Your issue with heat sinks and copper spreaders is already solved
and one can run simple fans instead of blowers. And combining two
devices is more complicated than combining 8 or 16? Huh?
I'm not saying it isn't possible - nor that is hasn't been solved.
Only that is it not less expensive than an 8877 and that those who
say a single 1.25 KW LDMOS is a suitable replacement for an 8877
are not considering other performance issues like IMD, etc.
The multiple module with combiner configurations have been around for
a long time. The LDMOS modules were designed for television broadcast
service in addition to ISM purposes. The broadcaster transmitters use
a large number of combined modules to reach quite high power levels
but each module is running less than 1 KW and the transmitters have
significant amounts of adaptive precorrection to maintain linearity.
If you want to step back and design precorrection capability into
the transceiver DSP (e.g., an input for a directional coupler/RF
tap at the output of the *system*) and can maintain linearity of
multiple modules "close enough" to use a single composite correction,
be my guest. However, the cost and complexity are an order of
magnitude greater than a good cathode driven triode - but then if
you add phase modulation to the precorrection, perhaps you can get
away saturated (pulse) amplifiers.
73,
... Joe, W4TV
On 2014-12-14 4:10 AM, Edward R Cole wrote:
Joe,
You are correct that the specs are for pulse and one can run CW/JT65 at
this level since there is no IMD produced by a single sine wave. So
combining two running 750w each gets you 1500w in linear operation with
2400w dissipation.
1500w mobile!!! Are you serious? I'd guess 200-250w would be adequate
for mobile. But I do not do serious HF'ing.
Your issue with heat sinks and copper spreaders is already solved and
one can run simple fans instead of blowers. And combining two devices
is more complicated than combining 8 or 16? Huh? If you think this is
all theoretical you are mistaken as these are out there as kits and
assembled amps on VHF to 1296. Several companies have them for sale (M2
is one). The kits come with the LDMOS already installed on a copper
spreader designed for the needed heat conduction. They haven't hit the
HF market as yet but no reason why not (be interesting to re-examine
this topic in dec. 2015).
I sure would consider a 50v PS a lot simpler and safer than a 4kV PS.
In fact I have one that I bought on e-bay for $31 made by HP (it will
run my 1100w 6m PA). I also have a HB 4kV - 1.5A PS for my 8877...cost
me a lot more than $31.
Once I use up my current 8877, I will likely sell my amp with the final
8877 "pull" that I have in hand and replace it with a LDMOS amp since I
can install it next to the base off my tower instead of having it inside
with all the noise and HV. Then I can haul the 1-5/8 inch Hardline to
the metal recycle'r as I will only need to run RG213 to the amp with
4w. Yes, 240vac will be needed but running that is not rocket science
as every home well pump is wired with buried 240vac.
73, Ed - KL7UW
From: "Joe Subich, W4TV" <li...@subich.com>
To: elecraft@mailman.qth.net
Subject: Re: [Elecraft] (no subject)
Message-ID: <548cee2e.7050...@subich.com>
Content-Type: text/plain; charset=utf-8; format=flowed
> But no longer do you need to run a dozen "pills" (man I hate that
> term). Here is all you need for 1250w 1.8-600 MHz:
Unfortunately, the LDMOS device data sheets provide *no* IMD spec's
for linear operation. The only data is for CW and pulse service. If
one is to extrapolate from similar LDMOS devices (single FET vs. two
FETs on a common die), one would need to derate to 800-900 W PEP in
order to achieve reasonable IMD levels. That conclusion is further
corroborated by the compression spec's (actual vs. ideal output power)
which show the onset of output compression above 59 to 60 dBm (59 dBm
=> ~800W).
The devices work at 1200 W CW (or JT65) because those modes are single
tone and work with saturated operation (class C amplifiers) where IMD
performance is not "tested".
At full output these devices would be as dirty in SSB operation as the
old FM "brick" amplifiers were when run in SSB service - perhaps like
the RMA Italia solid state amps <G>.
BTW, since these are 50V parts they are not suited for mobile use at
12V (13.8 V nominal) service.
> $241.50 compared with the new price of a 8877 is pretty cheap!
To even approach the IMD performance and reliability of an 8877 you
would need two devices and the cooling problems (cost of heat sinks,
heat spreaders, etc.) are much more difficult with two of these devices
than with a single 8877. Any cost advantage for even two of the LDMOS
devices over an 8877 will be more than offset by cooling system (in
addition to splitter/combiner and protection system) costs.
73,
... Joe, W4TV
73, Ed - KL7UW
http://www.kl7uw.com
"Kits made by KL7UW"
Dubus Mag business:
dubus...@gmail.com
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