[Lowfer] Matching coil

[Garry Hess]

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