20-05-2012, 08:28 PM
Thank you Lawrence: very informative.
First, your understanding of the significance of what, in effect, is the importance of signal + noise to noise ratio is the same as mine.
Secondly, the value of having a s.w. receiver with two tunable R.F. stages is also the same as mine. Provided the noise generated by the first R.F. stage is substantially less than the noise generated by the mixer stage, then a second tuned R.F. stage - unless it really is exceptionally noisy - will add nothing by way of improvement to the overall S/N ratio. The real benefit of a second R.F. stage is the reduction of image reception on the higher freqs., typically above 10 ~ 12 MHz. Obviously particularly relevant with a receiver with an I.F. of about 450 kHz. {Of course, you know all this, but others reading this Post might benefit from those remarks}
Thirdly, an A.M.U. / A.T.U., the desirability of for a receiver, seems to be a subject that not everyone agrees on. All I can say is that yes, I will agree that the noise level on the HF bands below about 20 ~ 25 MHz is such that the extra apparent gain provided by such a device will only provide a very small improvement is the apparent sensitivity of any receiver. However, I can only report from my experience, based on many different locations, aerials and receivers - and that is for the simplicity of what an A.T.U. / A.M.U is & the ease of construction of one - it does provide a worthwhile benefit. One particular case is evident here. My 60 ft. horizontal 'T' aerial, if connected direct to my Eddystone 888A, gives horrendous spurious responses on Top Band, simply because the Z-matching is so bad that it completely throws out of alignment the tuning of the first R.F. stage. And I have known similar effects with other receivers over the years, too.
Fourthly: I have found that with all types on end-fed 'long-wires', below about 7 MHz (approx.), a really good earth connection is important for bringing up signal strength. And despite what you say about your 'ground' there, I suspect that that factor, with your location and height a.s.l., has a significant bearing on why your relatively small and close to the ground aerial is performing so well. If, one day, you can get up a half-wave dipole at, say 30 feet, I would not be surprised if you found that you need an attenuator between it and a receiver to remove cross-modulation taking place at the 'front end' of any receiver that you hang on it.
And finally, on that subject of earthing, all the comms. radios I use here are fed with isolated, filtered, balanced mains with no 'mains earth' connection. I run a separate low-resistance R.F. earth to each receiver. I have found that that arrangement improves the overall noise performance and is worthwhile.
And that little lot, from me, will do for now!
Al.
First, your understanding of the significance of what, in effect, is the importance of signal + noise to noise ratio is the same as mine.
Secondly, the value of having a s.w. receiver with two tunable R.F. stages is also the same as mine. Provided the noise generated by the first R.F. stage is substantially less than the noise generated by the mixer stage, then a second tuned R.F. stage - unless it really is exceptionally noisy - will add nothing by way of improvement to the overall S/N ratio. The real benefit of a second R.F. stage is the reduction of image reception on the higher freqs., typically above 10 ~ 12 MHz. Obviously particularly relevant with a receiver with an I.F. of about 450 kHz. {Of course, you know all this, but others reading this Post might benefit from those remarks}
Thirdly, an A.M.U. / A.T.U., the desirability of for a receiver, seems to be a subject that not everyone agrees on. All I can say is that yes, I will agree that the noise level on the HF bands below about 20 ~ 25 MHz is such that the extra apparent gain provided by such a device will only provide a very small improvement is the apparent sensitivity of any receiver. However, I can only report from my experience, based on many different locations, aerials and receivers - and that is for the simplicity of what an A.T.U. / A.M.U is & the ease of construction of one - it does provide a worthwhile benefit. One particular case is evident here. My 60 ft. horizontal 'T' aerial, if connected direct to my Eddystone 888A, gives horrendous spurious responses on Top Band, simply because the Z-matching is so bad that it completely throws out of alignment the tuning of the first R.F. stage. And I have known similar effects with other receivers over the years, too.
Fourthly: I have found that with all types on end-fed 'long-wires', below about 7 MHz (approx.), a really good earth connection is important for bringing up signal strength. And despite what you say about your 'ground' there, I suspect that that factor, with your location and height a.s.l., has a significant bearing on why your relatively small and close to the ground aerial is performing so well. If, one day, you can get up a half-wave dipole at, say 30 feet, I would not be surprised if you found that you need an attenuator between it and a receiver to remove cross-modulation taking place at the 'front end' of any receiver that you hang on it.
And finally, on that subject of earthing, all the comms. radios I use here are fed with isolated, filtered, balanced mains with no 'mains earth' connection. I run a separate low-resistance R.F. earth to each receiver. I have found that that arrangement improves the overall noise performance and is worthwhile.
And that little lot, from me, will do for now!
Al.
