26-10-2018, 03:35 PM
Craig
A couple of other things come to mind:
1. The FS from the BBC stations is probably in the volts/metre range at about 1KM from the transmitter and attenuation at LF is not particularly high so you might still have hundreds of mV per metre at 12KM separation. With 17M of antenna wire that again becomes Volts applied to the receiver input. An easy target for straight front end overload. But the story doesn't finish there because that level can itself act as a local oscillator (LO) for the first mixer and hence more mixing products are involved. Further, you have two broadcast transmitters 90KHz apart and each one can act as a LO so even more mixing opportunities exist in your front end.
This is one of the reasons that most amateur radio equipment (rather than a SW receiver) uses front end bandpass filtering purely to reduce out-of-band signal levels.
2. The BC transmitters are close enough for you to hear transmitted intermodulation products at various points due to mixing at their base frequencies and then multiples of them. This can occur through design but also due to the 'rusty nail effect', which simply means that the signals applied across a corroded joint (which acts as a diode) causes mixing products. That 'rusty nail' could be rusty bolt on the transmitting array or even just near to it but the field strength is so high that the metalwork attached to it radiates sufficiently for you to hear it 12KM away, particularly given 17M of wire and a sensitive receiver.
(PS 17 metres of wire operated against virtual ground is resonant at about 4.3MHz)
The simplest way to isolate whether it is receive or transmitted intermodulation product is by substituting a different receiver, preferably using the same antenna. Transmitted products will be there on the same frequencies regardless of the receiver design. Received products (read overload into that too) will be only on the existing one provided the replacement has better front end characteristics.
You could also try taking your receiver and a moderate (eg 10 metre) length of wire (1) closer and (2) further away from the transmitter site. Go for a drive.. That helps obviate your wire antenna at home and its mounting as a 'rusty nail' intermod generator plus helps confirm whether it is front end overload (see my previous post re the pot to vary signal levels).
I hope that all makes sense to you.
Doug
A couple of other things come to mind:
1. The FS from the BBC stations is probably in the volts/metre range at about 1KM from the transmitter and attenuation at LF is not particularly high so you might still have hundreds of mV per metre at 12KM separation. With 17M of antenna wire that again becomes Volts applied to the receiver input. An easy target for straight front end overload. But the story doesn't finish there because that level can itself act as a local oscillator (LO) for the first mixer and hence more mixing products are involved. Further, you have two broadcast transmitters 90KHz apart and each one can act as a LO so even more mixing opportunities exist in your front end.
This is one of the reasons that most amateur radio equipment (rather than a SW receiver) uses front end bandpass filtering purely to reduce out-of-band signal levels.
2. The BC transmitters are close enough for you to hear transmitted intermodulation products at various points due to mixing at their base frequencies and then multiples of them. This can occur through design but also due to the 'rusty nail effect', which simply means that the signals applied across a corroded joint (which acts as a diode) causes mixing products. That 'rusty nail' could be rusty bolt on the transmitting array or even just near to it but the field strength is so high that the metalwork attached to it radiates sufficiently for you to hear it 12KM away, particularly given 17M of wire and a sensitive receiver.
(PS 17 metres of wire operated against virtual ground is resonant at about 4.3MHz)
The simplest way to isolate whether it is receive or transmitted intermodulation product is by substituting a different receiver, preferably using the same antenna. Transmitted products will be there on the same frequencies regardless of the receiver design. Received products (read overload into that too) will be only on the existing one provided the replacement has better front end characteristics.
You could also try taking your receiver and a moderate (eg 10 metre) length of wire (1) closer and (2) further away from the transmitter site. Go for a drive.. That helps obviate your wire antenna at home and its mounting as a 'rusty nail' intermod generator plus helps confirm whether it is front end overload (see my previous post re the pot to vary signal levels).
I hope that all makes sense to you.
Doug
Doug VK4ADC @ QG62LG51
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.