[Lowfer] TX Output Filters

[WE0H]

Will do. Thanks again.

Mike>WE0H


-----Original Message-----
From: lowfer-admin@mailman.qth.net [mailto:lowfer-admin@mailman.qth.net]
On Behalf Of Bill Ashlock
Sent: Sunday, August 03, 2003 12:18 PM
To: lowfer@mailman.qth.net
Subject: [Lowfer] TX Output Filters

Mike,

>I have noticed lately after running the beacon for several months that the
>output toroid in the lowpass filter is warm. Not hot but warmer than the
>outside temp all the time. The finals you sent run stone cold all the time.
>I was thinking of jumping around the filter and seeing what the loop
>current
>would do without it. I am pretty sure there is power being used to heat
>that
>toroid that could be more well used in the loop instead.

This brings back a standing disagreement I have with most of the guys. I say
the filter is not needed - at least more trouble than it is worth and almost
everyone else (I suppose all Hams) insist on using one. Two years back I ran
hours of P-spice output LP filter simulations and they all indicated there
is a frequency above the intended cut-off where a high current spike occurs
IF you don't match the filter with its proper load resistance. Your antenna,
because of its very high Q, has a resistive load only at the frequency of
resonance and this is reflected back to the filter at a value of ~70 ohms.
Only 1 kHz, either side of the loop's resonant freq, the load on the filter
approaches infinity. If you look at it on your scope the neat square wave at
the filter's input turns into a nasty ringing something-or-other at the
output.  This waveform consumes power and cuts down your efficiency no
matter what way you cut it!

The southern leg of my N/S loop (currently a #12 - 50'/50') resides only 8ft
from the house, yet I have a lot of trouble finding a harmonic on an AM
radio inside the house next to that same wall. I ran a mobile test at 40
meters, where the loop is most efficient, and found the harmonic of 185.3 in
this band (of course VERY narrow bandwidth) is into the noise within about a
mile radius from the antenna.

So get rid of it!!

Bill

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