Splitting a 50-ohm antenna cable.

[DogBone384]

I'm new to scanning and am installing an antenna on my roof. I am going to put BNC wall plates in two different rooms. Can I 'cheat' and solder my 'Y' at one wall plate, or do I need a splitter specifically for 50-ohm cable? I am using R8X cable. I'm not planning on using an SWR meter unless I have to.

Thanks, in advance, for your input.

 

[W2PMX]

You can do whatever works, but the right way to do it is to use a splitter (not the TV type, one made for scanner frequencies) and make sure that if you disconnect one of the scanners you plug in a 50 ohm resistor to that port.

For receiving fairly decent signals, just splitting the cable at one jack would probably work, though.

 

[K9WG]

Most good quality TV splitters will cover the scanner frequencies up to 900 MHz. Also make sure you get a splitter and not a directional tap. the splitter will reduce the signal strength by half for each output.

 

[jackj]

As K9WG shows in his picture, TV type splitters are 75 ohm and use 'F' connectors. Scanner splitters are 50 ohm (same as your cable and antenna) and most use BNC connectors. BNC's for RG-8x are a lot easier to find than 'F'. If you don't mind another suggestion, use a cable with less loss than RG-8x. You will loose half of your signal in your splitter (a lot more if you just use a 'T' connector) and 8x already has bunches of loss. There are a lot of sites that list cable characteristics so do a search.

 

[WA1ATA]

There is a way to do it without the loss of a splitter, and without the strange problems that can be caused by an unterminated length of coax attached to your scanner.

Rather than doing a hardwired Y, run a coax to your first wall plate, but make it a dual BNC wallplate. When you are using the scanner in that room you will have a direct connection to the antenna without any extra stubs.

The second BNC in that wallplate connects to a coax that runs from that room over to the other room. When you want to use your scanner in that room, you use a short jumper between from BNC to BNC on the dual wallplate in the first room.

With that setup you also have a connection to the antenna without any extra stubs, although the coax run is from antenna to wallplate, through a jumper back into the wallplate and over to the 2nd room.

Obviously, the system described above isn't suitable if you want to use scanners in both rooms at the same time.

 

[gmclam]

Antenna signals are part of a SYSTEM

There is a way to do it without the loss of a splitter, and without the strange problems that can be caused by an unterminated length of coax attached to your scanner.

Rather than doing a hardwired Y, run a coax to your first wall plate, but make it a dual BNC wallplate. When you are using the scanner in that room you will have a direct connection to the antenna without any extra stubs.

The second BNC in that wallplate connects to a coax that runs from that room over to the other room. When you want to use your scanner in that room, you use a short jumper between from BNC to BNC on the dual wallplate in the first room.

I would recommend that you DON'T do this, but that's my opinion. It really depends on the entire "system", how coax cables are run, lengths, and more. But what you must also realize is that the connections required in the first room to "jumper" the signal to the 2nd room has loss too. Sure it might only be 1dB, or more, but it is loss.

Standing Waves (measured with SWR meter) are really only applicable when transmitting and not a significant factor for reception.

The use of GOOD quality "cable TV" splitters is a GREAT idea. I also use that solution. Don't get cheap or low quality splitters though, and make sure you are using a "splitter" as described earlier. The splitters I use are rated to pass 5-1000 MHz.

There is an assumption that because you are using RG-8 it is better coax than oh let's say RG-6. It might be. But if you really want low loss why not use LMR-400 or LMR-600? I use Air-802 coax from my antenna (it is an LMR-400 clone) to a multicoupler. From the multicoupler I run RG-6 to desired destinations. Some of those destinations further split the signal with a TV splitter as described above.

HOWEVER, I found some very good RG-6 coax that is rated to 2250 MHz. It has less loss than typical RG-8 I found. I have one run that is 70 feet from my multicoupler and have no problems monitoring UHF on the other end (both 450 & 800 MHz bands).

The RG-6 coax also solves two other issues. One is that I can use F connectors for the TV splitters. Next is that it is flexible and managable to connect directly to the scanners.

Lastly, whatever you do, make sure each "antenna output" (splitter connection, etc) is terminated with either a radio or a 50 ohm terminator. Remove the terminator to connect a radio, or connect a terminator when the radio is not connected.

 

[WA1ATA]

I would recommend that you DON'T do this,

A splitter is a guaranteed 3dB loss, and is 4dB in real life.

What I described is nothing more that either 1) running a direct connection to the scanner when used in one room, or 2) running a direct connection to the scanner with the additional loss of a jumper cable and any additional run length if any. The loss from a short jumper and a couple of BNC connections is around 0.1 to 0.3dB, much less than using a splitter.

IMO, the only drawback to the system I described is having to remember to put in the jumper to use the scanner in the other room.

Obviously, if he intends to have two scanners, one in each room, then the splitter is the way to go.

 

[k9rzz]

How loud are the stations that you want to hear? If they are way above the noise floor, then 4db won't mean squat. Do as you please.

 

[gmclam]

A splitter is a guaranteed 3dB loss, and is 4dB in real life.

And that 1dB difference between 3 and 4 is the same extra loss you can get from connectors/etc via the patch cable method.

As I wrote, this does depend somewhat on how the coax is run. If the first location just happens to be "on the way" between the antenna and the second location, then it could make sense.

If the user intends to monitor UHF (such as above 800 MHz) then coax length is as critical of a factor as any. And by essentially daisy-chaining the coax from location 1 to location 2, I suspect the cable length will be longer than if coax was run as direct as possible from the antenna. So the real math is not whether or not there is a splitter involved, you need to consider the difference in cable loss via the two methods.

Also, this thread was started with the question of whether or not to just jumper the two cables together and not even use a splitter. Keep in mind that if you do that you will essentially be using all coax connected as part of the antenna "system". In other words, that "T" of coax will affect the signal, possibly in ways you don't want. RF is an AC signal and you don't want two different phases of the same signal mixing as it can reduce/cancel it.

 

[zz0468]

I would suggest taking a more scientific view of the problem. The OP is asking about a splitter vs a 'Y' (a TEE, really).

Figure a bit more than 3 db loss for a decent quality splitter, let's call it 4. That's the diffrerence between 0.1 and about 0.16 microvolts. Or the difference between 0.2 and 0.317 microvolts. Or the difference between 1 and 1.6 microvolts. Once the receiver is saturating, 3 db loss is undetectable, and if you CAN detect it, signals are already close to threshold anyway.

Those of you with a signal generator can observe the difference in quieting between two signals, 4 db apart. It's barely perceptible in an FM receiver.

Now, in the case of splitting with a TEE, what you'll see is a rippled response between the antenna and the receivers, with the peaks of the ripple 3 db down, and the valleys of the ripple 10db or more, depending on the quality of the coax - and it gets worse with better coax.

In the end, the 3 db loss from the splitter, plus the loss of the coax is the lesser of the two situations, and is low enough that unless you do a side by side comparison between two identical receivers, one with 3 db additional loss, and one without, you'll never see the difference. With just a TEE, you'll very likely notice performance differences between the two receivers, which varies with frequency. Bad idea all around.

 

[mpddigital]

If you Really want to run a splitter run some GOOD RG-11 (Times, Commscope et al) for your feedline and once you get indoors you can use good RG-6 satellite type coax for the shorter runs. This keeps you at 75 Ohms for your coax and minimizes your impedance mismatches and overall loss. Also, get a good splitter, not one from your local store. Use BNC connectors if you can. F are a poor second choice.

The above won't be as good as running LMR-400 to separate antennas but will be MUCH cheaper

 

[zz0468]

...F are a poor second choice.

Actually, F connectors are pretty decent, which is why cable systems make such heavy use of them. BNC's are better, F connectors are very acceptable for receive systems.

 

[Nap]

I'm new to scanning and am installing an antenna on my roof. I am going to put BNC wall plates in two different rooms. Can I 'cheat' and solder my 'Y' at one wall plate, or do I need a splitter specifically for 50-ohm cable? I am using R8X cable. I'm not planning on using an SWR meter unless I have to.

Thanks, in advance, for your input.

It would be helpful to let us know *why* you want a roof antenna. Is the signal low because:

1) you live in an concrete w. rebar apartment that acts as a Faraday cage and you need to get out of it in order to receive an otherwise strong signal

or

2) the signal is low at your location even outdoors.

For 1) the splitters won't do much harm but if 2) then you want to avoid anything that could attenuate the signal.

 

[reconrider8]

not meaning to resurrect an old thread but where do you guys get the good wall plates from say im running N into the back then a bnc jumper from the plate to the scanner its self?

 

[gmclam]

Not using a wall plate

I don't use wall plates. In one installation, there is an N connector on the antenna end of the "LMR-400" and a BNC on the other. The BNC end goes through filters and into a distribution amp. Outputs from the amp are with high quality RG-6 and terminate with BNCs that connect directly to each radio.

 

[zz0468]

not meaning to resurrect an old thread but where do you guys get the good wall plates from say im running N into the back then a bnc jumper from the plate to the scanner its self?

You can buy a blank plate, then get bulkhead mount adapters that have N female on one side, and BNC female on the other.