The Variometer ?

[Dave UXB]

Adjustment of Inductance in such cases as a Pi-tank and similar is usually a tapped coil (or, if possible, a Roller-coaster inductor). However the old way was a Variometer (a scheme which features on coil rotating inside another).
I have a requirement for an adjustable inductance to match an aerial to the Tx, preferably over the whole HF band. [Yes, I know I could use a tapped coil, but I really would love to make one of these beasts.]

I have had a look at Harmsworth's Wireless Encyclopaedia (1924) and there is some information there, but of course, it is mostly for Medium Wave.

Has anyone any clues about such a beast please?

[Herald1360]

I reckon you would have problems at the HF end with parasitic resonances in any coils with enough inductance to work at the LF end of the HF band that relied on the cancellation technique of a variometer. ISTR it's hard enough to come up with a tame anode feed choke that will cover the whole band.

You might be able to copy the technique used by Racal in some of their mobile units- wind a braided conductor between an insulating screw former (the active bit) and a metal cylinder (shorts out all the unused braid).

[Alan Stepney]

At least one of the "Command" transmitters used one.
It was the set that covered, or could easily be modified for, 80m.
(I should know the number, but cant recall it right now.)

I dont have the set but do have the variometer from it.
What details do you need?


Whilst it would be possible to make one, a roller coaster would be easier.

[neon indicator]

The 19set used one.

I have seen one for 137KHz.

The Racal syncal30 uses a "better" scheme. (I'm sure what Herald1360 is referring to. I have one of these!)

Tapped, shorted coils and roller coasters have issue of losses due to coupling of unused part of coil.

So the Racal syncal 30 has a large insulated hollow drum with grooves. The braided wire is wound on/off a small metal spindle parallel to it. It will tune an 8ft whip from about 1.8Mhz to 28MHz. A CB whip on car roof from 3.5Mhz to 28MHz. There is a French set that has this motorised and an "auto" button.
In use the power meter has 2 LEDs, one with Left arrow and one with right arrow. You turn knob in direction of arrow. Both LEDs will dim, light or flicker at optimal point and power meter peaks. Tuning on Low power recommended, but some version automatically set low power in "Tune" mode!

The Variometer on 19 set was also for tank whip.

Neither tuner works well for longer aerials.

The KW107 is an interesting tuner that works best with a vertical for 17M to 10M and long wire for 80m to 20m. It usually needs an extra coil switched in for 160m. Both aerials are connected at the same time.

The best "modern" approach is:

7 or eight coils that can be switched in series air core and wound on toriods in binary sequence, giving 128 to 256 values of L (including pass through short)
7 or 8 capacitors than can be switched in parallel, giving 128 or 256 values, including (none = open).
a switch to put the C on aerial side or receiver side of coil.
Most auto-match "tuners" use this, but I have seen one manual unit.

[Dave UXB]

Originally Posted by Alan Stepney

At least one of the "Command" transmitters used one.
It was the set that covered, or could easily be modified for, 80m.
(I should know the number, but cant recall it right now.)

I don't have the set but do have the variometer from it.
What details do you need?

Whilst it would be possible to make one, a roller coaster would be easier.

I'd appreciate some photos, dimensions, etc..

I recall the WS19 device; I wish I still had it.