So I was asked this question last week about how a 'tuned circuit' relates to s.w.r.
I admit I was a little stumped. When I discussed it with my fellow Radio Amateurs at the club on Wednesday, I was inspired to dig a little deeper.
I first thought about phasing and 'power factor'. Then I considered an ATU (Antenna Tuning Unit). But then as the week progressed I thought of more points. Almost none of these are directly useful to the RAE (Radio Amateur's Exam)! But as a bit of background this might help.
With the advent of Compact Florescent light bulbs and L.E.D. lights as well, 'power factor' has all but disappeared. Florescent lights have a large inductor (coil) inside as a 'ballast'. To assist in striking the light gas - to basically light!
[If you want a more detailed explanation:- Here ]
When you have one or two of them on the 'mains circuit' the phase difference is not significant. But when you have 20 or more in a large area like a workshop or factory, it is something to consider.
Similarly when you have a lot of electric motors on the mains again the phasing difference between the current and the applied voltage becomes quite large. This for most electricity consumers never becomes an 'issue'.
All this was in my college notes from many many years ago. The power metering would read the power consumed incorrectly. And the supplier would check this and apply a 'power factor' correction circuit. Usually a capacitor that compensated for the 'inductive' load.
I am now going to check the new (not necessarily improved) power meters that are fitted to the mains supplied to the homes...
When the current and voltage are 'in phase' the circuit is resistive. Also the 'tuned circuit' is at resonance.
This is important because the electricity supplier wants to measure the power drawn accurately. For billing purposes.
Standing Wave Ratio - s.w.r.
In the early days of transistors a lot of radio amateurs used them for power output at radio frequencies. As they were expensive but light weight and didn't need a heater supply. But they were also terrified of 'blowing them' or 'letting the smoke out'. After all replacing them could mean a week's wages!
You will notice - if you look at the CB manuals - all of the CB radios of the 70s onwards had a 'reflectometer' in the output connection. Which would announce in no uncertain terms if you had forgotten to connect the antenna!
S.w.r became the 'bogey man' of the radio amateur. This has lingered till modern times. While valves would glow a different colour, transistors would silently give up. Most these days just ignore the s.w.r.. Just take a look at the LDMOS device demonstrations on YouTube.
BUT - s.w.r. is an indication of a non-resistive load (antenna or dummy load). Most 'reflectometers' use a coupled pair of 'transmission lines'. Some use (QRP) a 'directional coupler'. Sorry QRP is low power which is quite popular this century. This usually means a small ferrite transformer or two. [some more inspiration!]
[Last week RAE I mentioned that an antenna 'looks like electrically' a 'tuned circuit'. Being 'inductive' above resonance. And 'capacitive' below resonance in frequency.]
So this brings me to an A.T.U
An A.T.U. is an Antenna Tuning Unit. It is there to adjust the antenna to resonate at the desired (usually the transmitted) frequency. Which also means the antenna is supposed to be 'resonant' at the same frequency. If it is not it will be 'reactive' - either 'inductive' or 'capacitive'.
How is it like a 'tuned circuit'? Does it have a 'Q' factor? (Bandwidth? -3db frequency power points?)
Yes it does. Remember that the higher the 'Q' factor the narrower the bandwidth. The more 'selective' it becomes. Don't ask what is the 'best', nobody knows!
So usually you want the antenna for a particular frequency band. This is difficult as at h.f. (1 to 30MHz) as the antenna 'Q' will be high. Why? Because usually it is made of wire... And someone wanted to call this wireless!
[I am going to put a curve here detailing the wire diameter to length ratio. This will show the 'Q' factor.]
If you look at the wartime (WW2) pictures of h.f. stations you will see 'dipoles' of multiple wires looking like sausages. Otherwise known as 'thick dipoles'. These exhibit resistive matches over a greater bandwidth at h.f. This is how I made a dipole to cover the entire 2 metre band with a good match. I used two 20mm aluminium tubes cut to length.
These exhibit resistive matches over a greater bandwidth at h.f. - which means the 'Q' factor is quite low. Larger bandwidth = lower 'Q'. [less selective]
Please don't start putting these antennas in your back yard. Unless you are on a plot or farm. Your neighbours will complain.
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