Topband: wideband inv-L

[DAVID CUTHBERT]

We are confining our design space to an inverted-L at this time. I did manage to achieve 200 kHz (2:1 VSWR) in simulation for a 45 foot tall inverted-L having a ground loss of 4 ohms. I can fire up a working model at a few hundred MHz this week (initially without the impedance transformation). I don't know that Dave wants to build this particular model as it is essentially a flat sheet (made of many wires) 50 feet across for 1.9 MHz. However, with a 12 foot wide inverted-L the 2:1 VSWR bandwidth is 100 kHz and going with a 16 foot wide gives 125 kHz. I have incorporated the impedance transformation into the basic design so we do not need any bandwidth robbing L-network. 

The antenna ends up quite similar to a couple I have designed for commercial use. The new part is the modification of the impedance transformation to allow one to select the integer step-up ratio without regard to the physical antenna width.

I did experiment with shunt T-lines and LC networks for further extending the bandwidth (using WinSmith). I do see the double VSWR hump and I do see a modest increase in bandwidth. However, the slightest loss really extends the bandwidth and I suspect this is what many find. I think we will avoid this route.

      Dave WX7G