There is no need to totally disconnect the AGC. That risks cutting off signal. Some actually use it to get the best signal strength & set that way. Astor sets (as before) often have a plug at the back to take a meter probe and multi-meters like Peak 200H have a blocking cap on a separate socket just for that method.

Seeing that you have a CRO you can use that to set the input voltage, via an attenuator, or cap (HMV circuit in AORSM's).

What has to happen, as I have commented on several occasions, is that the signal strength applied must not be of such magnitude that it causes the AGC to operate. Delayed AGC /AVC has a set point. the voltage has to reach a certain point before it impinges on the valves.

Very few Radio's actually quote an input to the antenna voltage. Midwest (USA) do on some sets as they have amplified AGC. You set the IFT's at a certain voltage, then you wind it up to the next specified voltage and adjust the AGC.

Refer Markus & Levy voltages quoted previous. You can calibrate the Graticules on the CRO from its reference voltage of 1V PP

Marc

What I meant initially was, a grounded shield partially covering the loop antenna in the direction you think the noise is coming from. Why is it more expensive to move house?

If the radio doesn't have a ferrite loop antenna, and you have a spare one,
and a dual tuner gang like this:
Image Link
You can use that to tune before the radio sees it,
but it's back to pre-superhet days when you turn two dials to
change a station.

As loop antennas work by picking up the magnetic component of an electromagnetic wave, they can be shielded against electric field interference with a metal shield wrapped around the full circumference of the loop. A small gap is needed at some point in the diameter of the loop so that the shield doesn't make a shorted turn for the magnetic field part of the signal.

Direction finding loops are usually placed inside a metal tube for this reason, as well as to protect the loop from the weather.

This only applies to "open loops" such as the one described. Never heard of it being done to an existing loop-stick.

I'm about to try pointing one at Amberley airbase near Ipswich (Qld).
They have a robot voice transmit constant weather (ten minute updates) around 359kHz.
I understand that ferrite loops are highly directional anyway,
but I do hear a lot of noise for something so close.
Maybe they transmit at low power, but it's for aircraft pilots,
so you'd think it should be very receivable.
.. or maybe it's just because it's out of the range the radio is designed for.

Maybe you are in the path of the ILS, or Radar. The latter would tend to be uneven in signal strength but regular in the variance of its rise & fall. That would be picked ap by the AGC voltage rising & falling like a grandfather clock ticking.

Like a broom, it sweeps

Marc

Was that directed at me? I'm deliberately trying to receive it.
My radio now tunes down to zero kHz, though it stops working at the extreme low end of course.

I've had some success with a second ferrite loop connected across the tuning gang.
Interesting results since both are independently directional.

Hi All,

The signal Art is receiving is the Amberley NDB on 359 kHz. Callsign is AMB which is transmitted in Morse code at regular intervals interspersed with weather information from the automatic weather station situated nearby.

Power is output is generaly 100 watts fed into an omni directional vertical antenna giving an even radiation pattern.
This should give a constant signal to a stationary ground mounted receiver.

This beacon was slated to be decomissioned on the 30th of May this year, may have been cancelled.

Listen between 202 to 400kHz and you will hear hundreds
of these beacons scattered around oz.


Cheers, Graham...

Since interference from switchmode solar gear was mentioned,
would a wave trap be a viable method of filtering noise so long
as it has been determined that it is being fed through the antenna?

I understand a wavetrap is too large for a PCB, but it used to be mounted outside of the radio anyway between the antenna and the radio, which for vintage radio, would be desirable.

If the frequency you are trying to suppress is far from the frequency you are trying to receive it's all the better, and can have a large curve.

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I'm guessing at least the first method is already achieved with a tuned loop antenna.
I've yet to learn if they actually reject anything, or if that's a natural effect
of resonating at the desired frequency. I would guess that's true.

Where you can get into strife is being too close to a major airfield. The TV can often tell you.

The Instrument Landing System ILS is a transmitter and you also see interesting patterns on some TV's as the Radar sends out pulses as it rotates & despite frequency differences there always seems to be a way it gets in.

The Long wave NDB's just happen to be in a frequency band that cops a lot of interference. A lot of switchmode supplies run around 200kHz and I have had them get into a 175kHz IF.

Marc