[TowerTalk] Tuning 40M & other beam elements across the band1 rpm

[Richard W. Ehrhorn (W0ID)]

Here's a practical & realistic consideration I haven't seen mentioned -- but It probably ought to be taken into account when worrying about gain & F/B across the 40m (or 80/75m) band: 

IMHO not much is gained by worrying about optimizing gain & F/B on CW. CW has three advantages: very narrow transmit and receive bandwidths - roughly 1/8 to 1/10 as wide as SSB - and the human ear/brain's discrimination ability. Not only that, but it's typically easier to know when and where to drop in your call. I think most will agree that it's nearly always a lot easier to get through even severe expedition and contest pileups on CW than it is on SSB. The last few dB of signal strength and F/B simply aren't as crucial on CW as on sideband. And your receiver really doesn't much care about VSWR or line loss - at least not on the lower bands. 

So my approach on any band is to optimize gain & F/B in the part of the phone segment used most for DX...and remotely tweak matching of the DE to make the amplifier happy & minimize xmit line loss. If the coax is good and not much over 100' it may be quite satisfactory just to use a simple "L" network at the amplifier output to make it happy wherever you're transmitting. 

At W0AIH's contest superstation, Paul has successfully used the latter approach for decades. He inserts simple L networks housed in shiny new little paint cans at the input of each feedline, using mostly receiving-sized air caps. Since my antennas and feedlines are vastly fewer than Paul's (aren't everyone's?) I house multiple (reversible) little band-by-band L nets in a box right at the operating position and choose feedline & L network with 5 - or 6-way relay boxes. TopTen, DX Engineering, Drake and homebrew boxes in the shack with ALPHA 77Dx and ALPHA 87A amps have never been a reliability problem. Note that feedline and/or antenna feedpoint input impedance may and often do change enough across a given band to require reversing an '"L" matching network for wide excursions in xmit freq.. 
   
For long coax runs and/or purists, remote tweaking of antenna VSWR is easy & inexpensive to accomplish - except maybe in extreme environments like coastal or marine where a vacuum variable cap may make weatherproofing easier. But a housing of inexpensive 4" PVC or ABS pipe & end caps (Home Depot or Lowes) will take almost any small ~1 rpm gearhead reversible d-c motor (Grainger) & small "breadslicer" air variable cap (surplus, EFJ or Cardwell) and keep them (and relays, if necessary) quite dry & clean for many years. It's crucial to use only stainless hardware through the plastic. 

Good luck!

Dick  W4ETO/W0ID

 

 

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