On Fri, 7 Nov 2014 01:08:33 -0000, wrote:

In the last installment we got the input impdance up to the order of
2 Ohms with loading and a top hat.

2 Ohms will still lead to lots of loss in the matching device, so let's
see if the input impedance can be further increased.

Idea: There are times when matching to 50 ohms isn't the best
approach. I had that problem once and decided that the solution was
to move the power amplifier stage to the antenna. With an emitter
follower power output stage, I was able to get source impedances of
less than 1 ohms fairly easily. The base of the emitter follower
ended up about 100 ohms, which was easily matched to 50 ohms with a
transformer.

The big problem was keeping the emitter followers stable. The usual
ferrite bead on the emitter leads helped, but did not provide
unconditional stability throughout the range of antenna tuning. I
never did stabilize the amplifier mostly because this was new to me at
the time and I didn't have decent models of the components involved.

Another problem was the high currents required that everything be big,
silver brazed, silver soldered, and expensive. Using silver plated
tubing seems to work best. I was playing with milliohms and spent
much time with a Kelvin bridge measuring tiny resistances. The worst
problem was that the antenna also had to handle the high currents,
which required heavy construction, thick plating, and lots of silver.
Most of the current flows in roughly the first 3 * skin depth. At
1.8MHz, that would be about 0.15 mm. I didn't think the world was
ready for silver plated antennas, but it was worth trying.

Ever see a lock washer glow red? That was fun.

Fortunately, it was a symmetrical antenna so I didn't need to deal
with high RF currents through a counterpoise (i.e. grounding system).

Using cut-n-try and copious overtime, I managed to deliver a prototype
that mostly worked. The fatal flaw was that the customer wasn't too
thrilled with having a fairly complex system split between two
locations (transmitter building and tower shack) and supply high
currents via garden hose size cables between sites. When lightning
hit nearby and blew up the output stage and most of the attached
monitoring equipment, it was decided to use a more conventional
design. I was not disappointed as I was seriously worried how I was
going to make it work reliably. Fortunately, the split site might not
be a major problem for ham use.

For 160 meter operation, I think this can be a useful method if the
power levels were kept low enough to prevent meltdown.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558