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Re: Want to Homebrew a Preamp



Hi Matt

Glad to see someone else is interested in the home brew option!  It is one of the most
entertaining aspects of the hobby and is often sadly neglected.  It costs you very little
to think about what you are trying to do and the rewards can lead as far as new careers.

I hope you have had many responses on this issue.  If you have you will no doubt find the
diversity of responses and approaches a bit bewildering.

I have designed three pre amps.  One is heavily based on a Ramsey unit which in turn is
based on a classic class A amplifier configuration.  I built the the thing and derived the
equations for its operation so that I could see someone else's solutions to the design
problem in detail.  Another design has a home brew elliptical filter (from ARRL published
filter tables... why re-invent the wheel?) on the input in order to reduce the overloading
I get on my 70 cm receiver when my 2m transmitter kicks in.  The third is a two stage
design for low noise figure (NF) and higher gain.

I have not used relays to isolate the pre amps yet but as soon as I get some good ones I
will.  They can be pretty pricey.

I have powered pre amps with DC on the coax and with separate power supply lines.  If you
don’t mind running another cable, powering your pre amps separately adds some flexibility
since most high performance transverter installations require the transverters to be near
the antennas anyway.

Some folks have already mentioned that Noise Figure (NF) is important.  The NF of your
receiving system is set by the first amplifier of the system after the antenna, usually
referred to as a pre amp.  In the first two pre amps I have designed I have picked a bias
point that presents a 50 ohm load to the antenna or elliptical filter whichever the case
may be.  I have done this since the transistor I am using has a fairly low NF (< 1.7dB)
for the application (LEO satellites).  Since LEO RF hits the ground pretty hard (compared
to VHF, UHF DX and EME) and you need a beam antenna anyway, NF isn’t a be all end all.
Some may feel that my previous statement is a bit of a contradiction... LEO signals are
strong but you need a beam... seems mutually exclusive.  My point here has more to do with
the awkwardness and limited aperture of omni directional antennas than signal power.  You
can design an omni to do well between 10 degrees and say... 50 degrees or you can design
one to do well between 20 degrees and 90 degrees but you can’t get the thing to cover
horizon to horizon.  This is why folks go as far as putting egg beater antennas on Az/El
systems.  They want those last 20 or so degrees of the pass.  If you have the Az/El system
why not use a beam to pull in the horizon.  During the high passes you may be 10dB over S9
with the beam and NF is hardly an issue but the horizon stuff may be just tickling the S
meter.  Each S unit is usually 8dB so 1dB in NF is an eighth of an S unit. VHF and 70cm
are noisy compared to 23cm, 13cm and 3cm, these bands can rely make use of low NF.
Besides, the higher NF transistors are much easier to get a hold of, unless you buy your
pre amp as a kit.

It boils down to where you point your antenna and what the background noise is on your
target band in that direction and 180 degrees from it if you are using a Yagi.  There is
no point building a super low NF amp, out of devices you can’t get for quantities less
than 1000, if your background noise is well above the noise floor of the pre amp anyway.
Good for bragging rights but doesn’t do much for you operationally.

Folks who take NF seriously are EME, scatter and DX types.  The signals that they bounce
off the moon are about as week as you get in HAM radio.  In such cases getting the noise
out of the NF can make or break a contact.  Here is where getting those precious GaAs FET
transistors with < 0.2dB NF at 2m is important.

The one design I have built so far, has worked well.  The unfiltered version has almost
30dB of gain in the HF bands, about 19dB at 2m and just under 10dB at 70 cm.  This is a
crude pre amp built to be a jack of all trades but master of none.  The filtered version
is no different except that by the time you are down to 300 MHz you have zero gain, by the
time you reach the 2m band you are down 27dB.

I am working on a two stage 70 cm pre amp with a front end high pass filter which will
have a total gain of 20dB or greater in the pass band.  NF depends on which transistor I
can get hold of.  A friend of mine just bought some fancy 0.2dB NF pre amps for 2m and I
would like to do at least that well with home brew at 1/10 the cost.  Nah nah nah nah
nah... gets those competitive juices flowing...

The trick to designing pre amps is to choose enough NF for your application and then
finding the transistor in the market place.  The major manufacturers (NEC, HP, Motorola,
Zetex...) are pretty good at providing data on their transistors, especially the ones that
are used in the cellular phone bands.  They also provide good application notes on
employing their transistors.  The problem is that they don’t make enough for their vendors
to stock them or the devices are in such demand that the vendors run out of stock in a
hurry.  The data sheets are important because they tell you where to bias the device for
best NF or gain.  You will never have both, it is always one or the other.  If you want
gain you pay with NF.  If you want NF you pay with gain.

Also available for pre amp use are MMICs (Mini-Circuits, HP...) .  These in general are
useful for the second stage of a pre amp and do not offer low enough NF for use as a first
stage.  For the best performance a GaAs FET with a noise figure well below 1dB is used as
a front end amp after a filter of some kind and then the MMIC.  MMICs ease the parts count
and design tasks since about all you have to do is drop them in with DC blocking caps and
a bias resistor... presto... 11dB to 20dB of gain.  Remember the filter is there to
prevent overloading the pre amp and or your receiver in the presence of strong out of band
signals.  Toko makes many off the shelf filters that can be used in the HAM bands for
small signal work.

Dig around HP’s and NEC’s web sites for app. notes.

HP:     http://www.semiconductor.agilent.com/cgi-bin/morpheus/home/home.jsp

NEC:   http://www.cel.com/

For more fundamental background info “Introduction to Radio Frequency Design” by Wes
Hayward (ARRL pub) is a good start without too much math if you are intimidated by it.
The RSGB has a series of books on VHF, UHF and microwave techniques, I have found them
helpful.  Also published by ARRL are the “UHF/Microwave Experimenters Manual” and the
“UHF/Microwave Projects Manual”.  There are lots of pre amps, transverters, filters...etc
in these volumes.  Don’t forget your local library.  If they don’t have the book you want,
maybe they can get it for you.  I recommend “Fields and Waves In Communications
Electronics” by  Ramo, Whinney and Van Duzer published by Wiley.  It is an “academic” text
which most technical libraries have.  If you are not familiar with Calculus and
differential equations it will be hard going but it covers the theory behind all that is
RF.

You can go far in design without knowing all the math and the details of the applicable
theories.  All you have to do is read and think.  If you want to push the limits you will
have to work at understanding the details of theory so that you can figure out when you
are doing something dumb or thinking the wrong way.

One final point.  You do not need fancy CAD packages to build pre amp circuits for VHF and
UHF.  It is even possible to build microwave circuits for PCBs without much more than a
drawing package for your computer.  You will have to pay attention to detail though.  Some
of the RSGB books and ARRL books talk about construction techniques.

This response has been a bit long winded but building your own stuff is an important topic
to me.  I want to encourage folks to try some.  If I can be of any help email direct.

--
/***************************************
Bronson Crothers
17 Charles St.
Orono  ME  04473

Phone: 207 866 0405
Fax:.....207 866 0405

Radio Call: N1ZAQ

--... ...--

Email: bronson@eece.maine.edu
**************************************/


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