DIY RF amp?

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Occurs to me that I could use a small RF amplifier to improve my DTV
reception (it's marginal on some stations, PITA to constantly readjust
my indoor antenna).

So how about a reeeealy simple DIY amp? Anyone know any designs out
there that would be suitable for DTV (VHF/UHF) signals? I'm thinking one
transistor, battery powered.

Could probably just buy one but it would be fun to build one instead.


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Re: DIY RF amp?


On 10/19/2009 7:45 PM David Nebenzahl spake thus:

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Like, maybe, this one?  http://english.cxem.net/rf/rf37.php


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Re: DIY RF amp?



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That will work.
It did for me, point to point wiring, using parts from the back of a
scrapped VCR.
Check eBay for a 1980s ARRL VHF manual.



Re: DIY RF amp?



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Much much simpler than that, is anything with a MiMIC in it. Basically one
(cheap) four pin device with a small (random junkbox) choke for supply
decoupling, and a couple of small caps at the in and out pins to isolate the
DC from the coax, if you're not going to line feed it. As Mr Cook says,
ideally, it should be up at the antenna and hence line fed for DC, but if
the signals are nearly good enough and just a bit marginal under some
weather conditions or whatever, you could probably get away with putting it
wherever is convenient in the line.

Arfa



Re: DIY RF amp?


On Tue, 20 Oct 2009 10:04:09 +0100, "Arfa Daily"

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It's got to be near or ideally right on the antenna outfeed or it will
amplify noise. My HF preamp is located on the mast below my horizontal
Log Periodic Yagi.

Re: DIY RF amp?


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More like produce noise, and with a low signal, you get that noise.

I don't know the state of TV's and converer boxes as far
as sensitivity. I heard its bad. I have made amps before
using some small chips, and etched a board to form feedlines.
It worked. Many of the old time amps used inductors and tunned
circuits, and a spectrum analyzer was used to set the gain
as flat as possible.

I had problems at the old house and used a line amp
to feed one drop. I also had to use the special one for
the internet. The cable guy made me take it off, and
the reseption was pretty good after he replaced the 30 year old
feed from the pole.
greg
 

Re: DIY RF amp?


On Tue, 20 Oct 2009 13:56:51 GMT, snipped-for-privacy@zekfrivolous.com
(GregS)wrote:

wrote:
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With even the best shielded RG6/59 you'll get noise infiltration. If
you stick a high gain low noise GaAs FET preamp at the end of the feed
you -will- amplify that noise. Hams don't use 75 ohm  double or triple
shelled feed line but rather RG8, 9913 or hard line at 50 ohms. Some
with long wires use 300 ohm ladder line or twin lead. If you're a DX'r
looking for that rare contact on a CW band you'll want the noise to be
minimal by locating your pre amp at the antenna feed point and a rig
with DSP.

Re: DIY RF amp?


On Tue, 20 Oct 2009 10:04:09 +0100, "Arfa Daily"

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Ummm.... I guess that's considered a good idea because that's what's
inside some of the Radio Shack mast mounted inline amps.  However,
they're not a universal solution.  The devil is in the details:

If you have 10-20dB worth of coax cable losses, then such a 20dB MMIC
amplifier will work quite well.  The gain of the amp compensates for
the loss in the coax cable.  However, the noise figure of the typical
cheapo MMIC is not so wonderful and there will probably be no net
improvement in sensitivity.  The real benefit is in the ability to
locate the antenna on the roof, without incurring coax cable losses.

Another problem is too much gain.  Some Radio Shock amps have as much
as 30dB gain.  Coupled with a typical receiver, and with minimal coax
cable loss, all this gain does is reduce the dynamic range of the
receiver.  30dB of gain will simply increase the base line noise level
and additional 30dB.  However, the overload point of the receiver
remains the same, thus reducing the dynamic range.  This overload is
fatal in strong signal area.  For example, if you have a fairly strong
nearby local station, on any frequency within the range of the
amplifier, it will "overload" the receiver making it difficult to hear
distant weak signals.  The "overload" can appear in a variety of ways
such as intermod, blocking, densensitization, adjacent channel
leakage, etc.  Incidentally, this is also one reason why an FM trap is
often included in tower mounted amps.  The gain should only be enough
to compensate for the coax losses, no more.

The NF (noise figure) of most MMIC amps are not as good as separate
VHF and UHF discrete designs.  In general, broadband amps have a worse
NF than comparatively narrow band amps.  The absolute best are
amplifiers tuned to amplify only one channel.  (Actually, they're
usually not that narrow and will cover several channels).  They have
the best NF.  For the low VHF channels, bipolar xsistor amps are fine.
For the upper VHF channels, I like DGMoSFET's.  For UHF, I like GaAs
front ends.  I live in the hills with lots of trees.  Line of sight
just doesn't happen.  Prior to getting DirecTV, I had two single
channel amps attached to two separate antennas (on CH 35 and 54).
There was no way to reliably watch these stations with anything less.

Bottom line:
1.  Antenna location is what makes things work.  All the gain in the
world won't amplify a signal that isn't there to amplify.
2.  NF is more important than gain.
3.  Too much gain is a bad idea.  Match the gain to the coax cable
losses.
4.  Overload is a problem with all broadband amps.  Plug in a CATV
spectrum analyzer to check for problems.

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Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: DIY RF amp?



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All of which was why I was careful to say that if the problem was just an
occasional marginal signal level, then a MiMIC based solution anywhere
convenient in the line (although up at the antenna is best) would be a
solution that the OP could "probably get away with ..."

I agree that the noise contribution figure of these devices is not the
lowest that you're going to come across, but neither is that from a single
transistor solution where there are many other considerations such as the
choice of other components in the circuit, layout, supply voltage level and
so on. I have always found MiMICs to be reasonably well behaved devices that
can (almost) be rats nested. Their noise figure is adequate for all but the
most demanding cases, but above all, any amplifier using them, is cheap and
*simple*, which is what I believed the OP was looking for.

As far as the device having too much gain, for most practical cases, this
need not be a problem. Cheap plug-in in-line attenuators will sort that.
Personally, I'd rather have more signal available than I actually needed,
and then attenuate it, than find that I was trying to squeeze every last dB
out of a discrete transistor amp.

Arfa



Re: DIY RF amp?


On 10/20/2009 2:04 AM Arfa Daily spake thus:

 >
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Something like this, perhaps?
http://users.belgacom.net/hamradio/schemas/preamp_HF_VHF_UHF_SHF_wideband_MAR6.htm

One problem is that this device (MAR-6/MSA-0685) cost something like
$16-17, from what I've been able to find online. I was looking for a far
cheaper solution. I think I'm just going to build that 1-transistor
preamp and see how it works.


--
Found--the gene that causes belief in genetic determinism

Re: DIY RF amp?


On Tue, 20 Oct 2009 12:53:14 -0700, David Nebenzahl

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$2.28 plus shipping:
<http://www.angelfire.com/electronic2/index1/mar6.html

--
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: DIY RF amp?



David Nebenzahl wrote:
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http://users.belgacom.net/hamradio/schemas/preamp_HF_VHF_UHF_SHF_wideband_MAR6.htm
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<http://www.minicircuits.com/cgi-bin/modelsearch?search_type=model&model=MAR-6%2b&tb_no=TB-432-6%2b

$1.40 in single quantity

http://www.minicircuits.com/pdfs/MAR-6+.pdf datasheet.

http://www.minicircuits.com/pcb/98-pl262.pdf Sample PC board layout.

http://www.minicircuits.com/pcb/WTB-432-6+_P02.pdf Evaluation board.


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The movie 'Deliverance' isn't a documentary!

Re: DIY RF amp?


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Radio Shack (In the USA) has an in-line coaxial amplifier that would
allow you to place the power supply at the tv and the amplifier at the
antenna.  You probably want more than just a UHF amplifier as digital
tv (in the USA) covers more than just the UHF frequencies as far as I
know.

Re: DIY RF amp?


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Whatever it is has to go right up at the aerial, while there you may as well
replace the existing aerial with a much higher gain one and new down lead.


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Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
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Re: DIY RF amp?


On Mon, 19 Oct 2009 19:45:11 -0700, David Nebenzahl

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The problem with indoor DTV antennas is that you get plenty of
multipath reflections.  These trash the signal in much the same manner
as insufficient signal.  The current DTV tuner chips do a heroic job
of dealing with multipath, but they're not prefect.  My guess(tm) is
that you're not aiming the antenna for maximum signal, you're aiming
for minimal multipath.  If the best signals end up in a direction
other than the direction where the xmitter is located, you probably
have a multipath problem.  If moving around the room causes the signal
to change, you're part of the reflection problem.

The problem with adding an amplifier on an indoor antenna is that it
will also amplify the reflections equally.  The result is no net
improvement.  There are really only two situations where an amplifier
will help.  
1.  If you have a really long length of coax cable coming from the
antenna and need something to compensate for it's loss, an amplifier
at the antenna will work well.  
2.  If your choice of DTV receiver or converter is stone deaf and
insensitive, an amplifier can improve the sensitivity.  Putting a high
gain and low noise amp in front of a quality receiver, which already
has a high gain and low noise front end, will do nothing for the
sensitivity, but will reduce the dynamic range of the front end.
(Translation:  Amplifiers can sometimes make things worse).

Your best bet is an external directional antenna and possibly a
rotator with a TV amplifier at the antenna.  That may not be possible
or economical in all situations.

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If you do, I suggest a Channel Master 7777 antenna mounted amp:
<http://www.crutchfield.com/S-hzGI9abFisi/p_6597777/Main.aspx?i65%97777&tab=review
Separate amplifiers, inputs, and outputs for VHF and UHF (needed to
keep the noise figure low), FM trap, and coax attenuation tilt
compensation.  That's stuff you won't find in most homebrew
installations.  $55 to $75.  However, it's really only appropriate for
a mast mounted external antenna.



  
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Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: DIY RF amp?


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Thats very correct in theory, but I have seen improvement for
whatever reason.


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There is only one VHF channel thats broadcasting digital here and thats 13.
There is no need for low VHF, but the FM trap is OK. When you need
VHF at all may depend on location, as VHF high has some different signal
paths, and is better in some locations.

You can make or get a small directional UHF antenna that will
work indoors. Receiving the digital channels is so much better with me with
the requirment of longer elements and just using rabbit ears on VHF low.

greg

Re: DIY RF amp?


On Tue, 20 Oct 2009 16:54:26 GMT, snipped-for-privacy@zekfrivolous.com (GregS)
wrote:

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Theory is usually right.  If you've seen an improvement with an
amplifier, it's possibly because the receiver or converter was a piece
of insensitive junk and *ANY* amplifier in the front end, no matter
how noisy, would constitute an improvement.  It's difficult to tell
without knowing the circumstances, topology, equipment, levels, specs,
measurments, etc.


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Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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