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Old 3rd Apr 2018, 12:58 pm   #1
stevehertz
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Default Distribution unit terminators - beware, two types!

A warning for those who like me, didn't know about this. Having just bought ten 75 ohm terminators for use with a distribution unit, I now find out that you need different types depending on whether you're terminating an input or an output port?! I've since read that DC passing types are for inputs, and DC blocking types are for outputs.

Sellers seem to sell these things 'willy nilly' without stating clearly that there are two types, particularly, for TWO different applications. The ones I bought were described thus: "F Male 75 ohm terminator closes off any open ports on splitters, taps, amplifiers, multiswitches etc. . Open ports can cause signal reflections and unwanted interference. These can result in poor signal quality leading to poor pictures." In fairness, that description did include two extra words, "DC passing" at the end. But that is not quite the same as explaining what I point out above - you just think it's a normal function of 'a terminator'. Poor selling in my opinion.

In general and in practise how much difference can/do terminators make on 1) inputs and 2) outputs? In simple terms can someone describe what they do, and why does one have to be DC blocking and the other DC passing? Thanks.
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Old 3rd Apr 2018, 3:19 pm   #2
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Default Re: Distribution unit terminators - beware, two types!

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Originally Posted by stevehertz View Post
A warning for those who like me, didn't know about this. Having just bought ten 75 ohm terminators for use with a distribution unit, I now find out that you need different types depending on whether you're terminating an input or an output port?! I've since read that DC passing types are for inputs, and DC blocking types are for outputs.
It's only a concern when DC is present to power a masthead amp, and it would be the input that would carry the DC, feeding power downstream seems unlikely in a domestic situation.

Your amp probably doesn't have the facility anyway.

Incidentally I don't consider terminating unused multiple outputs of a distribution amp to be of great importance, If you take that to its ultimate conclusion, you would be fitting them to unused wall plates in blocks of flats where the tenants get their entertainment from other sources.

I am talking about multiple padded outputs, if you have an unused high level output, termination is important.
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Old 3rd Apr 2018, 5:58 pm   #3
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Default Re: Distribution unit terminators - beware, two types!

I thought cable TV systems in blocks of flats would use a “Tap” to feed the outlet giving isolation to the outlet. I have no idea what is in the distribution amp so termination would seem easy and worthwhile.
The terminators you have would seem to do the job, why 10 though, I thought you only required a couple?
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Old 3rd Apr 2018, 9:11 pm   #4
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Default Re: Distribution unit terminators - beware, two types!

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I thought cable TV systems in blocks of flats would use a “Tap” to feed the outlet giving isolation to the outlet. I have no idea what is in the distribution amp so termination would seem easy and worthwhile.
Yes, the rigger would carry a box of normal 75 ohm resistors in his van, and use one across the unused trunk output on the last tap-off unit in each cable run.

When I started work, I did a lot of TV repairs in council tower block in Salford. There were three VHF channels BBC1 & ITV on 405 lines and BBC2 was added by downconverting it to an unused VHF channel that roughly coincided with Channel 8, (but 8 megs wide of course)

This was done so the original thin lossy coax could be retained. There were no separate tap units they used special wall plates in the flats with an in and an out cable from the flat above and below respectively. Different sockets with progressively lower attenuations applied to the outlet as you went down the chain.

The sockets on the ground floor had a 75 Ohm resistor wired between the unused output and the saddle-clamp screw.

Needless to say, anyone tampering with their socket would likely disconnect all the flats below their own.
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Old 3rd Apr 2018, 9:39 pm   #5
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Default Re: Distribution unit terminators - beware, two types!

We had council flats in the area where I worked, similar system of 2 405 VHF channels and BBC2 down converted to a VHF Channel. In the 60’s the signal quality was generally appalling and that system never got fixed.

Later a new system was installed in flats in the borough after many complaints and that was much better. I cannot remember if the 3 UHF channel were down converted or left on frequency.
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Old 4th Apr 2018, 9:30 am   #6
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Default Re: Distribution unit terminators - beware, two types!

Hi,
I seem to remember that my Grandparents were at one point using a communal system that required a small box ( vhf to uhf , was it Labgear ? ) to receive one ? of the three television channels. this would have been the late 1960's. They also had a Colour TV, if only I could remember what make it was !
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Old 5th Apr 2018, 11:44 am   #7
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Default Re: Distribution unit terminators - beware, two types!

Labgear made one, the name Tele-Verta, it converted the VHF channels in communal aerial systems to UHF. It was just Frequency conversion not standards.
https://www.vintage-radio.net/forum/...d.php?t=114193
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Old 5th Apr 2018, 5:26 pm   #8
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Default Re: Distribution unit terminators - beware, two types!

On CCTV distribution systems years ago I remember all the monitors except the last one on the common cable run were switched to "through". The last one was switched to "term" which put a 75 ohm resistor across the input. Makes sense I suppose. I certainly have never bothered with terminating unused outputs on TV distribution amps as I didn't really think they were designed to be that critical. On the other hand, on the microwave baseband distribution systems I worked on it was very important to terminate correctly, otherwise you could get all sorts of funny effects. Even more important when you have RF circulators and duplex working.
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Old 6th Apr 2018, 9:07 am   #9
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Default Re: Distribution unit terminators - beware, two types!

Because I didn't know that there were two types, I now have qty 10 'DC passing' designated 75 ohm F type terminators. Can I use these for outputs too? How does a terminator being 'DC passing' or 'not DC passing' affect how it is used?
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Old 6th Apr 2018, 9:22 am   #10
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Default Re: Distribution unit terminators - beware, two types!

It doesn't. No terminator can be DC blocking because it is a terminator. All it is, is a 75R resistor in an F type cap.

What you appear to have is attenuators, not terminators.
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Old 6th Apr 2018, 9:39 am   #11
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Default Re: Distribution unit terminators - beware, two types!

No way, you're getting confused. They are terminators, I bought them as such. I may be struggling a bit here but I'm not that stupid! To repeat myself, some sellers offer two types of terminator; DC passing and non-DC passing for input or output port use. FYI, terminators are a physically capped, 'dead end' device, they do not allow connection of another connector as do attenuators. I know what an attenuator is and looks like, and I know a little about terminators and what they look like. I want to know more about the two different types of terminator.. Cheers.
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Old 6th Apr 2018, 9:45 am   #12
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Default Re: Distribution unit terminators - beware, two types!

The type for use on connections which have DC 'masthead' power on them simply has a capacitor in series with the 75R resistor. It only needs to be an effective terminator at frequencies greater than about 100MHz so the fact that the capacitor makes it not a perfect 75R at low frequencies doesn't matter.

DC masthead amplifier power would quickly overheat and destroy a straight 75R resistor.

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Old 6th Apr 2018, 10:00 am   #13
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Default Re: Distribution unit terminators - beware, two types!

Right, so are you describing a 'DC passing' or 'not DC passing' there? And would my 'DC passing' ones be ok for use on output ports at VHF?
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Old 6th Apr 2018, 10:30 am   #14
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Default Re: Distribution unit terminators - beware, two types!

From what I remember the unused normal outputs from the dis. amp weren't terminated but an unused full output socket (if fitted) was.

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Old 6th Apr 2018, 10:35 am   #15
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Default Re: Distribution unit terminators - beware, two types!

The terminology is a bit unclear, but I suspect that 'DC passing' means 'suitable for use on circuits which have masthead power on them'. That's how the phrase would be used with regard to other aerial wiring accessories. Therefore I suspect that a 'DC passing' terminator actually has the blocking capacitor in it, but a 'DC blocking' one doesn't. This has the potential to be very confusing.

In any case, the type of terminator with a blocking capacitor in it will work everywhere on any reasonable antenna system, from 50MHz-ish Band I all the way to 850MHz-ish Band V.

If you really want to be sure, you can use a multimeter to check whether the socket you're terminating has a DC voltage (of more than about 1V) on it. If it doesn't, then you have nothing to worry about, and you can use any old terminator - blocking, passing, whatever.

If the socket you're trying to terminate does have a DC voltage on it, then you need a particular type of terminator which can handle that. Such a terminator should measure as an open circuit with an ohmmeter, whereas a regular terminator would measure as 75R.

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Old 6th Apr 2018, 11:07 am   #16
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Default Re: Distribution unit terminators - beware, two types!

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From what I remember the unused normal outputs from the dis. amp weren't terminated but an unused full output socket (if fitted) was.

Lawrence.
Yes the full output if fitted requires termination if not used. There are various views on the other outputs from no terminator, only terminate if half or more outputs are not used and terminate any unused output.

Rightly or wrong I go for full termination, that way if there any problems with performance, termination would not be one of them.

But at the end of the day you pay your money and take your choice.
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Old 6th Apr 2018, 11:36 am   #17
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Default Re: Distribution unit terminators - beware, two types!

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Originally Posted by cmjones01 View Post
The terminology is a bit unclear, but I suspect that 'DC passing' means 'suitable for use on circuits which have masthead power on them'. That's how the phrase would be used with regard to other aerial wiring accessories. Therefore I suspect that a 'DC passing' terminator actually has the blocking capacitor in it, but a 'DC blocking' one doesn't. This has the potential to be very confusing.

In any case, the type of terminator with a blocking capacitor in it will work everywhere on any reasonable antenna system, from 50MHz-ish Band I all the way to 850MHz-ish Band V.

If you really want to be sure, you can use a multimeter to check whether the socket you're terminating has a DC voltage (of more than about 1V) on it. If it doesn't, then you have nothing to worry about, and you can use any old terminator - blocking, passing, whatever.

If the socket you're trying to terminate does have a DC voltage on it, then you need a particular type of terminator which can handle that. Such a terminator should measure as an open circuit with an ohmmeter, whereas a regular terminator would measure as 75R.

Chris
Chris, you're a shining light. At last, clarification! Thanks.
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