Hello.
Modern over-the-air TV antennas are typically 'pre-amplified'

If those antennas work with say a 12VDC wall adapter, can instead a choke inductor be placed from internal +12V of the TV to the RF input jack; to supply power to the antenna only while TV is turned on ?
Same power injection at the antenna to apply the power to its circuitry.
All assuming there is coupling capacitors at both...

Or, are new TVs already supplying DC at their 'F' jacks for active antenna operation ?



Image 'borrowed' from the web.

 

I would just get a decent passive antenna instead.

https://www.solidsignal.com/rca-ant111-indoor-vhf-uhf-passive-tv-antenna-ant111

 

I have a passive antenna on the roof. It's big and receives plenty of signal. And you don't need a special antenna to receive digital signal. It's all "airwave signal".

 

Then I guess your roof is a good place for it

 

That's not the kind of antenna that goes on a roof. I also have a roof antenna. I get about 70 channels. Much of those are Hispanic and a few are shopping channels. A goodly number of those are junk channels, so out of all those channels I probably watch less than 10 of them. ABC, CBS, NBC, Fox, MyTV, H&I, AntennaTV, Cozy, and PBS and the wife watches a few of those crime channels, probably about 2.

Then my Sony Smart TV gives me a whole bunch more. So CNBC, PowerBlock and AskThisOldHouse. Once in a while one of those channels will have a marathon of James Bond or Star Trek movies. Then when I want to watch MSNBC, HBO, NetFlix, Amazon Prime or Hulu. So I spend about $300 a year on television entertainment as opposed to the $100 a month for a few prime channels with cable or satellite TV. Rarely do I listen to music because most of what I listen to I already have in my collection.

But this little antenna doesn't go on the roof.

 

That's not the kind of antenna that goes on a roof. I also have a roof antenna. I get about 70 channels. Much of those are Hispanic and a few are shopping channels. A goodly number of those are junk channels, so out of all those channels I probably watch less than 10 of them. ABC, CBS, NBC, Fox, MyTV, H&I, AntennaTV, Cozy, and PBS and the wife watches a few of those crime channels, probably about 2.

Then my Sony Smart TV gives me a whole bunch more. So CNBC, PowerBlock and AskThisOldHouse. Once in a while one of those channels will have a marathon of James Bond or Star Trek movies. Then when I want to watch MSNBC, HBO, NetFlix, Amazon Prime or Hulu. So I spend about $300 a year on television entertainment as opposed to the $100 a month for a few prime channels with cable or satellite TV. Rarely do I listen to music because most of what I listen to I already have in my collection.

But this little antenna doesn't go on the roof.
View attachment 223362

Image 'borrowed' from the web so we have no idea what the actual antenna is assuming that's just an example. If you're in the city then you might not need a outdoor antenna with a good indoor unit.
Most of the amplified antennas are worst than a passive set of rabbit-ears with a UHF-loop. The secret of good TV reception is simple, a good signal with a low noise background. No amplifier will create a good signal from crap like you see inside most of the decorator HDTV antennas. TV amplifiers are great when used correctly as a pre-amp for deep-fringe reception on a low background noise outdoor antenna or to boost the signal for long cable runs. For indoor use, it's mainly a gimmick to make a really bad passive antenna a little better.

 

A friend of min swore this works - take a short length of unshielded wire, single conductor, and connect it to the cable antenna feed and then take a matching length of single conductor wire and plug it into the back of the TV and you can get free cable.

OF COURSE YOU CAN'T! An antenna needs to be the right length for a given frequency. And the receiving antenna needs to be that same length; which is probably why he said it works. But the amount of radiated energy from a cable is so low that it's not going to act like an antenna transmitter.

Those power amp'd antennas start off receiving a weak dirty signal. All you do - I believe - is amplify a weak dirty signal.

 

A friend of min swore this works - take a short length of unshielded wire, single conductor, and connect it to the cable antenna feed and then take a matching length of single conductor wire and plug it into the back of the TV and you can get free cable.

OF COURSE YOU CAN'T! An antenna needs to be the right length for a given frequency. And the receiving antenna needs to be that same length; which is probably why he said it works. But the amount of radiated energy from a cable is so low that it's not going to act like an antenna transmitter.

Those power amp'd antennas start off receiving a weak dirty signal. All you do - I believe - is amplify a weak dirty signal.

Let's forget the comments about free cable for a while.

That's a misconception that antennas (especially receive antennas) need to be the right (I assume you mean resonant electrical length) length for a given frequency. It's important when you design antennas that depend on a relationship between antenna elements for some sort of directional gain or pattern. I've transmitted and received HF to UHF on a random length (adjusted for the band of frequencies needed) of steel rope before using a tuner to match the random reactance of the antenna to a load that expects to see X resistive impedance for max power transfer.

 

I don't profess to know everything about antenna construction. I know, have first hand experience with this with CB radio and with FM reception. With CB radio the antenna needs to be matched to the radio and cable length. A typical 40 channel radio will have an antenna who's best balance is to balance for either the primary channel one will be operating on OR on channel 20. Channel 20 will give the best match for both extremes of the spectrum of 40 channels. When an antenna is on the long side, the lower frequencies benefit. When it's short, the high frequencies benefit. For the best overall operation the antenna needs to be tuned by adjusting the length to mach the mid channel; in this case channel 20. I have done that for over a decade.

With FM reception, while working within a steel building inside a steel building virtually no radio frequencies got in except the local - VERY local Country & Western broadcasting channel. By running a coax from my radio to outside the building, the length of unshielded cable had to be tuned to the mid frequency of the receiving radio. When the antenna was too long only the low channels were received. As the antenna was shortened incrementally the frequency continued to move toward the higher frequency. When a balance was struck I stopped trimming the antenna. So length does play a role.

Now, I don't mean to imply or condone piracy of cable. My intent was to focus on these tiny antenna systems and their inefficacy. Amplifying a noisy signal is going to give you a stronger noisy signal.

[edit] I should add that I do understand resonant lengths. I've installed close to a dozen "Quarter Wave" whips. It's been such a long time I don't recall the exact length of a quarter wave antenna but it is around 9 feet. Yes, I agree, correct length doesn't mean the exact correct length. There are coil loaded antenna as well as quarter wave whips and such.

Another point I should clarify, this friend of mine used a length of copper wire approximately six inches long. A random guess at a tunable length which might have resulted in even the very littlest bit of possible reception. Since I did not see it I can't say for sure it didn't work, but I suspect if it worked at all the results must have been frustrating at best.
[end edit]

 

What you really describe are matching problems with transmission lines and radio antenna connections to the antenna impedance not the actual performance of a X length antenna when properly matched at the desired frequency if the length is at least in the general range of the resonate frequency length (the antenna radiation resistance is a big factor on effectiveness). I've transmitted thousands of miles across oceans and continents using wire antennas and received signals using wide-band system with things like rhombics that have very wide bandwidths on a single antenna.