, the A F would be 9dB
The lower the A.F. the more sensitive the antenna.