RadioBanter - View Single Post

Richard Clark · March 2nd 07, 08:15 PM posted to rec.radio.amateur.antenna

On 2 Mar 2007 11:33:57 -0800, wrote:

Hi Jim

Most inexpensive amplifiers do not have "well behaved" properties at
RF (i.e. they change all their parameters as they heat up).. this
makes dealing with phasing and mutual coupling a bit challenging If
the output Z changes, then the network matching it to the element
needs to change..if the gain and phase through the amp changes, then
the drive needs to be adjusted.

If you are interested in a design for yourself, and maybe a production
run of a few hundred, then look to the HF/10M/6M/2M surplus repeater
market from Motorola and RCA designs of the 70s and 80s. Dirt cheap
decks with enough elbow room to make mods.

Fortunately, the problems are solvable, at least in a theoretical
sense.

One doesn't usually see fortunately paired with theoretical.
ironically this is how this thread started - was with a pig in the
poke explanation passing for science

All you need is a few kHz, so a single L and C might do it,
"good enough".

So, this daydream is on one band only?

But it turns out that you want a different kind of phasing for receive
than for transmit (not only a different pattern, but it turns out
you'd like to do it a different way... null formation being one
reason).. that pushes you away from a simple adjustable LC phasing
network for the receive array. For receive, you'd also like it fast
(and, potentially, multiple beams at once).

Ah, there's always a big but (as PeeWee Herman would say).

Programmable oscillators that shift immediately and start at any point
in the cycle (absolute phase AND frequency control) would be miles
further down the road.

That's available now.. it's a DDS. And you can buy a radio off the
shelf that has these capabilities (the FlexRadio SDR1000), although,
there are a couple difficulties with the flex (for one, they didn't
bring the sync input to the DDS out to the connector). You can buy a
eval board from Analog Devices for $200.

OK, Analog Devices is a star performer. I built a tube version of
this DDS back in '68 when it was called a coherent detector (could
have been called many names depending on where you developed the audio
output).

However, this is NOT what I was referring to, as that is distinctly
different. This is a software controlled oscillator whose frequency
and phase is immediately settable (within on clock, this is in the
nanosecondS range). If you need to phase each array element
independently to phase steer the combined system (also to take care of
phase matching through mutual coupling), the software solution spring
immediately to the front for a solution.

However, as I pointed out, it means N individual drivers following
those oscillators so that each can be phase controlled for the system
combination. The downside finds our current investment sitting in the
corner again, and our investing more design effort into SSB
generation, and hence, feature creep.

ordered to a single frequency. You still have the elemental clock osc
XTAL for processor and driver, but those litter the world for pennies.

Interestingly, in a very much higher budget arena (deep space comms
and ranging), they're also doing this. Until recently, you had to
special order the crystal for your spacecraft radio (with 18+ month
lead time!!), so if you had a channel reassignment, it was a real
problem. (One of the Mars Rovers and the Mars Reconnaissance Orbiter
are on the same channel.. wasn't supposed to be a problem because the
rover was not planned to survive long enough)

The software oscillator I described in the previous post would solve
that for the same cost as the custom XTAL.

If you are looking for a design and a market, I cannot think for the
life of me why that hasn't happened yet.

73's
Richard Clark, KB7QHC