[Lowfer] To Q or not to Q?

Wed, 13 Feb 2002 22:24:14 -0500

Mike,

>Yea, but every little bit helps. 

Well........... Wait till I set up the -20% experiment and you be the judge.
These super-Q coils aren't always the best of friends. They tend to be a
little snooty about their environment, so I'm told. 

>Did you read on the RSGB reflector about not bothering to use Litz wire on
a transmit loop antenna??? Interesting. Go for the higher Q coil.

Yea, but in the case of the TX loop, if your soil loss is very small (as is
John's and mine) the Rac for the loop is ~ proportional to 1/Q. With the
well-constructed monopole the earth loss is the dominate loss, and it's a
loosing performance battle with Q from the get-go. 

There are a number of other small issues in Dave's posting that I want to
address later. One of them is the use of speaker wire. This is 136kHz not
20kHz where this wire is supposed to work. The stated "just one foot above
ground" is way too close.  He indicates 0.5 ohms reflected by the earth is
trivial. Heck, my whole pipe loop Rac, including earth loss, was <0.5 ohms.
This would be >50% loss in power for me if I had dropped this loop that low.

The most interesting comment to me was from a guy stating 20Hz should be
significant in Dave's loop tuning. If the Q for any antenna at 136kHz is 200
(which is darn good), that's a -3db bandwidth of 136k/200 = 0.68k or 680Hz!
Worrying about 20Hz is off the scale! 

One basic problem our UK friends have compared to us is the difference in
loop efficiency with frequency. This varries to the 4th power of the
frequency ratio which amounts to 0.26x @190k or  ~12db. The reduction in
monopole effieciency varies only to the square of the freq ratio; less than
6db.

Bill      

Mike>WE0H
http://www.geocities.com/we0h/index.html

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Ashlock,William
Sent: Wednesday, February 13, 2002 6:24 PM
To: '[email protected]'
Subject: [Lowfer] To Q or not to Q?

Do you guys know what the effect is on your signal when you go from a plain
old garden pea variety coil with a Q of say...200 to an exotic coil with a Q
of 400? If you have a total system resistance of 25 ohms, excluding coil,
which is pretty good, the signal gain is ~20%. If the system resistance is
50 ohms the gain is ~10%. I think I will sequence my signal between normal
and -20% so you guys can see how small this difference is.

Killjoy Bill

-----Original Message-----
From: Mitch Powell [mailto:[email protected]]
Sent: Wednesday, February 13, 2002 3:10 PM
To: [email protected]
Cc: [email protected]
Subject: Re: [Lowfer] Coil question???
Importance: High

Mike:
I wound that same coil last week - following ideas from Dex's pictures.
With 18 inch diameter, I used #14 plastic covered household wiring. I
Had 11 vertical 3/4" PVC pipes and did a basket weave.
Coil was 70 turns - height was 7 inches. Inductance measured on Boonton
Q meter was 2.4 mH and Q >400. I couldn't come within 2 feet without
changing the resonant frequency.
Then used a plastic pail to wind a variometer - 10 inches diameter, 4 inches
high, close-wound - same #14 plastic covered wire.
Can tune from 2.3 to 4 mH.  or over the entire 160-190 kHz range.

Calculated all these values with Bill Bower's formulas, and was within a
turn in each case - great stuff !

Hope this helps.

73
Mitch VE3OT

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