[Lowfer] Matching coil

[Andy - KU4XR]

--- On Wed, 10/13/10, Garry Hess <k3siw at sbcglobal.net> wrote:

> Andy,
> 
>  From earlier posts I gather your antenna with raised
> inductor of about 
> 2.3 mH is self-resonant around 290 kHz. That implies an
> antenna 
> capacitance on the order of 130 pf.
>
> 73, Garry, K3SIW, EN52ta, Elgin, IL

_______________________________________________________________

Hi Garry:  That's the same figure I came up with today, but didn't 
try to factor in the antenna capacitance itself. Using a resonant
frequency calculator; it showed a needed 5.68 Milli Henries to be
resonant at 185.28 KHz... The total Inductance needed to resonate
this antenna is right now 4.88 Milli Henries, which, if you take
the antenna self capacitance into consideration; this appears to 
be pretty darned close. I knew I would need more Inductance, I 
just didn't know how much. The smaller top hat - farther from 
the ground really made a big difference. If I can afford the wire,
I really need to rewind the inside matching coil, and get rid of 
the very small wire. Way too much resistance in it, but at the
least, it is working right now until I can do better.

73, and thanks for your comments:



Andy - KU4XR - EM75xr - Friendsville, TN. USA
LOWfer Beacon " XR " @ 185.29875 KHz ( QRSS-60 )

*** Temporarily " TESTING " the new antenna ***

Coordinates:  N:  35º 43' 54" - W:  84º 3' 16"
http://www.myspace.com/beaconxr

*** The 5 most important words ever spoken ***

http://ku4xr.webs.com/5mostimportantwords.htm


--- On Wed, 10/13/10, Garry Hess <k3siw at sbcglobal.net> wrote:

> From: Garry Hess <k3siw at sbcglobal.net>
> Subject: Re: [Lowfer] Matching coil
> To: lowfer at mailman.qth.net
> Date: Wednesday, October 13, 2010, 7:56 PM
> Andy,
> 
>  From earlier posts I gather your antenna with raised
> inductor of about 
> 2.3 mH is self-resonant around 290 kHz. That implies an
> antenna 
> capacitance on the order of 130 pf.
> 
> As a check, consider the rule of thumb for the capacitance
> of a vertical 
> antenna of 6 pf/m, or about 2 pf/ft (see 
> http://strobbe.eu/on7yd/136ant/#CombTop).
> Your 40' mast can thus be 
> expected to present something on the order of 80 pf. But
> your tophat 
> adds to this. Your complicated structure isn't addressed
> directly by the 
> previous URL, but a second rule of thumb is that a single
> horizontal 
> wire at the top adds approximately 1.5 pf/ft. From the
> picture of your 
> antenna it looks like you have something on the order of
> eight 3' wires, 
> or 36 pf. Since they are all connected at the periphery the
> net 
> capacitance would be somewhat greater than 80+26=106 pf.
> 
> One last estimate of antenna capacitance can be made using
> your comment 
> that 4.76 mH total is needed to tune it to 185 kHz. That
> implies a 
> capacitance of 155 pf.
> 
> Conclusion: your large inductance does seem in line with
> the antenna 
> structure you've built.
> 
> I don't have a lot of experience winding variometers but
> I've certainly 
> learned that Litz wire is essential (see 
> http://en.wikipedia.org/wiki/Litz_wire).
> Winding a coil with 2.5 mH and 
> low DC resistance isn't hard and need not result in a huge
> structure. 
> Unfortunately, all the coils I've wound with magnet wire,
> while having 
> great DC resistance, have significant resistance at AC
> (meaning 185 
> kHz). This isn't because of the skin effect, but rather is
> the result of 
> the proximity effect. Even using 20/40 Litz wire, the best
> I've done 
> with a 2.5 mH coil is a Q of about 250 because the length
> of wire needed 
> has a DC resistance on the order of 10 ohms. Litz wire
> keeps the AC 
> resistance at about the same value but a good ground system
> should 
> present no more than something on the order of 35 ohms of
> resistance in 
> series with the capacitance mentioned earlier. If the PA
> drives that 
> load via a variometer with 10 ohms resistance, the true
> efficiency is 
> only about 80% even when the PA is 100% efficient. It seems
> clear why 
> really good lowfer beacons like WEB use 175/40 Litz wire.
> 
> 73, Garry, K3SIW, EN52ta, Elgin, IL
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