DIY RF amp?

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

Like, maybe, this one?

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Reply to
David Nebenzahl

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.

Reply to
bw

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

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

Reply to
Arfa Daily

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.

Reply to
Meat Plow

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

Reply to
GregS

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.

If you do, I suggest a Channel Master 7777 antenna mounted amp:

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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Reply to
Jeff Liebermann

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     jeffl@cruzio.com
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Reply to
Jeff Liebermann

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

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

Reply to
GregS

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     jeffl@cruzio.com
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Reply to
Jeff Liebermann

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

Something like this, perhaps?

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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.

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Reply to
David Nebenzahl

$2.28 plus shipping:

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Jeff Liebermann     jeffl@cruzio.com
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Reply to
Jeff Liebermann

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$1.40 in single quantity

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datasheet.

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Sample PC board layout.

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Evaluation board.

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Reply to
Michael A. Terrell

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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.

Reply to
hrhofmann

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

Reply to
Arfa Daily

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.

Reply to
Meat Plow

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