Re: LF: Variometer question

[Andy Talbot <andy.g4jnt@gmail.com>]

There's also another issue with having a variometer in the shack, driving a length of transmission line then the loading coil / antenna.   I discovered this by chance, and almost wrecked a 73kHz transmitter (back in those days)  before realising the error.

With a shack variometer you have the following network :

Series L (the variometer) 

Shunt C (the feeder)   In my case 12m of RG58 with around 1000pF

Series L (the loading coil)

Shunt C (the antenna)

As with any multi element L/C network you get several poles / zeros which manifest themselves, here, as two points when reactance is zero, but with wildly different transformed impedances.  In my case it was around 200 ohms (the wanted one) and also one at 6 ohms or thereabouts.   Since I had been adjusting the variometer using phase angle displayed on a scope, guess which point I found first.   The amp was a MOSFET audio amplifier bridge  and fortunately I spotted very high drain current in time and it survived.

The capacitance of a few metres of coax may seem insignificant at LF, but when introduced into part of a multi L/C network it can throw up surprises.  Play with a network analysis programme and see what happens

Andy  G4JNT

On 20 December 2015 at 10:47, DK7FC <selberdenken@posteo.de> wrote:

Hi Chris,

Am 19.12.2015 12:28, schrieb Chris Wilson:

Christmas last minute work and home jobs have got in the way.

Same here. There are always open projects :-)

  It's
electrically and mechanically complete and working save for the
microswitch end stops, which I rushed and in the light of day was
relegated to the bin :) I will redo these in a more elegant manner!
   

Uuh, so something must have been wrong connected?!

Is there any hope the variometer could be in the shack at the amp end
of the co-ax feed to the loading coil?

Yes this is possible too and has been done by some in the past. But it is not the best choice to match the system. There can be significantly higher losses. Also the voltages across the coax cable can be very high, depending on the reactance that you want to compensate.
We are talking about LF, don't we?!
Also you cannot use a transformer any more between coax and loading coil because it will saturate at higher voltages...
You can use that method for a fine-fine-tuning if you want to have an SWR of 1:1.00000000000000000000000000000000000000000 but it's not a good choice to tune a narrow band antenna from 137.7 to 136.17, for example.

Thanks for your help Stefan, and a happy Christmas and a healthy and
prosperous New Year, I have to say I am disappointed you haven't put
an angel at the top of "your" tree ;) All the best I am always amazed
at your ingenuity and courage in the woods, take care.

Maybe i can find an angle before xmas is starting. But we have +16 °C here these days.
What i thought i could do up there in the tree is a nice breakfast with some coffee! :-)
When is the time for teatime in UK? And do you really ALL have teatime to that time there? ;-) Probably not. Same like here, we are not at all eating Sauerkraut all the time :-)

73, Stefan