[Elecraft] LDMOS for QRO

[Edward R Cole]

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" <lists at subich.com>
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] (no subject)
Message-ID: <548CEE2E.7050400 at 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:
     dubususa at gmail.com