Antenna Coupler (or ATU)

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Antenna Coupler (or ATU)

Postby G8UYZ » 04 Dec 2006, 00:38

I've been given a nice piece of ex-military kit in the form of an ATU and it is a 'remote control' so I should be able to use it in my limited back garden. It's part of a 'long-wire' aerial kit of a large RAF aircraft installation (STR18b), whose stated band was 2.8 - 18.1 MHz, and used at 100w AM (400w SSB). The usual aerial was a shortened Windom.

Bearing mind that the RAF's definition of 'long wire' is/was not in keeping with standard HAM practice (80m on 60 ft of wire?), it obviously has a few limitations.

The circuit is basically a Pi, but the shunt elements are 100uH roller-coasters (sliders to earth) and the capacitors are fixed, switched in for the appropriate part of the required band.

I'd like to find out just what it can do. Such huge inductors should indicate handling stuff a lot lower that 2.8MHz with even a slightly longer aerial (100ft?) is used.

Can anyone shed any light on the problem please?. I cannot even find this type of circuit in ARRL or RSGB manuals.

Many Thanks

G8UYZ
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ATU Questions

Postby sparky » 05 Dec 2006, 21:03

1) Do I understand correctly that separate adjustable inductors connect between the input and ground and the output and ground, and that one or more fixed-value capacitors connect between the input and output? If so, that component arrangement is seldom used, because that circuit forms a high-pass filter that doesn't attenuate harmonic energy.

2) What are the capacitor values and does capacitor switching allow the capacitors to be used both individually and in series and/or parallel combinations?

3) If you measure the coil diameters, maximum number of turns, and maximum coil lengths we can estimate maximum coil inductances. That information together with the capacitor information will make it possible to calculate an estimated tuning frequency range with a given length of antenna wire.
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Antenna Coupler (or ATU)

Postby Jess » 05 Dec 2006, 23:40

Good reply. Sounds like a gold mine for building a more convential antenna tuner with a PI network having the inductor in series and using shunt capacitors.

Have fun.

Jess, w4pqk
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Postby G8UYZ » 06 Dec 2006, 00:30

Thanks for the replies, guys.

Sparky:

The inductors are about 100uH each 'roller coasters' and are connected to deck with input to the top (or output) . The capacitors vary from 20pF to 0.01uF and are individually switched. The ceramic switch is something to see!

Jess:
This is a "remote controll" unit with three 24v motors and a Wheatstone bridge circuit to detect the setting of the external resistor. It's not an easy job to convert it to a remote unit if I modify the thing to a more conventional Pi. Apart from anything else, I'm not sure what value of capacitor/s to use, if it would fit and what of the fixed caps to use. The mechanical connections are standard quarter-inch shafts.

----------

I realise it might not be too efficient (the RAF wasn't bothered too much in those days (late 1950s), but would the thing work ?
I'd like to get it at the base of my aerial in the garden, rather than next to the radio. The aerial could either be a half-sized G5RV or a "Radio Works" Windom (it struggles a bit at 80m) and I have to use my MFJ 949 VersaTuner. I have both at my disposal. If this beast will work on a shorter wire, it would suit my garden better, if not the DX.

Remember that this thing was somewhere in the back of a bomber with a short (!) wire aerial. What I'd like to do is to see if I could get it to work on 15m as well.
G8UYZ
 

Antenna Coupler (or ATU)

Postby Jess » 06 Dec 2006, 07:03

G8uyz, as far as estimating capacitor and inductor values you might consider looking at the schematics of some of the auto tune tuners available.

I have one that I think uses discrete capacitors and inductors with values in a 1, 2, 4, etc ratio where the small values give fine tuning and larger and intermediate values set the frequency range. Can't think of the name of mine right at the moment. If you are interested I can go look at it. Don't know if a schematic is on the web or not.

I understand you may not want to pull the inductors out of the unit considering the mechanical aspects. I guess the RAF wasn't too concerned with the higher harmonics Sparky mentioned. Have fun.

73, Jess, w4pqk
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Postby G8UYZ » 06 Dec 2006, 10:39

Thanks Jess,
The circuits that I have seen, apart from not featuring this type of Pi filter, all seem to be controlled by microprocessors.

If you can find the circuit you mentioned, I'd be grateful.
I'll find my circuit and let you know all the values.
73
Dave
PS: Thanks to all.
G8UYZ
 

Postby G8UYZ » 06 Dec 2006, 20:11

I was looking at an old ARRL manual the other day;
I wonder if I could make it into a "Tee" ?

I reckon that should not be impossible, but would it work at all ??

Dave
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Postby G8UYZ » 06 Dec 2006, 20:46

I've had another look at the 'chokes' (which is what they are defined as on the lable), and written on one is "22uH".

So much for "the book".

Any chance ?
G8UYZ
 


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