So, if modern receivers are not superhets what are they? Sure they are a few direct conversion types on the market. SDRs are growing in popularity but they are still in the minority. Yes early single conversion superhets had image problems. I'll be willing to bet you cannot name a current production single conversion superhet receiver, DC yes, single conversion superhet no.
What your problem is - is that you are too young to understand what I am talking about, and you failed to read the links I posted. OK
The frequency allotments for Forrest Fire, Game Commission, Some Ambulance, Fire were distributed after the second World War.
After the Second World War - the only type of radios we had were all TUBE TYPE.
The Scheme might have been the same, but the design was much different then the solid state - PCB stuff we have today. The filtering - and there wasn't a lot of filters to speak of - other then Collins Mechanical Filters, was poor at best.
Collins Rockwell's biggest customer was MA BELL - The Phone Company.
As most telephone companies is phasing out their COPPER lines, going to fiber optic and cellular coverage, the demand for these filters shrank and Collins went out of the filter business a number of years ago.
Unfortunately, our amateur radio exams given today are nothing more then match the questions to the answers, and most licensed hams are nothing more than Appliance Operators.
Do you even understand HOW A RADIO WORKS?
The most basic radio is nothing but a detection circuit. A pair of high impedance head phones, a diode, an antenna, a razor blade and a sewing needle - Fox Hole Radio - is a good example.
A more complex example of a Fox Hole Radio is a Yaesu FRG - commonly referred to as a FROG.
Yaesu FRG-7 Product Reviews
1. The first receiver built by a hobbyist is usually the plain old crystal set. If you are unfamiliar with the design then check out the crystal set page.
2. The T.R.F. (tuned radio frequency) receiver was among the first designs available in the early days when means of amplification by valves became available.
The basic principle was that all r.f. stages simultaneously tuned to the received frequency before detection and subsequent amplification of the audio signal.
The principle disadvantages were (a) all r.f. stages had to track one another and this is quite difficult to achieve technically, also (b) because of design considerations, the received bandwidth increases with frequency. As an example - if the circuit design Q was 55 at 550 Khz the received bandwidth would be 550 / 55 or 10 Khz and that was largely satisfactory. However at the other end of the a.m. band 1650 Khz, the received bandwidth was still 1650 / 55 or 30 Khz. Finally a further disadvantage (c) was the shape factor could only be quite poor. A common error of belief with r.f. filters of this type is that the filter receives one signal and one signal only.
Let's consider this in some detail because it is critical to all receiver designs. When we discuss bandwidth we mostly speak in terms of the -3dB points i.e. where in voltage terms, the signal is reduced to .707 of the original.
If our signal sits in a channel in the a.m. radio band where the spacing is say 10 Khz e.g. 540 Khz, 550 Khz, 560 Khz.... etc and our signal, as transmitted, is plus / minus 4Khz then our 550 Khz channel signal extends from 546 Khz to 554 Khz. These figures are of course for illustrative purposes only. Clearly this signal falls well within the -3dB points of 10 Khz and suffers no attenuation (reduction in value). This is a bit like singling one tree out of among a lot of other trees in a pine tree plantation.
Sorry if this is going to be long but you MUST understand these basic principles.
In an idealised receiver we would want our signal to have a shape factor of 1:1, i.e. at the adjacent channel spacings we would want an attenuation of say -30 dB where the signal is reduced to .0316 or 3.16% of the original. Consider a long rectangle placed vertically much like a page printed out on your printer. The r.f. filter of 10 Khz occupies the page width at the top of the page and the bottom of the page where the signal is only 3.16% of the original it is still the width of the page.
In the real world this never happens. A shape factor of 2:1 would be good for an L.C. filter. This means if the bottom of your page was 20 Khz wide then the middle half of the top of the page would be 10 Khz wide and this would be considered good!.
Back to T.R.F. Receivers - their shape factors were nothing like this. Instead of being shaped like a page they tended to look more like a flat sand hill. The reason for this is it is exceedingly difficult or near impossible to build LC Filters with impressive channel spacing and shape factors at frequencies as high as the broadcast band. And this was in the days when the short wave bands (much higher in frequencies) were almost unheard of. Certain embellishments such as the regenerative detector were developed but they were mostly unsatisfactory.
In the 1930's Major Armstrong developed the superhetrodyne principle.
3. A superhetrodyne receiver works on the principle the receiver has a local oscillator called a variable frequency oscillator or V.F.O.
This is a bit like having a little transmitter located within the receiver. Now if we still have our T.R.F. stages but then mix the received signal with our v.f.o. we get two other signals. (V.F.O. + R.F) and (V.F.O. - R.F).
In a traditional a.m. radio where the received signal is in the range 540 Khz to 1650 Khz the v.f.o. signal is always a constant 455 Khz higher or 995 Khz to 2105 Khz.
Several advantages arise from this and we will use our earlier example of the signal of 540 Khz:
(a) The input signal stages tune to 540 Khz. The adjacent channels do not matter so much now because the only signal to discriminate against is called the i.f. image. At 540 Khz the v.f.o. is at 995 Khz giving the constant difference of 455 Khz which is called the I.F. frequency. However a received frequency of v.f.o. + i.f. will also result in an i.f. frequency, i.e. 995 Khz + 455 Khz or 1450 Khz, which is called the i.f. image.
Put another way, if a signal exists at 1450 Khz and mixed with the vfo of 995 Khz we still get an i.f. of 1450 - 995 = 455 Khz. Double signal reception. Any reasonable tuned circuit designed for 540 Khz should be able to reject signals at 1450 Khz. And that is now the sole purpose of the r.f. input stage.
(b) At all times we will finish up with an i.f. signal of 455 Khz. It is relatively easy to design stages to give constant amplification, reasonable bandwidth and reasonable shape factor at this one constant frequency. Radio design became somewhat simplified but of course not without its associated problems.