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Splitters, Filters and Multicouplers For discussion of all inline devices used to split, combine or amplify a receive signal. This forum is not for any bi-directional (transmit) device. Use the Amateur or Commercial Radio forums for those.

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Old 12-02-2012, 10:09 AM
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Default Multi Scanners One Antenna?

Love seeing everyone's shack pictures, but I have a question about antenna's. Lot's of scanners going, but just what do you do for antenna's? Can you use a splitter to add on more than 1 scanner to one antenna? I use a Discone but have 4 scanners and want my 2 base's on one antenna. Will it workout ok?
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Old 12-02-2012, 10:28 AM
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Quote:
Originally Posted by BlackHawkH60 View Post
Love seeing everyone's shack pictures, but I have a question about antenna's. Lot's of scanners going, but just what do you do for antenna's? Can you use a splitter to add on more than 1 scanner to one antenna? I use a Discone but have 4 scanners and want my 2 base's on one antenna. Will it workout ok?
I use a Stridsburg Engineering multicoupler, MCA204. Google them and see they have several different modles. Works fine to use several scanners with a single antenna. You can purchase direct or from one of their dealers such as ScannerMaster. Pricey but a good product.
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Old 12-02-2012, 10:34 AM
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Thank-you for the lightning fast info. That will be my next purchase. I though so, but needed an answer not just my guess that you could do it. Again thankx.....
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Old 12-02-2012, 11:22 AM
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Receive and transmit are totally separate functions, and don't play well in the same environment.

Receivers are trying to detect signals down to -120 dBuV or 0.000002 uV EMF volts. A 25 watt transmitter puts +151 dBuV or 7.07e+7 uV EMF into the coax. A typical transceiver has switching circuitry that isolates the internal receiver from the transmitter during a PTT, but a second radio connected to the same coax without an expensive RF tuned transmitter combiner doesn't have that protection. Putting the performance and mis-match issues aside, without protection the result will be a lovely curl of smoke rising from the unprotected equipment, very much like looking at the sun through binoculars.

Multiple receivers on a single antenna is a different issue. There is only so much energy comming down the coax from the antenna. A 2-way splitter sends half each way, a 4-way sends one quarter to each receiver. This reduces the amount of signal any of the receivers will have to act on.

(Note: each 3db change halves or doubles a signal.)

To put it simply, if your receiver can barely hear a transmitter that has -116 dBuV at your location while not sharing the antenna, if you share the antenna four ways your receiver will only see -121 dBuV and won't hear it at all. If a closer/stronger transmitter has -100 dBuV at your location while not sharing the antenna, if you share the antenna four ways your receiver will see an adequate -106 dBuV signal (about eight times more than it needs) and won't know the difference.

Broad-frequency amplifiers will boost the signal to compensate for the splitter loss. Also, even receivers transmit a small amount of energy, so the rule of thumb is to provide 30 db of isolation between receiver ports. "Amplified Receiver Multi-Couplers" (read as expensive) are designed for this but are subject to the same overload from nearby transmitters, even those on differing frequency bands.

"Real" sites use large (read as expensive) RF filters to only allow specific frequencies to pass from their source to their destination and ground out everything else. When you see a tower with two dozen antennas, you can be sure that a substantial portion of the site cost will be filters to keep unwanted RF energy out of where it doesn't belong. Strong transmitters on close frequencies even miles away can overload receivers if not filtered properly. The RF filters are tuned to a single frequency and do not allow for multi-frequency operation, making it useless for multi-frequency scanners. Typically each radio at a commerical site is a single-frequency device.

Simple answer to your question: If there are no transmitters within several blocks, use a broad-band amplifier and splitter to share a single antenna between several receivers but keep the amplified level to 'just enough'. Before you key up your own transmitter, ground out the other receiver systems. Keep each transmitter on its own antenna, as far away from the receiver antenna(s) as possible.
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Old 12-02-2012, 11:49 AM
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Quote:
Originally Posted by BlackHawkH60 View Post
Love seeing everyone's shack pictures, but I have a question about antenna's. Lot's of scanners going, but just what do you do for antenna's? Can you use a splitter to add on more than 1 scanner to one antenna? I use a Discone but have 4 scanners and want my 2 base's on one antenna. Will it workout ok?
What lep said is good info. Touching on what jeatock said, good multicouplers do have low gain amps built into them to overcome the mini splitters (mini versions of a cable tv splitter but designed for 50 ohms) inside the MCA204 lep mentioned from Stridsberg.
I live in a very high RF environment and use multiple Stridsberg multicouplers. I've never had an issue caused by them from overload. If I do, that issue is also present without the stridsberg multicoupler. Another words, the Stridsberg multicoupers do not make the problem worse for me.
I actually have two of their MCA208M's which have 8 outputs each.

And as jeatock said, yes, they are not cheap! It's the proper way to do multiple scanners if you cannot dedicate an antenna to each scanner however. I wish I could but I'm in an apartment and they did give me a key for the roof hatch so I can work on my antennas but I don't push my luck and keep my scanner antennas to just two. One multiband and a 12 element Yagi beam on a rotor for 800 MHz. The setup works very well but GRE scanners do not play nice with all the very near VHF paging signals here on top of all the hospitals. The paging signals cause the GRE's to go into desense mode and thereby reduce the front ends sensitivity to the point I can't hear other VHF signals in many cases. I must use a PAR 152 MHz paging filter to correct that.

My Uniden's are not bothered by the VHF paging signals so they don't really need the PAR filters but I place the PAR filter at the input to the MCA208 so it's little amplifier is not affected by the paging signals also.

I removed the PAR filter the other day when a new statewide VHF site came online here so I could hear it. Our state is using several frequencies that fall in the 152 MHz paging band. With the PAR filter, I can't hear the sites that are not right here. So far, the GRE's are handling that sites signals without the PAR filter OK but there has been no voice traffic yet.

I may end up needing a 2nd VHF antenna hooked to another multicoupler for or state's new VHF system and let that feed just the Uniden's with no PAR paging filter. Then use the GRE's with a PAR filter but do not use them for the statewide sites that the PAR filter will reduce the signal strength on. It's going to be a bit of a challenge for me getting everything where I need it with multicouplers and PAR filters only for the GRE made radios. Then a new coax run and antenna and yet another Stridsberg multicoupler to feed the radios I dedicate for the state system.
It may be cheaper for me to purchase a PAR filter for each GRE made radio and place them at the mutlicouplers output jacks that feed the GRE's.

The PAR filters drop the signal levels within their designed range by about -35 dB and then that tapers off quickly as you move away from the designed filter range so they don't also attenuate other frequencies like say the 155 MHz range.
I've never tried placing individual PAR filters on the outputs from the multicoupler that feed the GRE's. In theory, it should work as the ports are isolated from one another so the PAR filters should still work as designed.

edit: to add, many have used simple splitters made for TV. While they may work for strong signals, they will reduce the signal levels as jeatock pointed out plus they often do not have good port to port isolation that a true multicoupler will have. Add that to the fact that you would also need adapters for each connection which can also cause a small amount of loss, It's been said that an adapter or simple connector can each add about 0.5 dB of loss. Get enough of them inline and you may end up doubling your loss which is not good if you are after distant signals that are already weak levels. Many also use Cable TV drop amps. Some of those work but many are a cheap design and have high noise figures that will wipe out any chance of decoding a digital control channel. They also do not pass any signals below 54 MHz so you would lose VHF Low Band if you are into that.

Last edited by kruser; 12-02-2012 at 11:58 AM..
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Old 12-02-2012, 12:35 PM
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Building on what kruser said, the longer you are in the business the more you discover that radio is one third theoretical physics, one third black magic, and one third pure luck, all factored by the difference of the last Superbowl scores.

You also discover that there is no black and white, only infinite shades of gray. If you are only trying to hook up three or four receivers to hear strong local signals, a coathanger stuck into a TV coax splitter will probably work. Just don't expect to directly hear mobiles thirty miles away with that setup.

If you are trying to hear distant weak signals on multiple receivers sharing the same antenna system be prepared to break out the debit card, and do so knowing that part of the baby always gets thrown out with the bathwater.
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Old 12-02-2012, 2:45 PM
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Quote:
Originally Posted by BlackHawkH60 View Post
Can you use a splitter to add on more than 1 scanner to one antenna? I use a Discone but have 4 scanners and want my 2 base's on one antenna. Will it workout ok?
If the signals you want to listen to are strong, a splitter will do fine.

Where you will start having problems is with weak signals.
For example a 2-way splitter will reduce the signal to each scanner by about 3.5 dB.
On marginal signals that will be enough to put them into the noise so you won't even know they're there.

Try it. It's cheap enough.
Make sure to use a good quality splitter that's rated for the desired frequency range (or better).

I have seven antennas. Each of them has a pre-amp at the base of the antenna because many of the signals I want to hear are quite weak.

One of those antennas feeds 32 scanners through multi-couplers. The others have splitters on them.

Rich

By the way, you can listen to a sample from one of the 32 scanners sharing that one antenna here:
(The actual event starts around 1:00 into the audio)
http://www.youtube.com/watch?v=ETW3z4mj5rM

Last edited by rbm; 12-02-2012 at 3:02 PM..
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Old 12-02-2012, 3:12 PM
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Default Lots of info Here

http://forums.radioreference.com/pas...-scanners.html
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Old 12-02-2012, 7:37 PM
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Moved to the Passive forum.
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Old 12-03-2012, 5:48 PM
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Wirelessly posted (Mozilla/5.0 (X11; Linux i686; U; en-US) Gecko/20081217 Vision-Browser/8.1 301x200 LG VN530)

Out of curiosity, can you do the opposite, feed multiple, band-specific antennas into one scanner, by simply flipping the multicoupler upside down? Or is the amp a one way affair?
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Old 12-03-2012, 10:51 PM
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Quote:
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Wirelessly posted (Mozilla/5.0 (X11; Linux i686; U; en-US) Gecko/20081217 Vision-Browser/8.1 301x200 LG VN530)

Out of curiosity, can you do the opposite, feed multiple, band-specific antennas into one scanner, by simply flipping the multicoupler upside down? Or is the amp a one way affair?
No, but you can use a diplexer for that. And yes to your second question.
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Old 12-05-2012, 4:52 AM
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So for 2 antennas you use a diplexer, and for 3 antennas you use a triplexer. Is there a quadplexer or a quintplexer? LOL
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Old 12-05-2012, 10:18 PM
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I currently use a 2 port and 8 port catv amp. It works great. I use a mix of 75 and 50 ohm cable without issue. (yes, i know but it works fine for my receive only setup) Here is a thread on my 8 port that a friend of mine modified.

Electroline Drop Amp Modification
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