Terminating unused taps on a coax splitter

[lilroop]

Are you required to terminate any unused taps on a coax splitter with a terminator cap?

 

[jumper]

Required-no. Good idea-yes.

Here is a similar thread that discusses the topic.

http://forums.mikeholt.com/showthread.php?t=124019

 

[egnlsn]

Required-no. Good idea-yes.

Here is a similar thread that discusses the topic.

http://forums.mikeholt.com/showthread.php?t=124019

Good thread!

 

[GoldDigger]

Required-no. Good idea-yes.

Here is a similar thread that discusses the topic.

http://forums.mikeholt.com/showthread.php?t=124019

All that an unterminated port on a splitter will do is mess up the impedance of the transmission line leading to the splitter, causing ghosting there. But a cable attached to a port that is then left unterminated can cause an apparent short circuit at spaced RF frequencies, and only the presence of the splitter will keep that from messing up the other ports totally.
A coaxial cable at RF frequencies is a transmission line with a characteristic impedance, which also acts like an impedance transformer in some situations.
If you have a piece of coax which is exactly 1/4 wavelength (or any odd multiple of 1/4) long at a particular frequency, then an open circuit at one end of the cable looks like a short circuit at the other and vice-versa (at that one signal frequency). Whereas for a multiple of 1/2 wavelength a short looks like a short and an open looks like an open. But any length with a proper terminator on one end looks just like a terminator at the other end.
The chances of causing a problem later that will be a royal pain for you or someone else to fix argues very strongly for using terminators.
(The above detail was not in the other thread.)

 

[lilroop]

That is a lot of good information! Thank you to all that responded, I really appreciate your time, and the good information.

 

[GoldDigger]

That is a lot of good information! Thank you to all that responded, I really appreciate your time, and the good information.

Pay it forward when you are able.

 

[egnlsn]

All that an unterminated port on a splitter will do is mess up the impedance of the transmission line leading to the splitter, causing ghosting there. But a cable attached to a port that is then left unterminated can cause an apparent short circuit at spaced RF frequencies, and only the presence of the splitter will keep that from messing up the other ports totally.
A coaxial cable at RF frequencies is a transmission line with a characteristic impedance, which also acts like an impedance transformer in some situations.
If you have a piece of coax which is exactly 1/4 wavelength (or any odd multiple of 1/4) long at a particular frequency, then an open circuit at one end of the cable looks like a short circuit at the other and vice-versa (at that one signal frequency). Whereas for a multiple of 1/2 wavelength a short looks like a short and an open looks like an open. But any length with a proper terminator on one end looks just like a terminator at the other end.
The chances of causing a problem later that will be a royal pain for you or someone else to fix argues very strongly for using terminators.
(The above detail was not in the other thread.)

It isn't a short that is caused -- it is a standing wave is caused. That standing wave can totally disrupt one particular frequency, but it's not like only one frequency is affected. That standing wave has its peak at X-frequency, but it's a sine wave. Adjacent and nearby frequencies are affected as well -- just not to the same degree.

Ghosting is caused by one of two things -- either ingress or reflections. Reflections are caused by impedance mismatch (which is usually an unterminated port or line, but sometimes is defective equipment), and ingress is caused by poor (or a loss of) shielding. Either way, a symptom is two instances of the same signal traveling down the cable -- one behind the other. It shows up on the TV or monitor as a ghost. Say you have a 2-way splitter with out ports A & B, and port B is unterminated at the outlet, which is 30 feet from the splitter. The signal coming from the source hits the splitter and goes down both legs. When the signal hits the unterminated outlet on port B, it is reflected back towards the source (usually an amplifier). It goes through the splitter to the source (amplifier), where it is reflected again and goes back downstream. If the Return Loss of the source (amplifier) is high, there's not too much to worry about, because not much will be reflected. But, not all amplifiers and splitters are created equal, and too many people use the cheapies. The RL on those is horrible.

As for unterminated ports being a source of ingress: Several years ago, I read an exchange in a CATV forum (S.C.T.E.-List) between 2 CATV industry engineers with differing opinions. One was an engineer for an equipment manufacturer (who argued that unterminated ports do cause ingress) while the other is a leading engineer in the industry. He lectures and authors the technology column in an industry magazine. He argued that unterminated ports do not cause ingress. Both gave convincing arguments.

 

[GoldDigger]

It isn't a short that is caused -- it is a standing wave is caused. That standing wave can totally disrupt one particular frequency, but it's not like only one frequency is affected. That standing wave has its peak at X-frequency, but it's a sine wave. Adjacent and nearby frequencies are affected as well -- just not to the same degree.
See a more detailed discussion below.

As for unterminated ports being a source of ingress: Several years ago, I read an exchange in a CATV forum (S.C.T.E.-List) between 2 CATV industry engineers with differing opinions. One was an engineer for an equipment manufacturer (who argued that unterminated ports do cause ingress) while the other is a leading engineer in the industry. He lectures and authors the technology column in an industry magazine. He argued that unterminated ports do not cause ingress. Both gave convincing arguments.
Unterminated cables (especially Type F connectors assembled with a really long center conductor sticking out) can be much more of a problem, even if they do not accidentally touch something.

Is light a particle or a wave? Is a square wave a single complex waveform or a superposition of an infinite number of odd-harmonic sine waves? The answer, if there is one, depends on what you are trying to do.

In the case of a cable (transmission line) with an open end, that end point must be a current node and a voltage anti-node in the standing wave pattern, since no current can flow. A center-to-shield short would be a voltage node and a current anti-node in any standing wave pattern, since no voltage can be sustained.
Now when you have a 1/4 wavelength section, if the open end is a voltage anti-node, the other end must be a voltage node. That is, no voltage present regardless of the amplitude of the applied waveform and the signal current flowing. It looks exactly like a short circuit (at that frequency.)

Now if you suddenly apply the test signal, the line will look like a perfect 75 ohm resistor until the signal has reached the far end and bounced back. From that time on until you turn off the signal, it looks like a short circuit at the connected end where you are observing. But when you stop the signal, you will see the transmission line as a signal source of an opposite voltage waveform (with a 75 ohm characteristic impedance) until 1/2 cycle has gone by and it all stops.

True, adjacent frequencies are affected, but the resulting "circuit" is a good enough approximation to a high-Q filter to make it useful in nulling out interference caused by nearby transmitters in an antenna circuit.
And yes, it really looks like a comb filter, since it affects all higher frequencies for which the line is an odd multiple of 1/4 wavelength.
For a single frequency of interest, using a 3/4 wave or longer section of cable makes the filter more selective.

Bottom line: when you have a 20 or 30 foot length of unterminated cable and you are working with frequencies whose wavelength is less than a foot, it is going to be hard to predict just what will happen, but it will usually not be good.

 

[mike_kilroy]

I think the answers given so far make a major assumption: that some coax of >= 1/4wavelength is attached to the unterminated port. In that case, sure, terminate it to prevent reflections/stand-waves/ghosts.

But if it is a splitter with NOTHING attached to the unused ports, why in heaven would you want to cut your signal down by terminating them?

My vote goes to leave unused ports alone, no wire attached, no terminator attached to be a signal sucker.

 

[GoldDigger]

I think the answers given so far make a major assumption: that some coax of >= 1/4wavelength is attached to the unterminated port. In that case, sure, terminate it to prevent reflections/stand-waves/ghosts.

But if it is a splitter with NOTHING attached to the unused ports, why in heaven would you want to cut your signal down by terminating them?

My vote goes to leave unused ports alone, no wire attached, no terminator attached to be a signal sucker.

My answer to that, particularly if somebody put in a splitter with too many ports compared to the number used, is that the characteristic impedance that the splitter itself presents at its input port to the incoming cable will be correct if and only if all of the output ports are either in use or terminated.
You might get a slightly higher signal voltage at most frequencies on the remaining splitter ports by leaving the unused ones unterminated, but it will be at the cost of causing standing waves in the cable attached to the input port.
If you are only generating standing waves on a single cable run back to the splitter used by the cable company to feed the signal, they may not do much harm to the signal, and the variance they cause in the impedance on the other side of the cable company splitter will be pretty small. (The cable company designs to protect their signal from screwed up connections at any individual subscriber, but you better believe that they put terminators on their own unused ports anyway.)
But if there are several splitters cascaded, with cables longer than a few inches joining them, then unterminated ports on a splitter can actually end up reducing the signal on other splitter ports at some frequencies.
Wrapping your head around good practices when transmission lines are involved takes awhile and often the gut instinct answers are wrong.

 

[mike_kilroy]

i've edumacated myself after ur post gold. - thanks for correcting my gut feel wrong answer. u'd think as an extra class ham & old first class commercial licensee i'd know better! 73, ac8v

 

[GoldDigger]

i've edumacated myself after ur post gold. - thanks for correcting my gut feel wrong answer. u'd think as an extra class ham & old first class commercial licensee i'd know better! 73, ac8v

Quite likely you never needed a splitter unless you separated your transmit and receive antennas. Splitters on high power feeds don't make much sense.
Did you ever need to use an SWR bridge? One way to correct SWR, even in high power circuits, is to insert a carefully calculated length of wrong impedance cable into the line (e.g 1/8 or 1/4 wave of 50 ohm cable in a 75 ohm line.) But exactly where you put it relative to the antenna or the xmitter matters too.
73, (null).

 

[egnlsn]

But if it is a splitter with NOTHING attached to the unused ports, why in heaven would you want to cut your signal down by terminating them?

My vote goes to leave unused ports alone, no wire attached, no terminator attached to be a signal sucker.

The voltage is reduced by half (if it's a 2-way, 1/4th if a 4-way, and so on, exponentially) merely by virtue of the splitter. Whether ports are terminated or unterminated doesn't change that.

While I've not used splitters on transmit antennas, I have on receive antennas. Doing so increases the received signal quite a bit, which you then cut down by ~3.5dB, but you also increase the SNR by that same 3.5dB. Makes a world of difference.

 

[GoldDigger]

The voltage is reduced by half (if it's a 2-way, 1/4th if a 4-way, and so on, exponentially) merely by virtue of the splitter. Whether ports are terminated or unterminated doesn't change that.

While I've not used splitters on transmit antennas, I have on receive antennas. Doing so increases the received signal quite a bit, which you then cut down by ~3.5dB, but you also increase the SNR by that same 3.5dB. Makes a world of difference.

Thanks for the information on the practical aspect of combining receiver antennas. It looks like what you are getting is an increase in the SNR because the signal you are looking for is identical from both antennas while the noise is random between them. It may also be that what you are doing is actually constructing a phased array directional antenna if the wavelength of interest is not large compared to the antenna separation.

But as to the voltage/power ratios: A resistive divider rather than a true power splitter will have a voltage ratio of 2:1 from input to output, but an RF splitter which uses impedance transformation will actually give you a power ratio of 2:1, with the addition of a hopefully small insertion loss because the splitter is not perfect. That is the kind of design in which leaving a port unterminated has the possibility to make things worse.
You might want to take a quick look at the overview of microwave splitters which is given here. I am sure that there are better references, but that is the first one I found which is not Wikipedia.

 

[egnlsn]

Stacking antennas has been used in rural CATV systems for decades. Mount 2 single channel antennas on the same mast 1/4 wavelength of the desired channel and combine them utilizing a 2-way splitter, making sure that the leads from the antenna to the splitter are the exact same length. One of the things that causes the SNR to increase is the splitter itself. It drops not only the desired signal, but the undesired signals (noise) as well.

You can combine 2 antennas that are on adjacent masts in the same manner. That makes it a phased array directional antenna. Those are highly directional.

There are a couple of real good articles about r.f. splitters at http://www.cencom94.com/Download.html. Right-click on the link to open or download the article(s).

 

[GoldDigger]

One of the things that causes the SNR to increase is the splitter itself. It drops not only the desired signal, but the undesired signals (noise) as well.

If the splitter drops the signal and the noise by the same amount, it does not change the SNR at all. It may help reduce overload in the receiver stages if there is a strong local signal, but if you drop the level too far you get to where the noise floor on the input stage will cause the SNR to increase as you drop the signal further.

 

[egnlsn]

True on dropping the signal too far and the SNR decreasing. All I know is that I once had an engineer (not mine) suggest I stack a couple of antennas as I was having difficulty receiving a particular FM radio station (we used the SCA carrier of that station). The antennas were large VHF antennas and it was fun mounting the 2nd below the other, but it worked just fine. A few years later I mentioned that to my systems' engineer, and his comments included that it raised the SNR.

A few years later, I did the same thing to receive ch.12 out of Phoenix, as there was not a nearby translator for ch. 12. Channel 12, which previously had been filled with noise, then looked as good as the others, which were coming off nearby translators.