Question: How can an injector improve reception without preamp?


DTVUSA Jr. Member
In the process of troubleshooting a reception problem, I bypassed the preamp, but not the injector. This was a huge improvement. I took apart the preamp and found that a connector had broken at the circuit board (not worth soldering due to crappy design). Since I needed a new preamp, I removed the injector. Strangely, I lost about 20% signal strength across the board. I tried different configurations (with and without the surge protector, I changed cables), but the signal was still not as good as when I first removed the preamp. So, I reconnected the injector, and bingo, I got a 20% improvement. I turned the injector off, and again the signal strength drops. Clearly the injector is somehow improving the signal without a preamp. How the hell is this possible? I'm flabbergasted.

P.S. To be clear, the signal flow is: antenna > grounding block > injector > surge protector > digital converter box > TV.
A) Maybe your converter box is interpreting DC on the line as RF "signal." Does reception actually improve, or are you looking at some goofy meter? What model converter box?
B) What model antenna? Are you sure it doesn't have an internal pre-amp, like most "indoor" antennas?

If everything is right with all parts of the antenna system, and receiver this can't happen. In my short travels sometimes working with junk converter boxes and old TV's I have ran into similarly baffling phenomenon. While the cause could be anywhere in the antenna system ( balun, coax, antenna, connectors). I've twice found the problem to be a worn out antenna connector on the converter box. The power injector, and plug is acting as a part of the antenna system. It should not work that way.
You need to:

!) Post your TVFool plot so we can see what signals are in your area and what their signal powers might be.
2) Identify the specific make and model of your amplifier. There are some two-part units out there that have an amplifier stage in each of the physical components.

The converter box should not be seeing any DC power as the "injector" *should* have a capacitor on the output of that F-connector.


DTVUSA Jr. Member
A) Interesting theory. The reception does improve. I lose a half a dozen channels without the injector. I'm not sure of the model of the converter box. It's at my parents house in another state.
B) The antenna is a massive directional rooftop I installed myself. It has no internal preamp. I don't remember the exact make and model, but it's similar to this: Terk : Outdoor TV Antennas : TV38.

Thanks for your thoughts.



DTVUSA Jr. Member
I could see that happening. The injector/preamp are cheaply made Radio Shack junk so I wouldn't be surprised if there was some signal creeping in from somewhere it shouldn't.

Thanks for your ideas,
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DTVUSA Jr. Member
Here is my plot:


The make of the injector is Radio Shack. I'm not sure of the model. It's at my parents house in another state. Maybe there is an amp stage in the injector itself. I'll have to look into that. However, as junky as most of Radio Shack's hardware is these days, I wouldn't be surprised if there was some signal sneaking in from somewhere it shouldn't.

Thanks for your insight,


DTVUSA Jr. Member
I've also been also wondering if leaving the injector on without a preamp at the other end could be dangerous in any way. There is some current going out with nothing to handle it at the other end. Could it possibly heat something up enough to start a fire? It seems unlikely since the current is very low and the antenna is grounded, but it still makes me a little uneasy.

Thanks again,

Project brought up an interesting possibility that I was unaware of. A two stage amplifier.
My original line of thinking was that something else is broken causing the power injector, power supply, and to some extent even the house wiring to act as a part of the antenna system kind of turning every thing into a random wire antenna system. Not a good situation.
While unlikely to start a fire some baluns are a dead short at DC, not a good circuit to be feeding DC voltage into. Something else is likely to fail on the power supply side. Flames are unlikely. Might not even be any smoke.
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The make of the injector is Radio Shack.
That explains it. Radio Shack, back in the 90s sold a two piece amp that had an outdoor preamp plus an indoor unit consisting of a power injector, second amp (variable, IIRC), and a two port splitter in a single indoor housing. Probably the Archer 15-1108 or 15-1109. I had one back in the day. IIRC, the cable connection in the outdoor unit corroded from exposure to air and the thing became more useless.

The power on the coax is low voltage and is not hazardous in any way. If you were to short the coax, the power injector would shut down the power, probably by using a thermistor. If you're worried about it, install a voltage block. Cable TV In-line Voltage Blocking Coupler: Electronics

You'd probably improve your system's performance by simply trashing the Radio Shack stuff and installing a much better quality pre-amp. You can pick up an RCA unit for under 25 bucks that works quite well for its price point. Menards, if you have one in the area, usually carries it in stores. Otherwise, you can have ship it to a local store for pickup or you could order it from any number of online sources.
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Fringe Reception

Super Moderator, Chief Content Editor
Staff member
:welcome: citizen!

If the Terk example you posted is similar to what you installed on your Folk's home, that's a big honking antenna to receive WGVK-5 (PBS) whereas WNIT-35 (PBS) probably repeats the same shows. You may not need that enormous periodic-Yagi.



That explains it. Radio Shack, back in the 90s sold a two piece amp that had an outdoor preamp plus an indoor unit consisting of a power injector, second amp (variable, IIRC), and a two port splitter in a single indoor housing.
Radio Shack still makes those amps and it works almost as well as the older CM 7777 did. I know because I own one of each and bought both four or five years ago.

RS redesigned the 15-321 amps housing recently but the specs are the same.
RadioShack Antenna-Mounted High-Gain Signal Amplifier -


DTVUSA Jr. Member
The amplifier is a Radio Shack 15-259. I can't find whole lot of information about it, but I'm guessing there is an amplifier in what I thought was purely a power injector unit. That would explain everything. Problem solved as far as I'm concerned. Thanks for your help everybody.

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