Topband: Hairpin Matching Coil Questions

[Charlie Cunningham]

Should work just fine, Mike, since your inverted L is > 1/4 wavelength and
has some series inductive reactance. You're tuning out a portion of that
series inductance, leaving the remainder to work with the shunt capacitor to
form an L network to transform the resistive component of the inverted L up
to 50 ohms. Even at full power, I wouldn't expect enough voltage at the 50
ohm point to arc capacitors of any reasonable spacing!

Regards,
Charlie, K4OTV




-----Original Message-----
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Mike
Waters
Sent: Tuesday, September 17, 2013 1:24 PM
To: topband
Subject: Re: Topband: Hairpin Matching Coil Questions

I didn't read everything in this thread, but there's a good alternative to
a matching coil. On my 160m inverted-L, I just use two air variable
capacitors, and they work great. One adjusts the impedance, the other tunes
out the reactance. The interaction between them is fairly small.

Like this:
http://www.w0btu.com/files/antenna/
http://www.w0btu.com/files/antenna/Inverted-L_L-network_100_3760.JPG

One cap is in series with the coax center conductor and the antenna (a 155'
inverted-L).
The other is in parallel between the coax shield (and two 1/4 wave 10' high
elevated radials) and the point where the first cap is connected to the
antenna. They don't arc or heat up at 1500 watts, and I'm sure I could have
used caps with a smaller spacing.

There's no ground rod. And not shown is a choke made from several turns of
RG-6 feedline through 4 or 5 stacked 2.4" OD #31 ferrite cores.

I use an MFJ-259B between the coax connector and the choke. Just keep your
antenna analyzer up off the ground lying on a dry 2x4 (or other insulator)
while you're adjusting the capacitors, or you'll get screwy readings.

There's also a small spark gap in parallel with a 33K Ohmite OX resistor,
connected between the antenna and radials.

I've got some more photos and a schematic with capacitor values here, if
anyone is interested.

73, Mike
www.w0btu.com
_________________
Topband Reflector