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 Conrad Hoffman 12th April 2012 03:34 PM

Esoteric question on tuner inputs and RF design

A properly tuned antenna will look resistive at its operating frequency, hopefully with the desired characteristic impedance value. (tell me if this is wrong!)

My assumption has always been that said antenna is designed to match a tuner input for maximum power transfer. Thus, I'd also expect a tuner input to be essentially 300 or 75 ohms, close to resistive at the frequency of interest.

I recently probed a tuner input with a vector impedance meter and was surprised to see about 75 ohms, but with +90 degrees phase, so it was essentially a 122 nH inductor.

Not being an RF guy, can anybody explain how antenna coupling actually works?

 Osvaldo de Banfield 12th April 2012 04:11 PM

The antenna (Aerial) input normally is a single coil called "a Link". Iīll try to explain it. The tuned circuit is normally wired to the input of the active device (Tube, MOSFET, IC, etc), directly (called Series feed) or capacitive (parallel feed), at the top of the tuned circuit (TC) if the active device matches the impedances under question, or at a tap in the coil if not.

To obtain the width of band desired, the antenna coil is normally inductively coupled to this TC, and its reactive reactance is normally design to obtain the 75 or 300R to match the antenna load. This low Z coil normally isnīt too much coupled to the TC so variations in antenna conditions donīt affect the TC itself. Say, supose you have a high coupling factor, and the antenna is properly matched. So, a bird puts in the antenna, resulting in a detune of the FM section: a bad idea. In FM front ends rarely is used some "rare circuits" like a Pi coupled.

I hope this info is useful. Iīm ham radio from 1987.

Regards.

 Elvee 12th April 2012 04:26 PM

1 Attachment(s)
I see two possible reasons:

-In good quality tuners, there is normally a tuned circuit at the input, coupled to the main tuning control (cheaper receivers have a fixed, heavily damped tank circuit).
Such a circuit has the argument of the impedance equal to ~+90° or ~-90°.
Only at the resonant frequency is the impedance resistive, but it is a spot frequency.
If you make a random measurement, you'll see either + or - 90°.
See below

-Many tuners have an inductive matching component, balun or similar between the tuner and the antenna connector, and this inductive component will dominate the impedance

 Elvee 12th April 2012 04:54 PM

1 Attachment(s)
Here is a typical front end, to illustrate what I said above:

 Joe Roberts 12th April 2012 05:26 PM

Losses in slightly mismatched antenna are very small and very easily made up for by front end gain.

A more or less close match does terminate the antenna cable system in its characteristic impedance which helps reduce common mode effects and reflected power in the cable.

Reflected power is a bit more problematic for transmitting where power transfer to the load is paramount but here the antenna is the generator and the receiver is the load. Although there is obviously power developed across the load, the front end is a voltage gain instrument. Power across a 75 ohm load with antenna signals is quite miniscule.

The impedance measurement you will get depends on the frequency you choose. Antennas work the same way. Move up or down a few mc and the picture will change.

...A ham since the 70s.

 Conrad Hoffman 12th April 2012 08:59 PM

Thanks, that all does help. I didn't carefully tune to the measurement frequency (or vice versa) but will try that to see what happens. It makes sense that this is a voltage sensitive system, rather than a transmitter where we don't want to lose any radiated power, but it seems foolish to worry about carefully tuning an antenna and then connecting it to an unmatched load.

... my dad has been a ham for near forever, but it didn't rub off!

 DF96 13th April 2012 10:38 AM

Most low-noise receiver inputs are a mismatch. This is because lowest noise does not coincide with highest signal. Sometimes people talk about a receiver input being power-matched (i.e. normal conjugate match for maximum power transfer) or noise-matched (lowest noise). For VHF and up noise-matching is normal, as external noise is low.

The impedance you measured will depend critically on the length of cable between your instrument and the receiver. The only time the cable length will not matter is if you have a perfect match, which is unlikely. To work out what the actual receiver impedance you would need to use a Smith chart.

 Conrad Hoffman 13th April 2012 01:09 PM

Interesting! It's counter intuitive, to me anyway, that max power and lowest noise wouldn't coincide. Now we're getting somewhere. Time to break out the Smith charts- it's been years since I messed with those, but I still have 'em. I have a nice working HP 4815 vector impedance meter to play with, without which I'd just be groping in the dark, he says, groping around in the dark.

 Osvaldo de Banfield 13th April 2012 01:52 PM

Yeah!, it appears that there are several hamīs here. Iīm LW1DSE, from Argentina.

I never had an APRS station!!!

 DF96 13th April 2012 04:33 PM

Reducing noise by having a mismatch surprised me when I first heard of it, but having gone through the maths for one simple situation (here) I now understand it a little better. Some active devices benefit from a reactive input, so as well as an impedance mismatch the first tuned circuit might be slightly mistuned.

For broadcast MF and HF receivers low noise is not an issue as external noise is so high, so an impedance match is more likely.

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