TUNING UP

After doing the calculations and creating your TX antenna, the next major issue involves getting it tuned up and resonated, while presenting a 50 ohm non-reactive load to the transmitter.

When first getting on the band in 2012, I constructed a simple 50 ohm SWR bridge consisting of a single turn of heavy wire passing thru a toroid with 50 turns on the secondary, a capacitive bridge, and associated diodes, meter and sensitivity control.  The generic HF bridge, CB Radio Shack meters and the like just don’t work accurately at LF and MF frequencies.  If you can afford a Bird slug for your Model 43 at these frequencies, go for it!  So initially, this meter allowed tuning for minimum SWR at 50 ohms and worked well enough to avoid blowing anything up.  I’ll post the actual schematic in a bit.

IMG_0347

Quick and dirty MF SWR bridge, sheet copper panel in a plastic box.  Visible in the background is a homebrew transmit converter driving the 2x frequency doubler (aluminum box) that drives a 500W Class D PA.

RIG EXPERT AA-30 Antenna Analyzer

A real antenna analyzer really helps!  I purchased the AA-30 and it has proven invaluable.  It tells one everything they need to know on a series of screens – SWR as well as actual resonance point, real R, real Z and actual feedpoint impedance at a glance.  It will also produce a Smith Chart on a PC via a USB interface.  Best money I ever spent on a single piece of test gear.

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RIG EXPERT AA-30, connected directly to the feedline

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Closeup of the AA-30 in SWR mode.  Here I’m running a ‘mushy’ feed as I have several capacitors on-line at the tuning box at the feedpoint to compensate for a soaked radial system.  Normally SWR is 10:1, 5 kHz off resonance at either side.  Operation is always lower than the resonance point, as the actual WSPR signal transmitted is at least 1.5 kHz above ‘dial’ frequency.

SCOPE MATCH

If the voltage and current are in phase on the transmission line one can be confident that a reactive component of antenna Z is not present in a properly set up tuning system.  A Scopematch is simply a bridge that isolates the E and I phase relationship on the transmission and displays them on an oscill0scope.

good

E and I in phase – good!

goodnot

OUT of phase, reactive load

 

 

MORE TO COME, Please stay “tuned”.

 

myweb.tiscali.co.uk/wgtaylor/LFTA.pdf

Cut/paste above into your browser …

Above link to the test gear – TU Jim Moritz   M0BMU !!