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THE AM BULLETIN BOARD => Technical Forum => Topic started by: WU2D on January 20, 2008, 07:42:20 PM

Title: Analyze or Cut and Try?
Post by: WU2D on January 20, 2008, 07:42:20 PM
Maybe if I owned a network analyzer or one of those fancy bridges or an MFJ antenna tester, I would be able to get my Inverted L to match on 10 - 20M.

Being the old cut and try guy, I cut the antennas to the formula or a take a guess at the impedance and eventually I get an answer.

My 240 ft inverted L has presents a high impedance on 160M - 40M, that much I know. For years I have used a simple L match with a motorized cap on the end along with a big old switch. This has worked FB on 160M - 40M. On 20M and up, I never could get the L match to do better than 3:1 VSWR. My guess is that it is a low impedance on these bands.

Well today I was fooling around with a cap and some clip leads and I think I have invented something - I think I will call it the Pi match- Hi. Tunes those hard to match bands real nice. I even found a way to make the other poles on my switch put the new variable in when and where I need it.

Mike WU2D

Title: Re: Analyze or Cut and Try?
Post by: N3DRB The Derb on January 21, 2008, 12:52:11 AM
if it works, it don't matter. use it.  8)

I'll print that out and save it. Thanks!

Title: Re: Analyze or Cut and Try?
Post by: k4kyv on January 21, 2008, 04:35:21 PM
I have always been able to do it more quickly using the cut-and-try method.  I have tried calculating the values of all the elements in the networks, down to the number of turns in the coil, and inevitably my most careful calculations, which may have taken a while to do, end up being off by 30-50% by the time I get the circuit working.  In the same amount of time or less, by intuition I can guesstimate the capacitance, number of turns, whether series or parallel, where the taps go, and cut-and-try it from there, and come up with a network that works as well as I could ever hope for.

That's what you are basically doing with a tuner like  the Johnson Matchbox.  You have all the elements the in front of you, with numerous switching capabilities, and use trial-and-error to find the appropriate settings.

Of course, it takes some hands-on experience to develop the intuition.
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