[Lowfer] Pill bottles...

Tue Sep 15 20:07:47 EDT 2009

One of the questions Ive been mulling over is how far  a screened
"room/box" relative to the dimension of the coil and operating frequency
and secondly the net of having the coil too close to the ground whereby
the flux is either compressed, short-circuited or dumped.  I think I
remember as a ballpark least 1 x D and H...?

In large Loran and some larger ATU rooms they are screened with
foil/copper just to stop such variations but I'm guessing there must be
a min dimension/space between the coil and the screening
material.,...where my Termans/sliderool...

I do know I needed to keep my LF AtU a couple of feet off the ground not
to see an impact on the Q (keep it off concrete especially with rebar!)
, though I never got as far as screening it to stop local capacitive
affects detune.

Not all plastics are the same - one trick I did in Singapore when trying
to design a stable LF oscillator was to make a free running LC coil on
137khz and then offer up differing types of plastic with a long non
metallic grips to the tank coil and note the detuning affects of each
type of plastic monitoring the freq change using a nearby loosely couple
SSB rx. 

I guessed the smaller the change in freq and interaction with the
material the better it was as a coil former etc...The result was  the
choice of a  strong clear plastic pill bottle which I drilled out excess
plastic in vertical channels, though still rigid enough for  coil
stability. There was a lot of variance - and not just the color of the
material....

When I took all my materials under test and loaded them into a microwave
oven there was little to no difference after 30 secs of irradiation in
temperature at microwave...so I don't think the microwave test is fully
repeatable at LF..well not with the materials I tested.

Laurence
WE2XPQ Wasilla Alaska
BY3A Tianjin China.

-----Original Message-----
From: lowfer-bounces at mailman.qth.net
[mailto:lowfer-bounces at mailman.qth.net] On Behalf Of Andy - KU4XR
Sent: Wednesday, September 16, 2009 4:00 AM
To: European & UK) and MedFer bandsDiscussion of the Lowfer (US
Subject: [Lowfer] Definitely did something wrong !! But fixed it.

Hey lowFERs:

I'm posting this just for " this is what happened sake "
I put a paperbacked book about 3" thick on top of the 
variometer bucket. and I could then lay my hand on the book
and the tuning would change only slightly. Remove the book,
and place a hand within 3" from the top side ( hot end of coil ,
or within 5" above the top, and the tuning changed dramatically.
And just to see what would happen; I placed a sheet of aluminum
foil over the bucket, and the output droped to ZERO, and could 
not be tuned with the variometer.
I am not an antenna specialist, nor a coil guru, so I'm not
really sure that this is a higher " Q " issue or not. My thought
would be that if the coil " Q " went up, then my tuning would
become very sharp, and critical. After removing the bottom from
the bucket, the tuning sharpness didn't increase nor did the
output change when peaked. The increased proximity effect around
the variometer was the only thing noticable.

Still having fun; 73 to all ::

Andy - KU4XR - EM75xr - Friendsville, TN.
LOWfer Beacon " XR " @ 184.322 KHz ( QRSS-60 )
Coordinates:  N:  35* 43' 54" - W:  84* 3' 16"
http://www.myspace.com/beaconxr

--- On Tue, 9/15/09, Peter Barick <pbarick at niu.edu> wrote:

> Date: Tuesday, September 15, 2009, 1:08 PM
> Hi Mike, Andy,
> 
> Interesting on that bottom cutout, and surprised me. If the
> pvc bucket is thought to be nearly RF inert then why that
> effect? As described, it appears the Q has risen greately
> for such effect. Mike talks to this too.
> 
> I believe Mitch (MP) spoke of it also, causing the need for
> an extension rod with which to adj. the vair coil.
> 
> High Q is usually thought to be the goal (or bragging
> point) and causes reactions as above. But too much can lead
> to wx affected untuning and demands for attention to keep it
> spot on. Ha, Teslas we are Not!
> 
> Just some observations -- Peter

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