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[AMPS] Choke resonance

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Subject: [AMPS] Choke resonance
From: w8jitom@worldnet.att.net (Tom Rauch (W8JI))
Date: Wed, 12 Mar 1997 08:01:41 -0500
One way to test a choke (lacking measuring equipment) is to use a T
connector (or a simple splice) to access the line from your rig on the
on a way to a standard dummy load.

Ground the shield to the amps chassis and tap off the center of the
cable with one lead of a small (about 30-100 mA) wire leaded 12 volt
bulb.

Disconnect EVERYTHING external to the choke from the top of the choke
and connect the free bulb lead to the top of the choke, while leaving
the bottom of the choke connected.

Put the tank on 160 (or lowest band) and mesh the capacitors, so it
doesn't act like a suck out trap. LEAVE THE AMP UNPLUGGED.

Now drive the dumy load with RF and watch the bulb. Adjust the Rf til
the bulb just lights,  and spin the VFO dial on your rig.  

The bulb will get real bright at series resonant points of the choke, or
wherever the choke looks like a low impedance. When you find a series
resonate point near or in a desired band, move the blade of a small
scredriver along the choke coil. When the blade is next to the hot spot,
the bulb will go out.

If you remove choke turns at that spot, the series resonance will move
up rapidly, while the other choke characteristics will barely change. 


This is not like using a sweep impedance, but it works well. You can
learn where the choke is resonant and what spot of the choke is the "hot
spot" at that frequency without blowing things up (and mistakingly
thinking you had a parasitic, when it was really a series resonant
choke).

By the way, the choke resonance moves all over the place as the choke is
placed near other objects. Check it in it's final position.

73 Tom



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