EMRFD Message Archive 14379

Message Date From Subject
14379 2017-10-05 13:39:30 Ashhar Farhan A 435 mhz preamp
As a precursor to my UHF experiments, I started with building a doubly tuned band pass filter. I made the two inductors are made out of three turns on 1/4-20 bolt each. These are mutually coupled by keeping them 3mm apart. The resonating capacitance was 2.2pf from the hot-end to a mid-point, and a 10 pf from the mid point to the ground. This worked well. It should 6 db loss. That was not so bad for a first attempt in UHF with enamelled wire (not tinned).

Next, I built a 435 MHz preamp. I had a strip of BF989 SMD mosfets. I used one of them and it shows 14 db gain (This quite good, considering that the DTC in the output has 6db loss). I used a simple shield between the input and the output inductors. The pictures, response and the circuit are all in https://groups.yahoo.com/neo/groups/emrfd/photos/albums/1584932827

Note that the spectrum analyzer's tracking generator has -20dbm input.

- f
14380 2017-10-05 18:54:48 kb1gmx Re: A 435 mhz preamp
tinned wire... silver plated is better.  A good source is 18 or 22 guage from craft suppliers
for "beeding" and bracelets.   That's how I get what I need.  the alternate is 18ga solid wire
as often that is tinned and does work well.

 the BF989 and friends are very good at UHF, properly matched they do under 1db NF.

FYI if the adjacent channel stuff is not extreme... use a single tuned at the input for 
best noise performance and at the output the double tuned as image stripping filer.
The result is a lower overall noise figure as less signal is lost before the gate.

Another device if you can find them is the MiniCircuits pma5453+  good for 
.6db noise fiigure and 22-23db of gain at 432/440mhz.  Its 50 ohm in and out.

Plan B in the real of mmics is the MAR6 and MAR8 devices.  Inexpensive and 
fairly quiet gains of 20dB to 1ghz.

14381 2017-10-05 22:37:29 Ashhar Farhan Re: A 435 mhz preamp
There is a new kid on the block. BGA612. I have several of them. I wanted to use a 'traditional' device with low current. Last night I reduced the supply to just six volts and it still held the gain at 13 db.
I have asked a local jewellery maker to get me some solid silver wire. That will be the experiment for tonight. I have used x7r smds. The C0G caps from AVX along with other uhf devices are on the way. God bless Mouser for $20 to anywhere in the world in 48 hours deal.
- f

14382 2017-10-06 19:55:40 kb1gmx Re: A 435 mhz preamp
I've used it, way better than the MAR or ERA parts.  The PMA5453 is far lower noise part.  
we are taling .6 against 2.7db, even the bf989 is quieter. Then again for local FM work its 
fine.   For weak signal 432 SSB 2db NF different is important.

FYI solid silver is not important only plated as its less costly.  At UHF skin effect counts.
So even silver plated steel or aluminum is fine. Silver over copper is good as its easy to solder.
The hobby wire for bracelets and the like is silver over copper.

I'm from an older time when gain at UHF was not cheap like it is now.  But the tricks from then
can still apply.  However cheap gain makes some no tuning required designs easy to build 
and test. 

So now I build with UHF gain blocks and mixers.  For selectivity a half wave stripline in a 
trough made of PCboard can do well and is easy to make and tune.  Losses can be as 
desired for the level of selectivity.


14383 2017-10-07 10:54:34 Ashhar Farhan Re: A 435 mhz preamp
Thanks for the tiptip on PMA5453. I will check it out. More as soon as build some more.
- f

14384 2017-10-08 01:24:10 victorkoren Re: A 435 mhz preamp
Ashar, its hard to believe that few pF SMT capacitors are with X7R material. Are you sure?
Victor - 4Z4ME
14385 2017-10-08 01:31:26 Ashhar Farhan Re: A 435 mhz preamp
yep. it bought them for about 1/10th of a cent each. i am getting some C0G next week. I will swap them and report.

- f

14386 2017-10-13 14:01:39 kb1gmx Re: A 435 mhz preamp
For tuned circuits use the good stuff.  For bypassing usually it is not issue.  Coupling 
can be a bit tricky depending on their constructions as ESR at the frequency range 
is whats important.  Generally for UHF I reserve the ATC600B or 600F parts for the 
sections that are critical like tuned circuits and matching networks.  Bypasses and 
coupling are usually more common and cheaper ceramic preferably monolithic.

My expecience is around 600mhz and up one need to pay attention and above 1ghz 
it makes a difference. You just have to know what your putting into the circuit as 
then any effects noted are no surprise.   There are a lot of UHF "tricks" like to get 
lower ESR from caps running 2 or more in parallel can work measurably better.