Passive TV Repeater

I ran across this intriguing article about a passive TV repeater. The construction is extremely simple: connect two ordinary television antennas (pointed in opposite directions) with a length of coaxial cable. (I once saw a fairly complex installation just like this, although I didn’t realize what it was at the time.) The idea behind it is equally simple: the signal enters one antenna, travels along the coaxial cable, and then is retransmitted through the second antenna. The purpose of a passive repeater is to extend the reach of a signal beyond a line of sight obstruction. Unlike a powered repeater, the signal is rebroadcast on the same frequency.

Apparently a passive repeater can even use a small amplifier without requiring a FCC license, as long as it doesn’t exceed the power limits.

8 comments on “Passive TV Repeater

  • Tom Terrific wrote:

    Broken link on passive TV repeater but I like the concept.
    I’m up on a mountain and my friend is down in the valley can’t get HDTV reception.
    Would like to beam him down HDTV for Super bowl parties… etc.

    I was thinking of using old Satellite dishes to make it more directional.
    Let me know if there are more ideas in this area.

    Thanks in advance.

  • Russ Walsh wrote:

    Did this back in 1971, in the Cariboo region of British Columbia, Canada. Picked up channel 6 from Kamloops on one antenna, used a preamp, then ran the signal about 100′ over a rock bluff in order to provide isolation, and pointed the second ‘ransmitting’ antenna at Clinton. Reception was perfect, in color, all over town! Cost? $150. Works for UHF or VHF. At the time Clinton had only one other TV channel, so the doubling of TV service was much welcomed – by the community. Sadly, not by the DOC bureaucrats (Canadian ‘Department of Communications’) who shut the thing down after 1 month. The town was forced to spend $18,000 to get the signal back using standard repeater technology. The reception wasn’t any better; and the town was out $18,000 – in 1971 dollars = about $60,000 today. The same concept was used more famously in a town called Nakusp, BC, on the Columbia River. Nakusp had no TV at all. No radio, either. At least until night time skip kicked in during the evenings. It was discovered that channel 4 – KXLY – Spokane, could be received with good signal strength on a 7,500′ mountain. There were NO Canadian stations available at all. So a passive repeater was installed to provide Nakusp with its first ever TV reception. It worked superbly – until the Canadian bureaucrats moved in and tried to shut the system off. Since the operation merely ‘reflected’ channel 4 from Spokane down into the valley – still on channel 4 – the bureaucrats were skating on thin ice. More bureaucrats were hired – even supervisor bureaucrats, whose mission in life was to take away Nakusp’s only TV station. The battle went on for years. Much paperwork was generated – the life blood for bureaucrats. In the end the passive repeater was granted an exemption and allowed to remain operational. Since Saddle Mountain ‘repeater’ site was inaccessible by road and could only be reached via a 20 mile hike straight upward, and since there was no power available at the site, deep cycle batteries were installed late in the fall just before the snow moved in. The system then ran for the entire winter on these batteries, which lasted until spring, when they were replaced by fully charged ones. Now THAT’s dedication! The DOC bureaucrats, however, used helicopters to reach the site. The total DOC expenditure on trying to shut Nakusp’s passive repeater down was $5.2 million. The cost of the repeater? $250. Batteries included! On a technical note, the selection of both receive and transmit antennas is entirely dependent upon local circumstances. In order to increase the signal strength of a distant channel, high gain, highly directional antennas are best. However, if highly directional antennas are used for the retransmission, the signal will be focussed more narrowly. The transmit antenna depends upon how wide an area is to be served. Check the antenna’s specs for 3 dB beamwidth. 3 dB beamwidth is the aperture at which the transmitted signal has reached half power. So if an antenna has a 3 dB beamwidth of 40 degrees, 20 degrees in each direction from boresite will produce a half-power signal. And all antennas have lobes, which will produce signal anomolies. Passive repeaters are a great and inexpensive way to extend TV signals into unserved nooks and crannies. They work equally well for digital as well as analog signals. One further thing – if the receive site is capable of picking up multiple channels, either use a broadband receive and transmit antenna, and broadband preamp, or use a single channel antenna, single channel preamp, followed by a bandpass filter for the desired channel in order to filter out undesired channels, otherwise you’ll end up ‘repeating’ all of them.

  • Steven Andrew wrote:

    Hi, I am trying to rebroadcast a signal down into a valley without having the repeater powered. The input gain is 70 db and looking to transmit about 2 miles. Using two TCX 18 Antiference aerials with about 20m of cable from one aerial to the other direct. The collector aerial is horizontal and transmitter is vertical and the reason i am trying that is there is still a weak horizontal signal where the transmitter aerial is, but due to location it cant be moved to a non signal area, unfortunetly I am not picking up the signal, does anyone know how far 70db would transmit and if the weak signal could be interfering with the transmitter?

    TCX 18 Aerial
    Forward Gain: 15.0dBi
    Front To Back Ratio: 22dB
    Acceptance Angle: 18 degrees

    Many thanks

    Steve

  • Russ Walsh wrote:

    You say that the input gain is 70 dB. Let’s assume the signal being received has a strength of 0 db. That means 0 + 70 = +70 dB. +78 dB is 1 watt, so 70 dB is 8 dB down from 1 watt. Or about 1/8 watt. Don’t panic. It’s still usable. Here’s the calculation. The path loss for a signal is 32.2 + 20 log D(mi) + 20 log F(mH) In English, Boltzman’s constant – 32.2 plus 20 times the log of the distance in miles plus 20 times the frequency in megahertz. An essential factor you haven’t supplied is the FREQUENCY of the signal you’re dealing with So I’m going to have to ‘pick a channel. Any channel.’ Let’s choose channel 12, which is 213 mHz. Here’s the path loss:

    32.2 + 20 log 213 + 20 log 2

    Using a scientific calculator:

    32.2 + 20 x 2.3283 + 20 x .3010

    So: 32.2 + 46.6 + 6.02 = 84.82, or 85 dB

    The original signal was 70 dB. The system gains are: The original signal strength, at 70 dB. The transmit antenna gain – let’s use 9 dB, typical for a high band antenna. And a receive antenna gain of 9 dB. So 70 + = 88 dB.

    Losses are 85 dB. so 88 – 85 = +3 dB

    At a distance of two miles you’ll have TONS of signal to work with. +3 dB on a digital, over the air, 8VSB transmission will produce awesome pix. With 8VSB transmissions used by terrestrial TV you’ll get A1 signals right down to about -16 dB. So you could go much farther than 2 miles. Remember, I guessed at the channel frequency you’re dealing with. It’s such an important factor but you didn’t mention it. You can do the recalculation yourself using Google for the log calculations, or use a scientific calculator, using the frequency of the channel you’re working with.

  • John wrote:

    Great report by Russ. I’m sure the DOC foolishnes wasn’t funny to those involved, but it was quite entertaining to me. But to business….. My issue is similar to everyone else’s. Our house is isolated and in a bit of a valley. We can recieve only on signal, and that too weak to watch. I suspect that placing an antenna on our adjacent ridge might improve the situation (not yet tested), but there remains the problem of getting a signal from the antenna to the house. Question: does anyone know the limits of transmitting weak RF signals by COAX? This would be simpler than involving repeaters, but I don’t know if it would work. The distance would probably be around 600 to 700 feet at best. Comments will be appreciated.

  • cameron wrote:

    Before I try this out let me see if you can suggest something simpler.

    First: I’m not very confident in my skills at drilling holes in my house so the ‘obvious’ answer isn’t so attractive to me.

    My downstairs TV apparently sits in a veritable frequency hole; I bought some fancy compact antenna from Walmart for $50 and I manage to get about 5 channels; upstairs I can get probably 15 through it on my other TV.

    So, I’d like the antenna to be upstairs. But, again, I don’t want to have to drill holes when I’m not so good with the tools or knowing where to drill, etc.

    What I’m reading here is that I could place a couple of antennas in my attic and somehow manage to rebroadcast the signal through the house.

    My confusion here is that I presume that both antennas would be capturing signal and interfering with each other if they were hooked together. You mention ‘pointing’ them. So, I’d fix one of them for best reception and lock it in place. I’d point the other (and which end of an antenna is ‘forward’?) down into the house and somehow connect them together and then this might give me TV goodness, correct?

    Since I’m only going 100 ft or less I shouldn’t need an amplifier.

    I guess, living in a day when everything requires power, the concept of 2 pieces of metal hooked together solving my problem sounds too good to be true.

  • Rod wrote:

    I am seeking a Preamplifier or Amplifier to increase the power Received from Antenna #1 to Antenna #2, but having trouble finding a 12 VDC Amplifier.

    Can anyone recommend a 12 VDC Power Amplifier? VHF or FM only is fine, but broadband is acceptable, FM/VHF/UHF

  • Russ wrote:

    John – an easy one to solve. If you only need to go 700′ then coax with a line powered preamplifier located at the antenna would work fine. The losses for RG6 at channel 13 (213 mHz) is about 3 dB/100′. Use RG6 FOAM dielectric cable. The solid poly dielectric cable has higher losses. So a channel 13 signal at 700′. Calculations. Assume signal strength as measured by a dipole is -10 dB. Using a 30 dB gain preamp you would have a net gain of 20 dB PLUS the gain of the receiving antenna. Typically about 10 dB at channel 13. Here is a link to an excellent antenna site that tests various VHF and UHF antennas. There is a HUGE difference in gains between various antennas. Remember, low band VHF (Channels 2 -6) no longer exists. At high band VHF (Channels 7 – 13) as you can see from the antenna site, the Channel Master 3671B has the highest gain at about 9 dB on channel 13 and 11 dB at channel 7. Here’s the link: http://www.hdtvprimer.com/antennas/comparing.html

    Be sure to scroll down to the NET gain section rather than using the RAW gain part. More accurate.

    Back to the calculations. A signal of -10 with an antenna gain of 9 gives a net signal into the preamp of -1 dB. Typical low noise preamp gain is about 30 dB. So +29 dB into the coax. It has a loss of 3 dB per 100′, so 700′ means a loss of 7 x 3 = 21 dB. That leaves you with a signal of +20, which is a bit much into most digital TVs. Cut the preamp gain back to about 15 dB. Or use a splitter, which inserts a loss of 4 dB (2-way splitter) 7 dB for a 3 way on one of the ports. Power for the preamp is inserted into the downline at the receiving end using the power inserter provided with the preamp. Hope this helps. Note – I had to guess at the channel you’re trying to pick up. IF it is a UHF channel, use the antenna charts to select the best antenna for the channel you want. There are big differences. The UHF loss for RG6 foam is about 8 dB/100′, so recalculate using that number. However UHF preamps tend to have higher gain figures and so do UHF antennas. For example, as you can see from the antenna chart the Antennas Direct XG91 has a gain of up to 16 dB. Much higher than the 9 dB at channel 13.

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