Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
you would like to post something on RF Cafe's
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
Post subject: SUPER-DUPER PIN DIODE needed Posted: Mon Oct
17, 2005 5:58 am
I never used pin diodes in my projects,
hence looking for your, guys, advice.
The project is a magnetic
loop autotune antenna. Currently it is tuned by vacuum cap driven by
stepper and works fine with average tuning speed of 3-4 seconds. On
lower bands It needs additional caps and these added by jennings vaccum
relay that also controlled by the same CPU. These caps and relays are
6kV/20Amps rated, lower rated arching, checked. The loop itself is a
very high Q circuit and has reactive impedance. Although 100 watt PEP
is the power I have to multiply it by Q=900 in order to rate these components
Now I want to use my FH (freq. hooping) radio with
my antenna and it requires 4-5 millisec tuning time for the antenna.
This means that I have to switch from motor tuning to SPST relay switcheable
capacitor bank. However, all suitable relays have 2.5-3ms switching
time (it is measured, all of them rated 10ms) which means there is almost
no room left for calculations. These relays have 10-12 mOhms in closed
contacts which is next to bad as larger resistance causes antenna radiating
resistance less than sum of other resistances and lowers Q and efficiency
The question is:
Is the nowdays pin diode
technology advanced enough to offer following parameters: (reverse voltage
10-15kV, open diode resistance 0.01 Ohms, Current 20-30 Amps (at 30
MHz)). Are they available commercially? In what packages? (SMD is preffered).
I think switching speed is out of qustion, all of them are fast
Browsing on pin diode webs I noted that most powerful an
low resistance (best I found is 0.1 Ohm) are biased with quite high
current. I have only 3 Amps for 16 diodes.
Thanks and regards,
Post subject: Super-duper
PIN diodePosted: Mon Oct 17, 2005 10:52 pm
think you'll find what you're looking for.
To the best of my
knowledge, there are no SMT diodes of any variety rated at 10-15 kV.
One reason is that we're talking electric field densities near or over
the breakdown of air at STP for most SMT packages.
look at the values you name for current and resistance. Even though
you can't run a PIN diode at zero forward current, taking your low end
estimate of 20 Amps and 0.01 Ohms on resistance, P = I squared R gives
4 Watts of power dissipation: rather a lot for an SMT.
you have to have sufficient carriers in the intrinsic region to sustain
the currents you need. As a rule of thumb, that means a forward ON-state
current of more than 20 Amps - and you only have 3/16 A!
bright side, 1 millisecond for a calculation, even on a cheap uP, seems
like a long time!
Post subject: Posted: Tue Oct 18, 2005 3:00 am
Thanks for that.
look at http://www.microsemi.com/datasheets/hum3002-04 .pdf
These ones, if three or four in parallel are would be a subject
for experiment (20-30 mOhms) if were not packaged in these railways
styled bolts. Also, it makes me optimistic that since these already
come quite close to my specs I can hope on soon availability of better
However reverse volatge is still too low. Currently I'am
switching 7kV/12A rated multilayer ceramic caps by 6kV/12A/0.012Ohms
relays. These caps in most cases added to vaccum cap at nearly full
capacitance (typical situation) in parallel, thus current is divided
between them and they getting warm very little.
The nature of
entire circuit, as you can see, does not allow for conducting of precise
measurements. The q-factor is derived as calculation from measurement
of bandwidth at resonance curve. Thus, though powered by 100 Watt PEP
the reactive power has to be considered as much as 90 kW. Lots of guessing.
The fact that other guys managed to do similar job with 1 kW (!)
powered transmitter makes me very intrigued. By what means??? All known
relays are too slow, reed relays too weak and have huge capacitance
to it's coil... Magic.
Thanks and regards,
Post subject: Super-duper PI diodesPosted:
Wed Oct 19, 2005 1:52 am
Regarding those Microsemi PIN diodes: wow!
Thanks for bringing those to my attention. Maybe your task isn't
so impossible after all. But what's the problem with the package? They
obviously need to get rid of about 50W of heat.
Post subject: Posted: Wed
Oct 19, 2005 5:29 am
In my case I do not face trouble of heat dissipation
this much. First of all I do not have CW all the time, SSB heats the
diode by carrier that appears during modulation only, pauses are for
cooling. The other thing is typical fashion of communications is 10:1,
e.g. you listen more than speak. For that reason conventional air cooling
would be sufficient. Also construction, having SMD enveloped multilayer
caps and chip like pin diodes placed over copper clad flan (I think
FR-4 also will do, however) makes whole thing much more organized, compact
and technologically easier for assembling. With these bolts and strip
lided caps entire construction will look like 1930s SX-28 shassis.
Posted: Wed Oct 19, 2005 5:43 am
....and again, they need their
bolts exactly because 0.1Ohms resistance, should they have made three
same crystals on common piece of waffer and packaged into ceramic or
plastic cube shaped SMD, they would get 3 times less lossy unit with
no heatsink requirements for 50W for sure. Just rerate by frequency,
not all use them for microwave...I think this package is bit overengineered.