On Mar 1, 10:32 pm, Richard Clark wrote:

I can sense that this discussion is leading back to a separation of
driver from amplifier. You could still use a power divider to feed
the remote amps at the various array locations. Say 10W at 10 active
antennas, each with a 100W rating.

This probably the most practical thing.. and in fact what I'm working
on prototyping. The challenge is in the amplifiers (although the
recent FCC ruling doing away with the anti-CB amplifier rules will
help)

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.

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


This would preserve investment, and create and alternative to the
Henry market. Hams would have two purchase paths instead of
discarding their introductory base station and opting in for N number
of active arrays driven by a specialty item that looks like their old
rig gathering dust in the corner.

Yes.. this is exactly the growth path I would envision.


There's no reason to expect you need a large transmit antenna for
solid state amps either. The native source resistance of a transistor
is quite low, and has to be transformed UP to match 50 Ohms. If you
had a radiation resistance of only several ohms (a very short
radiator) all you have to pay attention to is cutting Ohmic loss and
providing flexible inductance. Unfortunately, this may be a
performance killer - but if you are demanding multiband performance,
you will have to answer for this for any size array element.

Indeed, yes.. fortunately, you already need to have an adjustable
impedance transformer (because the feed point Zs change with frequency/
steering), and even more fortunately, you don't need a broadband
match.. All you need is a few kHz, so a single L and C might do it,
"good enough".

There IS a strong signal IM
problem..so maybe active receive antennas aren't the right solution.

The same transceiver that survives IM would still handle it from
several phase active array elements. As you can see, redesigning a
new driver eventually leads you back to the gear you have. If you now
demand a separate receiver, separate driver, and separate active array
antennas, costs rise faster by the number of connections.

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).


Adaptive nulling is a bit weird to
work with as a user, especially if you expect to control it. And, for
hams, they want a bit more control.

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.

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)




Using this kind of technology is about the only reason I would trash
my current rigs and go for a custom driver (but it would have to
inhabit each array antenna's amp and thus render it a complete
transmitter. This would in turn cast all the features (like SSB
generation) into each of those elements. The unit cost of this would
climb because of feature creep, not component pricing.

Kind of depends where you divide up the building blocks. The raw
exciter cost (DDS plus mod plus D/A converter for modulation) is quite
low.


Jim