Topband: New Linear amp

[Tom W8JI]

Hi Bruce,

> The new Harris Flexiva 10 KW FM band (not pulse)  stereo transmitter is 
> not
> having a heat problem with air cooling, in a reasonably small package . It
> has individual modules that can be hot switched.

I'm sure Harris did a great job. The modules can be swapped while the TX is 
on, even power supply modules, which is a nice feature. But hot or live 
swapping does not mean the modules are "hot switched", as in allowed to have 
no load while amplifying. Generally a module is designed to remove power 
first when pulled, and apply power last when plugged in, which is nothing at 
all like some ham transmitting with an arcing tuner, or without a feedline.

You will also note Harris protects the modules, which is what everyone 
should do. VSWR ratings in data sheet are really useless marketing hype, 
because near full power in longer duty cycle modes, as little as a 2:1 VSWR 
can blow a device out.

My point is about Ham gear use where people are thinking they do not need to 
watch SWR or other parameters. There is little worry in a commercial 
transmitter that runs the device at 600 watts or less per device with 
adequate protection for SWR and thermal issues, and most likely current 
limiting.

We should not translate what Harris did into something with no thermal 
protection, no SWR protection, and twice the power on the device!!

> The Harris Salesman, Engineer, Ham, said there was already  a W2  using a
> pair, but had a small water cooled system.

I played with a thick copper heatsink and air cooling, but concluded it was 
too expensive, and it would be a manufacturing PITA to get the surface 
machining flat enough in production. With 1000 watts of heat in such a small 
footprint, the surface almost has to be polished flat. The copper spreader 
would have cost about $30-40, plus the aluminum heatsink would need to be 
reasonably flat all across the surface. I'm not even sure that would work 
without liquid cooling.

It makes sense hearing the W2 used liquid cooling.

At current costs for cooling and the devices, it is better to use more lower 
power devices and spread the heat around. Especially when the devices are 
just as reliable, have a history of good SSB IM performance, and cost less 
per watt.

My guess is the marketing blitz and creative specs are why we find a few 
amateur "soon to be released" products without any final specifications. 
They probably believed a single 1200 watt device would be linear at 1200 
watts, and would actually handle high SWR without protection. Anyone who has 
actually worked with high power RF semiconductors knows the VSWR specs are 
meaningless marketing drivel.

Like I said, I ran a pair of MRF150's in pulsed service, in deep class C, 
and they were safe and reliable at a kilowatt without any SWR protection, 
other than power supply limiting. Because they worked in pulse service deep 
class C in a low-Q system at 1000 watts without worry  doesn't mean 2x 
MRF150's would work on CW at that power, or be reliable at 200 watts CW 
without SWR and thermal protection.

Apples and oranges.

When the device price comes down a bit, or if they make a smaller device 
that allows spreading the heat around, you'll probably see the Freescale 
device in Ham gear.

73 Tom