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Author Topic: Newbie MMIC question  (Read 6132 times)
AC5UP
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Posts: 3864




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« Reply #15 on: September 02, 2012, 10:58:34 AM »

We still have a lot of users in the 30 - 50 MHz range in this area, and any TV stations still on the low VHF channels may swamp your preamp.  You certainly want to filter out the FM broadcast band, as that is one of the most saturated portions of the RF spectrum.

It's my understanding that since the switch to DTV there are no low band TV signals on the air. Neither Digital or NTSC, it's all Channel 7 and above... And I say that knowing there could be a temporary waiver somewhere in Montana or Puerto Rico. As far as NTSC, the only TV signals remaining are supposed to be low power UHF community outlets granted a delayed conversion date based on limited finances.

With FM, the signal density is somewhat different along the east coast with signals packed about as tight as the band will allow but typically < 10 kW ERP from 300' towers. In the Midwest you'll find less band saturation but plenty of 100 kW operations on 1,000 foot towers that definitely cover some real estate. Some of the biggest signals in the US are along the eastern edge of the Rockies in places like Denver where the listener density gets real thin outside of the metro.

My only comment regarding preamps is that if this one didn't need to cover 30 MHz a cable TV drop amp would have been an easy Plug 'N Play solution. 15 dB of gain for maybe $50 and reasonably flat from 50 MHz through 1 GHz.
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AB1JX
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Posts: 15




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« Reply #16 on: September 02, 2012, 09:58:31 PM »

I didn't know the 40673 was gone, it was just a number from memory.  Or that the BF988 was surface mount.  I probably could deal with it, but I use thru-hole when I can.  It's mentioned in the schematic at http://wavelab.homestead.com/HF_VHF_Rx_frontends.jpg, but I guess being smd is why I didn't spot it in the picture at http://wavelab.homestead.com/HF_VHF_module_view.jpg  I have a copy of the PCB artwork but it still didn't dawn on me.

I'd seen the zig-zag lpdas but I thought they were just some fad.  I've got Cebik's PDF on them now.  I could build a lpda out of wire, but we usually get at least one good ice storm most winters where there's about 1 inch of ice on everything.  I am favoring cheap and replaceable (come spring) though.

I hope to stay below 88 MHz and away from the FM broadcast band, but constant signals are just horizontal lines on a spectrograph that you can ignore.  If you take a look at a real spectrograph like http://ftp://www.ips.gov.au/data/Solar/Learmonth%20Observatory/Historical%20Data/Daily%20Spectrographs/02092012spectrograph.gif you'll see bunches of them.  I do see a band of trash from 40-45 MHz on most of the antennas I've tried, which may come from the house.  With a dipole like my fan dipole I can face the end of it at the house so that nulls out a lot, but with the T2FD it's back.  We do have police and fire on low band VHF here still, but it's gradually moving to UHF.  Electric companies and others use low VHF too.  I can see Mount Greylock and Mount Tom about 20 and 30 miles away, and both those (I think) have 6 meter repeaters.  Mount Greylock's 2 meter machine comes in on my HT with the rubber duck unplugged.

My T2FD is about 18 feet long and 7 inches wide.  MMANA-GAL comes with a few hundred antenna model files and I took a T2FD for 7-28 MHz and scaled it down by a factor of 3.  600 ohm output, so I wound an 8:1 balun to feed 75 ohm line.

I don't know about how much gain or what the noise floor is, I just know I'm not hearing anything yet.  That can happen for a few weeks at a time I gather, but I can't even detect anything from the sun for sure.  In the Radio Jove project at http://radiojove.gsfc.nasa.gov/ you put up a pair of dipoles and run a receiver with a pair of J310s in the front end and you should be able to hear/see something fairly often on your chart recorder.  It seems to get a little quieter at night, but that may just be because businesses shut down at the end of the day.  People do use other receivers not unlike what I'm running.

Preamps: I tried a cable TV amp and I could tell immediately that it was dead below 50 MHz.  I thought my antenna connection was intermittent, but it all depended on the frequency I was scanning at the time. I'm just a believer in putting a preamp at the antenna, especially running 200 feet of cheap coax. I used to install TV antennas in fringe areas and we used preamps on maybe 25% of the 300 ohm setups and 90% of the coax ones.

Heatsinking an MMIC noted, also a bias resistor as recommended in the datasheet.

I spent much of the day modeling a fat dipole and thinking about how to build one cheaply.  I've got a 1/4 mile spool of galvanized steel fence wire, also some used network wire I can separate into individual wires.  And a stack of useless "coaster" dead CDs.  If I can melt or drill holes in them near the edge, hang them from the steel wire by 1 hole then put network wires running parallel in 7 other holes it looks good.  I modeled one 14 meters long and it looks pretty good, but not perfect.  8 wires is a significant improvement over 4. I used to have a fat dipole built from old aluminum storm window casing that I used vertically as a scanner antenna and it was pretty amazing.  I'm not sure about wind and ice though.
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WB6BYU
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Posts: 13243




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« Reply #17 on: September 03, 2012, 12:25:46 PM »

Quote from: AB1JX

I didn't know the 40673 was gone...



Most of commercial production has shifted away from leaded parts, especially the
expensive metal cans, unless they are required for military applications.  They are
still available, though:  Dan's Small Parts and Kits sells several types of NOS ("new,
old stock", meaning unused but not recent manufacture.)  And I happen to have
a couple hundred in a jar somewhere...

The 40673 was a particular RCA part that the ARRL chose as a "standard" dual-gate
MOSFET because it was rated to 500 MHz, but especially below 100 MHz just about
any of the other similar parts will work, and they are often cheaper.

The BF998 is probably the easiest dual-gate MOSFET to find commercially these days,
and is capable of a lower noise figure than the older parts.  If you are building a
printed circuit board it easily solders to 4 adjacent pads:  I use a Dremel tool to cut
my boards and just put it at the junction of two cuts across the board at right angles.

There are still some leaded parts being made, including the J310 and 2N4416, which
are good general purpose JFETs.  The SMD version of the J310 would be good for
common-gate amplifiers in place of the old leaded U310 (same chip as the J310 but
with the gate tied to a metal can that could be soldered to ground.)


In the frequency range you are looking at, there are lots of different parts that
will work.  For ultimate sensitivity, once you get the local noise down, the critical
parameter is the noise figure of your preamp.  The MAR-6 is rated at 3dB, which
isn't bad for a broadband device.  If you happen to come across an old Plessey
SL-560 IC they are rated at 2dB or better, and good to at least 75 or 100 MHz. 
(Some of us have rather eclectic junk boxes...)

A Norton amplifier can provide a good combination of wide-band performance,
low noise figure, and resistance to overload, but you might have to experiment
to find one optimized for your desired frequency range.  Further information
here (and other locations around the web):

http://www.qrp.pops.net/preamp.asp


Quote

I hope to stay below 88 MHz and away from the FM broadcast band, but constant signals are just horizontal lines on a spectrograph that you can ignore.



The problem isn't with them showing up on the spectrograph, but with them overloading
the preamplifier, even if you aren't intentionally listening in that range.  When I put a
spectrum analyzer on an antenna, the FM broadcast band shows as a clear splotch of
RF, much stronger than anything else.  You'll probably want a low pass filter to reduce
the FM BC signal strength ahead of any wide-band preamp.  The same goes for other
signals in the passband:  very strong signals can overload the preamp and reduce the
sensitivity to desired signals.


Quote

Preamps: I tried a cable TV amp and I could tell immediately that it was dead below 50 MHz...



Preamps for over-the-air TV may cut off at 50 MHz, but those for cable TV service often
work down to 5 MHz, as even the HF bands are used to pass signals.


Quote

I spent much of the day modeling a fat dipole and thinking about how to build one cheaply.  I've got a 1/4 mile spool of galvanized steel fence wire, also some used network wire I can separate into individual wires.  And a stack of useless "coaster" dead CDs.  If I can melt or drill holes in them near the edge, hang them from the steel wire by 1 hole then put network wires running parallel in 7 other holes it looks good.  I modeled one 14 meters long and it looks pretty good, but not perfect. 



While the 14m antenna would have a reasonably low SWR from 30 to 60 MHz when used
with a 4 : 1 balun and 75 ohm coax, the radiation pattern isn't going to be suitable.  It
will have multiple lobes and, on some frequencies, a null broadside to the wire.  A length
of 14 FEET seems to work much better.  If I model an ideal 8" diameter dipole, the SWR
is under 4 : 1 across the frequency range (well, actually 15' worked better) using a
300 impedance when calculating the SWR.  Angling the wires out from the feedpoint to
a single wide spreader, then back together, likely will give better results than a fixed
diameter cage:  that is the basis of the true fan dipole that I proposed previously, with
the wires in a single plane to make construction easier.
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AB1JX
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Posts: 15




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« Reply #18 on: September 03, 2012, 10:52:15 PM »

I put my fat dipole modelling adventures online at http://ab1jx.webs.com/toys/jove/antennas/fat8/index.html.  It doesn't look so bad to me, and I can build it without spending any money.  I just haven't figured out the best way to drill holes in CDs.  14 meters is a full wave, sorry.  NEC2 file's linked on the page.

I just looked again at the BF988 datasheet: surface mount with leads isn't so bad as chip resistors and capacitors.  I've got a small collection of old satellite TV dishes from the dump and taking apart the LNAs often yields some interesting parts, but usually without part numbers. I haven't gotten my new parts into jars yet, I have to go through boxes of bags of parts to find anything.

I've got some J310s, also J309s which I thought looked "hotter" (Idss wise).  I've got one radio which had a Sony 2SK152 that got fried and I found it was almost legendary.  I've got a J309 in there now and it works OK, but it's a portable radio you have to hook an external antenna to for HF. I wish it had been designed around more mundane transistors.

If the MAR6 amps don't work out for some reason I might try something like the popcorn amp on the page you sent a link to but without the tuned circuit on the input.  Maybe rather than a tuned input I can use tunable traps to attenuate problem frequencies if I can find them.  The nearest broadcast station is about 17 miles away on the other side of a mountain.  The Norton amps look interesting too: I just realized they're broadband.

I just finished uploading my collection of spectrographs to http://ab1jx.webs.com/toys/spectra/archives/index.html  The 6th one down has a little bit of the cable TV amp which cuts out below 45 MHz.

I think I've seen what you're calling a fan dipole TV antenna, or maybe not.  They had a pair of elements in front then 6 more elements behind fanned out vertically as a reflector.  Never saw one up close, just on rooftops.  The same technique is also used in multiband scanner antennas.  What I'm also thinking of is the way hams used to cut multiband dipoles out of 4 and 5 conductor flat rotor cable.  Maybe those are called parallel dipoles.
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W8JI
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Posts: 9296


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« Reply #19 on: September 04, 2012, 06:58:21 AM »

I think we are engineering the hard way here.

I think I would start with a bowtie antenna 1/4 wave high at the highest frequency, and a reflector screen below the antenna.

If I didn't have the room for that, then I would start worrying about lossy antennas or active antennas. Starting with a lossy system, which might then require design work on amplifiers, seems like a bad approach.

By the way, I've used MMIC's as post mixer amps before filters, and had few issues with dynamic range. I generally avoid them for wideband RF preamps, but I wouldn't be too concerned with a bandwidth limited amp on what are normally such unpopulated frequency ranges.

73 Tom
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