Reducing gain on loop


PAUL LANGFORD
 

Hi all,can I reduce the gain on the loop by using a smaller loop say 2meters?
Regards 


Simon
 

Hi Paul

That question is abot like how long is abit of string.

What loop, what preamp, why you want “less gain”, what frequencies you interested in, what loop do you have? 2m circumference or diameter, etc etc??

Much more info please.

Simon


PAUL LANGFORD
 

Hi Simon,it's the newer version of the CCW amplifier.i use it for medium wave DX,I have it pointed east but where my location is it's pointed at the driotwich transmitters.and I get some ghosting signals on my airspy hf discovery.
Regards


PAUL LANGFORD
 

The loop is 3meters of westflex coax.
Regards 


Simon
 

Hi

If loop is at optimum size for mw, which if 3m circumference its not far off. making smaller will not help with its ability to rx on mw.

Lets assume the preamp is not being overloaded but your sdr rx..

So if above is true ( sdr issue) either buy a better sdr…(£200) or on output from bias to radio add some notch filter/s to remove the offending high power transmissions from said local bbc tx..
VERY easy to do..a coil and a capacitor tuned to say 999khz, coil and cap go from output to gnd..( radio side of T)
If multiple frequencies causing issues you need more traps.) traps will however give you “holes” in mw..

Reducing the size of loop will reduce said bbc transmissions likely, BUT will also reduce wanted signals at an equal amount, and still the issue remain.. ie yes no interference but can’t hear jack below ( ie) 8mhz..

Spectrum comms ( ebay) sell suitable coils to do this, they give cap values to suit..(£5? For 5.) 283429818976 ebay number.

IF BUT DOUGHT IT..the preamp is being overloaded then speak to Chris..But above I would do first..Leave loop as IS!!

Simon


Chris Moulding
 

Some good answers from Simon about this problem.

The AirSpy HF Discovery like most of the wideband SDR receivers is too sensitive for use at MF and HF frequencies as it's designed for low noise work at VHF frequencies.

Looking at the quoted spec online it has a sensitivity of -140 dBm at HF.

The blocking dynamic range is 110 dB so it will be blocked at -30 dBm.

It's very likely that the signals you see on 198 kHz and medium wave from the Droitwich transmitter are close to or over -30 dBm from your loop antenna.

Another simple solution would be to add a 20 or 30 dB attenuator between the Bias Tee unit and your receiver.

This will bring the range of the incoming signals within the dynamic range of your receiver and you will still be limited by external noise so you will not miss any weak signals

It is unlikely that the LAA++ amplifier is overloaded. During the on-air tests for every amplifier I run an IP2 check by checking the second harmonic of the local BBC Radio 5 Live transmission on 909 kHz by listening on 1818 kHz. The transmitter is at Moorside Edge 25 miles away with 400 kW ERP similar to Droitwich and the usual signal received here at the workshop on the test loop is -25 dBm on 909 kHz. I usually hear the real second harmonic of the transmitter way down in the noise on 1818 kHz. I do the test with a Yaesu FT-1000MP transceiver which has a far better receiver and filtering that most SDR receivers.

Regards,

Chris