Why is high power at high frequency so hard to achieve?

[Paul Burridge]

Hi guys,

It's pretty obvious that you can get some really good high power
transistors dirt cheap. And you can get some really good high
frequency transistors dirt cheap. But you can't seem to get high power
*and* high frequency devices cheap. And if you're talking about UHF at
anything above about 25W it seems to cost quite a lot of dough. So
it's safe to assume there's obviously some great difficulty associated
with producing a high power/high frequency device that necessitates
such high prices in this day and age. Question being: what exactly
*is* the problem??

p.

 

[Thomas C. Sefranek]

Hi guys,

It's pretty obvious that you can get some really good high power
transistors dirt cheap. And you can get some really good high
frequency transistors dirt cheap. But you can't seem to get high power
*and* high frequency devices cheap. And if you're talking about UHF at
anything above about 25W it seems to cost quite a lot of dough. So
it's safe to assume there's obviously some great difficulty associated
with producing a high power/high frequency device that necessitates
such high prices in this day and age. Question being: what exactly
*is* the problem??

p.

Winston Churchill

Simple illustration:
High power requires current density (Larger conductors),
Large conductors INCREASE capacitance.
Large capacitance means lower frequency response.

Tom
--
*
| __O Thomas C. Sefranek [email protected]
|_-\<,_ Amateur Radio Operator: WA1RHP
(*)/ (*) Bicycle mobile on 145.41, 448.625 MHz

http://hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

 

[John Woodgate]

I read in sci.electronics.design that Paul Burridge
[email protected]>) about 'Why is high power at high frequency so hard to
achieve?', on Sat, 20 Dec 2003:

It's pretty obvious that you can get some really good high power
transistors dirt cheap. And you can get some really good high
frequency transistors dirt cheap. But you can't seem to get high power
*and* high frequency devices cheap. And if you're talking about UHF at
anything above about 25W it seems to cost quite a lot of dough. So
it's safe to assume there's obviously some great difficulty associated
with producing a high power/high frequency device that necessitates
such high prices in this day and age. Question being: what exactly
*is* the problem??

There ARE process and packaging costs, of course, but I expect the main
reasons are:

- low volume sales;

- market is not price-sensitive.

Part of the gross on these exotic devices goes to support the costs of
making the devices that sell for $0.04.

 

[Mark Fergerson]

Hi guys,

It's pretty obvious that you can get some really good high power
transistors dirt cheap. And you can get some really good high
frequency transistors dirt cheap. But you can't seem to get high power
*and* high frequency devices cheap. And if you're talking about UHF at
anything above about 25W it seems to cost quite a lot of dough. So
it's safe to assume there's obviously some great difficulty associated
with producing a high power/high frequency device that necessitates
such high prices in this day and age. Question being: what exactly
*is* the problem??

Generally, high current means big chunks of stuff to
handle heat. Big active areas -> big capacitance which slows
things down. Parallelling more active areas or pieces ->
more leads which -> more inductance which slows things down
more. Getting around this requires some extreme (expensive)
thinking (remember when Hexfets came out?).

Toobs rule high power HF because metal handles heat
better than silicon and transit times are faster in vacuum
than in silicon.

Generally.

Mark L. Fergerson

 

[Georg Acher]

<RF and power>
|> Generally, high current means big chunks of stuff to
|> handle heat. Big active areas -> big capacitance which slows
|> things down. Parallelling more active areas or pieces ->
|> more leads which -> more inductance which slows things down
|> more. Getting around this requires some extreme (expensive)
|> thinking (remember when Hexfets came out?).

Another important problem: For NPN, the collector is usally the cooled side (open
an 2N3055...), AFAIK this is enforced by the fabrication process of the die.

In typical RF power amps, the collector is the hot signal side, and the emitter
is grounded, so you need to isolate the collector from the metal case (=emitter)
and try to get as less as capacitance as possible. That is where the toxic
BeO-stuff comes into play, since it conducts heat very good. But in the end, it
is a non-standard case, which makes it much more expensive.

 

[Michael A. Terrell]

Toobs rule high power HF because metal handles heat
better than silicon and transit times are faster in vacuum
than in silicon.

Generally.

Mark L. Fergerson

I guess that you haven't looked at the broadcast transmitter market
in the last ten years. Harris, http://www.harris.com/ has sold full
solid state AM, and FM radio transmitters, as well as VHF and UHF TV
transmitters. They area available to full legal power output for all
broadcast services in the US, and most other markets. The AM
transmitters are basically a modified modular high power switching power
supply that can isolate a defective module and continue working. They
seem to hold up longer than tube transmitters here in Florida, with all
of the power line and lightning problems. Another advantage. They are
80% or better efficient, compared to 30 % for a tube transmitter of the
same power level. I haven't heard of any radio station around her buying
a new tube transmitter in ten years. I haven't seen any new TV stations
built in the last ten years, but a friend of mine who worked for Harris
told me their solid state TV transmitters were very popular for upgrades
and new CPs


--
5 days!


Michael A. Terrell
Central Florida

 

[default]

Hi guys,

It's pretty obvious that you can get some really good high power
transistors dirt cheap. And you can get some really good high
frequency transistors dirt cheap. But you can't seem to get high power
*and* high frequency devices cheap. And if you're talking about UHF at
anything above about 25W it seems to cost quite a lot of dough. So
it's safe to assume there's obviously some great difficulty associated
with producing a high power/high frequency device that necessitates
such high prices in this day and age. Question being: what exactly
*is* the problem??

p.

True,

No problem unless you plan to be a commercial broadcaster.

At UHF frequencies range is severely limited unless it is line of site
and directional antennas are used - no matter how much power one
expends.

If I remember correctly it takes about 40 watts to broadcast a 100 MHZ
omnidirectional signal for about 30 miles with good signal strength.
A million watts buys you about ten miles greater radius.

The reason there are places where you can see six cell phone towers
from a single location . . .

 

[Paul Burridge]

I guess that you haven't looked at the broadcast transmitter market
in the last ten years. Harris, http://www.harris.com/ has sold full
solid state AM, and FM radio transmitters, as well as VHF and UHF TV
transmitters. They area available to full legal power output for all
broadcast services in the US, and most other markets. The AM
transmitters are basically a modified modular high power switching power
supply that can isolate a defective module and continue working. They
seem to hold up longer than tube transmitters here in Florida, with all
of the power line and lightning problems. Another advantage. They are
80% or better efficient, compared to 30 % for a tube transmitter of the
same power level. I haven't heard of any radio station around her buying
a new tube transmitter in ten years. I haven't seen any new TV stations
built in the last ten years, but a friend of mine who worked for Harris
told me their solid state TV transmitters were very popular for upgrades
and new CPs

What do these new solid state broadcast PAs typically put out then,
power-wise? (on a constant duty cycle).

 

[Paul Burridge]

If I remember correctly it takes about 40 watts to broadcast a 100 MHZ
omnidirectional signal for about 30 miles with good signal strength.
A million watts buys you about ten miles greater radius.

I don't see how that can be right. Capital Radio in central London
puts out only about 500W 50 miles from me and I have no problem
receiving the signal.

 

[Tim Auton]

At UHF frequencies range is severely limited unless it is line of site
and directional antennas are used - no matter how much power one
expends. [...]
The reason there are places where you can see six cell phone towers
from a single location . . .

There are other considerations with cell phone masts too - perhaps 3
to 5 networks covering the same area and each mast (well, transceiver,
of which there may be several on a mast) can handle surprisingly few
calls. It used to be 8, but I suspect it's more now.


Tim

 

[default]

I don't see how that can be right. Capital Radio in central London
puts out only about 500W 50 miles from me and I have no problem
receiving the signal.

Is their signal omni directional? A lot of the "antenna farms" of the
major American broadcasters are directional even in the AM broadcast
band. Build where the land is cheap - on the outskirts - and aim it
where the most revenue will develop - populated areas.

Doesn't matter where the studio is - just the antenna.

What is the frequency of operation?

Does everyone get the same reception you get?

Then I guess it may be a matter of antenna placement and type - they
sell fringe and super fringe antennas for broadcast VHF TV here that
they rate them up to "70 miles" (more - but it becomes hyperbole and a
sales tactic) but that is if you have a high location and can live
with some snow in the picture. I don't remember seeing any UHF "70
mile antennas."

 

[John Woodgate]

I read in sci.electronics.design that Paul Burridge
[email protected]>) about 'Why is high power at high frequency so hard to
achieve?', on Sat, 20 Dec 2003:

I don't see how that can be right. Capital Radio in central London
puts out only about 500W 50 miles from me and I have no problem
receiving the signal.

Isn't the antenna on the Crystal Palace mast? You are probably LOS at
just 50 miles.

 

[default]

At UHF frequencies range is severely limited unless it is line of site
and directional antennas are used - no matter how much power one
expends. [...]
The reason there are places where you can see six cell phone towers
from a single location . . .

There are other considerations with cell phone masts too - perhaps 3
to 5 networks covering the same area and each mast (well, transceiver,
of which there may be several on a mast) can handle surprisingly few
calls. It used to be 8, but I suspect it's more now.


Tim

Try hundreds of calls- I have a scanner that can pick up the analog
cell frequencies Takes it 10 minutes to scan all 800 . . .

"...... so there are typically 395 voice channels per carrier. (The
other 42 frequencies are used for control channels "

That is for analog cell phones. They get more channels on the digital
systems.

Yes, now that there is more competition and digital networks, it is
frequently cheaper to erect another antenna then pay a competitor to
lease space on their mast - but the fact remains cell phone signals
don't go very far.

Each cell covers about ten square miles. That is about a three - four
mile radius. Presumably our cell jamming friend would not have a 300
foot mast on his jammer or a power output of more than a watt or two
at the most.

The cell phone towers won't have any trouble picking up his sig, or
even triangulating the general area, once they install the equipment
to do it, but if he uses just a little restraint, stays around other
people, and doesn't brag or advertize the fact he's doing it, he's
probably safe. There isn't a "well oiled team" of anti-cell phone
jammers ready to "spring into action" and triangulate the signal down
to one person. (and probably won't be unless a lot of people start
doing it)

 

[Mac]

I don't see how that can be right. Capital Radio in central London
puts out only about 500W 50 miles from me and I have no problem
receiving the signal.

I don't think he was using exact figures. And he was ignoring altitude.

But the point is that VHF tends to have a line-of-sight range. Blasting
lots of power out will get you a little increase in range, but there is
definitely a point of diminishing returns.

Some FM stations in the US broadcast at the kilowatt level, and they
wouldn't do that if it didn't increase their range.

I have made contact with marine VHF over around 35 Nautical miles on the
open sea. This is with a 25 Watt (IIRC) transmitter with a masthead
antenna. The receive antenna was also a masthead antenna. (Say 50 feet
higher than sea level.)

I don't remember what frequency marine VHF is, but I think it is at around
150 MHz.

Mac

 

[Tim Auton]

At UHF frequencies range is severely limited unless it is line of site
and directional antennas are used - no matter how much power one
expends. [...]
The reason there are places where you can see six cell phone towers
from a single location . . .

There are other considerations with cell phone masts too - perhaps 3
to 5 networks covering the same area and each mast (well, transceiver,
of which there may be several on a mast) can handle surprisingly few
calls. It used to be 8, but I suspect it's more now.

Try hundreds of calls- I have a scanner that can pick up the analog
cell frequencies Takes it 10 minutes to scan all 800 . . .

Analog? Blimey, you Yanks really are in the dark ages when it comes to
mobile comms aren't you?

I just checked and the 8 figure is the number of calls per 200KHz GSM
carrier (channel), which is typically served by one "stick" on a mast.
Of course, you can have lots of them on a mast and usually do.

The capacity of the spectrum, the capacity of a cell and the capacity
of one transceiver are all very different things.


Tim

 

[default]

[snip]
At UHF frequencies range is severely limited unless it is line of site
and directional antennas are used - no matter how much power one
expends.
[...]
The reason there are places where you can see six cell phone towers
from a single location . . .

There are other considerations with cell phone masts too - perhaps 3
to 5 networks covering the same area and each mast (well, transceiver,
of which there may be several on a mast) can handle surprisingly few
calls. It used to be 8, but I suspect it's more now.

Try hundreds of calls- I have a scanner that can pick up the analog
cell frequencies Takes it 10 minutes to scan all 800 . . .

Analog? Blimey, you Yanks really are in the dark ages when it comes to
mobile comms aren't you?

I just checked and the 8 figure is the number of calls per 200KHz GSM
carrier (channel), which is typically served by one "stick" on a mast.
Of course, you can have lots of them on a mast and usually do.

The capacity of the spectrum, the capacity of a cell and the capacity
of one transceiver are all very different things.


Tim

Right, giving the impression that there are hundreds of calls that can
take place from the same tower at once is incorrect, mea culpa - that
would only be possible if it would not interfere with the nearest
cells to the tower you were talking about. Hundreds of possible voice
calls, but only fifty or so simultaneous duplex calls from one
system.

It may just be me living in the dark ages - I don't like cell phone
users (since the ones I see - notice - are generally doing something
that irritates me) so I tend not to use them myself. I got one as a
gift and used it once in the last 7 months.

 

[R.Legg]

I don't see how that can be right. Capital Radio in central London
puts out only about 500W 50 miles from me and I have no problem
receiving the signal.

This is within the Thames River Valley, with legislated building hight
restrictions everywhere.

RL

 

[Paul Burridge]

This is within the Thames River Valley, with legislated building hight
restrictions everywhere.

?? I beg your pardon, but I'm nowhere near that area! Admittedly, I
*am* on a high point looking out over (distant) London but that's
about all...

 

[James Meyer]

If I remember correctly it takes about 40 watts to broadcast a 100 MHZ
omnidirectional signal for about 30 miles with good signal strength.
A million watts buys you about ten miles greater radius.

If the transmitting (and/or receiving) antenna is high enough so that
the curvature of the earth doesn't come into play, then doubling the radius of
good signals requires four times the power, not millions of times.

Watts Radius
40 30
160 60
640 120
2560 240
10240 480

and etc...

Jim

 

[John Woodgate]

This is within the Thames River Valley, with legislated building hight
restrictions everywhere.

RL

So are Canary Wharf, what used to be the NatWest Tower and the Glass
Gherkin. What height restrictions? In any case, the restrictions don't
apply to broadcast antenna masts.

Incidentally, it's 'River Thames'. The Thames River flows the other way.
(;-)