Click here to go to our page on bias tees
New for July 2024, with content thanks to Srinivas!
Update for August 2024: we've added a video from Antenna Man, to help you determine if an LNA would help your TV reception, and we added a link to a website where you can find all of the TV transmitters that can reach your zip code.
We also added a comment from Lou, on the danger of running an active device at the end of a long coax cable....
Many times, you would have an antenna on a tower or a building far away from a receiver. It is important to have the low noise amplifier (LNA) close to the antenna to improve sensitivity and reduce noise figure. But how do you power a remote LNA without having to put a power supply next to it? That's where the ubiquitous bias tee comes in.
You need two bias tees to power a remote LNA. One bias tee is placed next to the LNA on the antenna mast and one is placed before the receiver. The RF port of the bias tee next to the LNA is connected to the RF output of the LNA. The RF+DC port of the two bias tees are connected with a long RF cable in-between. The DC port of the bias tee next to the LNA is used to power the LNA. The diagram below explains the schematic. DC current is injected into the left bias tee, travels up the coax cable and is pulled off by the bias tee on the mast and routed to the LNA.
Such a scheme is used in many products, such as satellite television dish set-ups and remote controlled, amplified TV antennas. In many cases, there are additional control signals that are also multiplexed through the RF cable.
EMP generator?
This comment came from Louis, who has served as a subject matter expert for years on our discussion board... (thanks again!)
On the How to Power a Remote LNA, you should add a note about putting diodes or varistors at both the LNA and bias tee, or you WILL blow the LNA with long cable lengths due to voltage transients; don’t ask how I know. All that energy in the transmission line and bias tee choke has to go somewhere when the source/load disconnects. This is how EMP generators are made; charge up a coax to a high voltage then short-circuit the end.
Hopefully, of you buy an antenna-mounted LNA it will include this feature!
Does a preamplifier make sense for your television setup?
Update August 2024... the answer to this question takes some expertise, and here to shed some light on the topic is Antenna Man. Antenna Man's motto is, "tired of being robbed by cable? Cut the cord!" He explains the difference between pre-amplifiers and in-line amplifiers. He makes the case that an amplifier might degrade your reception, as it could over-amplify strong signals, or raise the noise level for weak signals.
TV Antenna Signal Amplifiers Explained: Do They Improve Reception, by Antenna Man
One thing that is not discussed is the need for a high-gain antenna mounted on a rotator system. By throwing some gain in the direction of a weak station, you can amplify it without saturating your system with strong signals coming in from a different direction. How do you know where the broadcast stations are, relative to your neighborhood?
Of course, there's a web site for everything... and there's one that will tell you the signal strength of TV and direction of transmitters you could receive. Antennas Direct is one site that can provide these data.
https://www.antennasdirect.com/transmitter-locator.html
We tried to find a similar site that would provide data for Europe. This is the best we could find, but it is not very good:
https://www.freeview.co.uk/corporate/detailed-transmitter-information
Maybe someone could do us a solid and send us some links (or an app) that can find transmitters in Europe, Asia and Australia.
For an example we typed in Billerica, Massachusetts. New England has always had a mess of broadcast stations, and with the right equipment you can pull in over 100 channels today. Antennas Direct generated the map below, which shows 35, 50 and 70 mile radii and antennas in five states. A table that is generated shows relative signal strength for each channel. Trust us, you will need a good rotator setup to pull this off!
Billerica Massachusetts, was the home of Thomas Ditson, who inspired at least one verse to the song "Yankee Doodle Dandy" for his failed effort to buy firearms from a British garrison and ended up tarred and feathered:
Yankee Doodle came to town,
For to buy a firelock,
We will tar and feather him,
And so we will John Hancock.
Some say Ditson was a mite "tetched", but that might be considered politically incorrect these days. Let's just say he might have been a man of "special needs." He served the Continental Army throughout the Revolutionary War, and lost an eye to smallpox just before George Washington mandated that all solders be vaccinated, which no-doubt helped during the creation of the United States.
If you are near Billerica in September, check out the Yankee Doodle Homecoming!