Alignment Advice.

[Joe]

Thanks for that, Alan. The basic circuit is in the E2 manual but that doesn't give the value of L.

- Joe

[AlanBeckett]

I know. It took me a long time to track that down.
Actually there are other designs, but as that supposedly matches the Advance Generators it's probably best to use it.
Somewhere I've got a little diecast box. I suspect some carbon resistors would be an advantage.

Alan

[Joe]

Most of my resistors ARE carbon.

- Joe

[Skywave]

Perhaps I'm missing something, (not unusual), but that 'Advance' pad / dummy aerial combo. confuses the hell out of me.

Why is the input terminal, that is connected to a generator whose source impedance is 75 Ω, labelled 37 Ω? I did some sums: assuming that the load on the DA and E output terminals is a very high impedance (which, itself, may not necessarily be so), the impedance that the generator 'sees' looking into the "37 Ω" socket is 75 Ω - as it should be, to match the generator's source impedance.* On that basis, that "37 Ω" input socket should be labelled "S.G. input: 75 Ω". Then everyone will understand that that socket should be joined to a generator of that impedance through a co-ax. cable whose impedance is also 75 Ω . . . . which is what is required and what 'Advance' clearly state is necessary and what their 'box' is designed to do.

* Moreover, that calculation of 75 Ω justifies the assumption that the load on the DA and E terminals is a very high impedance, otherwise that 75 Ω figure would be smaller).

Anyone care to enlighten my darkness, please?

Al.

[PyrusBoy]

Morning all, #
Saw this thread over night so nipped out to the "Dog House" (my workshop) and looked through the draws and found enough to make one up. Trialed it on a couple of sets, but still prefer my set up, being lazy. I have a home built wide band RF amp, enough to cover LW, MW, and most common SW on vintage sets, this is fed from the signal genny. This amp is terminated into a loop which is actually arround the window frame above the bench. A signal taken before the attenuator on the sig gen is taken direct to a digital frequency meter. I check once in while the frequencies against known frequency broadcast signals (in spite of the frequency meter having built in crystal calibration), I like the belt and braces feeling.
Being a meany, I also aquired a very small FM transmitter with digital frequency setting, the range is only a few feet, but it a very handy for aligning FM IF's and RF.
All the best.

[Joe]

I know that impedance is √R²+(XL-XC)² but how does this relate to co-axial cable? It can't be 75Ω a yard nor be the impedance between the core and screen as this would be frequency-dependant.

- Joe

[Skywave]

Your stated equation for impedance relates to 'lumped' reactive components: transmission lines do not posses reactive components of that nature: the L and C are uniformly distributed throughout the line's length. Hence, that equation is not applicable for transmission lines.

The characteristic impedance (Zo) of a transmission line is independent of its capacitance and inductance. It is dependent on the physical size of the conductors, their separation and the nature of the material between them. Zo is not determined by the d.c. resistance of the line.

This link provides a simplified explanation:
http://www.allaboutcircuits.com/vol_2/chpt_14/3.html

Alternatively, you can learn all about transmission lines in much more detail in most text books on radio theory.

Al.

[AlanBeckett]

Joe,

See http://en.wikipedia.org/wiki/Characteristic_impedance
Actually the instructions say use as short a length of co-ax as possible, which is a bit odd because unless the co-ax is several wavelengths long you'll never 'see' it.

Al,

I find it logical if not sensible.
Ignoring the L, C and R4 I make the Impedance 78.16 - (270//(99+11)) - close enough I expect.
They say that the DA terminals want to 'see' 10R.
Again, ignoring L, C and R4 - less logical this time - I make it 10.282.

I agree that it's not clear why the 37R and 10R are there at all.

Alan

[Joe]

Thanks, Al. I won't claim to fully understand it (yet) but now have a better idea. I've got 'Foundations of Wireless' so will have another go at reading that.

I know maths is usually used to explain things but it's just something I can't seem to grasp: V=IR is meaningless but tell me that the greater the current flowing through a resistance the bigger the potential across it will be and I understand.

Even going back to school we'd be set questions like "If your are filling a fifty gallon bath and the tap is delivering five gallons a minute but the bath is leaking two gallons a minute how long would it take to fill?" any my immediate thought would be "Fix the leak" followed by "Use a stop-watch."

I like to think I'm improving; just saying that it isn't something I find at all easy.

By the way, I got a CSE Grade 2.

- Joe

[PyrusBoy]

Hi, Joe,
Maybe your approach is much like mine, I make things work, I don't always understand or want to understand the theory. I have alway been a practical type of engineer. Maybe thats why I hold or have held several patents and design copyrights in my time. I worked at Marconi's for a while on R & D EW and PAR Radar, my practical approach was often far quicker and cheaper than the theory guys answers, who wanted to prove to the n'th degree that the world was round. I well remember them wanting to design an azmuth gearbox which was so accurate and expensive (£80,000 in 1970), I upset several when I pointed out that with a beam angle of 1.5 deg a far cheaper option (£7,000) was as good if not better, it still hit the target at 200 mls. A bit of tax payer money saved. I won the argument and got the order. Many won't agree with my approach but I am happy seeing end results without pain.