Labgear FM distribution unit gone noisy?

[stevehertz]

In my loft I have a Labgear four output distribution amp/unit that I use to provide FM signals to three different parts of the house. Just lately I've been doing a review of things as I have not been happy with my FM signal. Yesterday a rigger came and remade the connection on my roof mounted FM aerial using low loss satellite cable, so that's eliminated now, and there was an improvement. But now, on weaker signals there's quite a bit of hissy background noise. So, I bypassed the disty unit and the signal meter on the tuner - as to be expected - displayed a lower signal level, but the noise situation was noticeably better. My take on that is that the dist unit is still basically working (distributing and amplifying) but has gone noisy? Any views or experiences of these units? Does that seem a reasonable assumption? It is at least twenty years old, more like 25.

Oh, also, our rigger has a habit (?) of putting a loop in the down lead as it travels down the mast. To me, this is an inductor that will limit the passage of HF? What do you think, have you seen this done before and why?

[Philips210]

Hi

I may be completely wrong on this but I seem to remember low loss coax with foil screen intended for satellite use may perform not so well at lower frequencies such as broadcast FM radio. I think I read this in Television magazine many years ago.

Regards
Symon.

[stevehertz]

http://aerialsandtv.com/cableandleads.html#LowLossCoAx

[Nuvistor]

May be worth looking inside the distribution amp, it may have lockfit transistors, check it’s PSU as well.
It’s worth terminating the unused output, if it’s a standard splitter for the outputs it can upset the other outputs if not terminated.

Presume it used to work correctly.

I don’t know about the loop in the downlead, 50 odd years ago when I was rigging we used to put a small loop where it exits the junction box on the aerial. Two fold, one to attempt to keep water out of the box and two if the aerial required replacement there would be enough wire without rewiring. It needs to be small though, if it is big enough to flap around it can disturb the signal, found that out the hard way.

[Nuvistor]

On the link in post 3 (photo) it suggests a service loop be introduced in the cable to facilitate adding an amp or splitter in the future.
Similar idea to that what I had so many years ago.

[Andrew2]

Nothing wrong with a loop in the coax, as it does not affect the currents flowing inside it. It does have the useful effect of 'choking off' any unwanted currents flowing on the outside of the braid.

Satellite coax is just as good at lower frequencies as it is for UHF.

[Graham G3ZVT]

Also a good installer will leave a "drip loop" in the cable so water runs off it rather than continuing into the building.

Usually placed at the point of entry, but I imagine one at the tail end of a vertical run makes sense too.

For a different reason it's considered good practice to leave a loop of extra coax at the aerial end, in case it needs reterminating in the future, perhaps to an aerial with a longer boom.
This is not often evident in domestic installations though.

[stevehertz]

Originally Posted by Nuvistor

Presume it used to work correctly.

I don’t know about the loop in the downlead, 50 odd years ago when I was rigging we used to put a small loop where it exits the junction box on the aerial. Two fold, one to attempt to keep water out of the box and two if the aerial required replacement there would be enough wire without rewiring. It needs to be small though, if it is big enough to flap around it can disturb the signal, found that out the hard way.

Oh yes, it used to work properly. I'm not going to try and fix a distribution amp, I'll try it by substitution and if it proves to be faulty just bin it. I've got a later Labgear LDA204L that I bought last year and I'll try that when I can round to the job (in my loft). Good tip about terminating the unused outputs though. What do you terminate them with? are their commercially available 'terminators'? How much difference does it make?

The loop in the cable is halfway down the mast about a metre down from the centre of the aerial and about 6-7 inches in diameter. The downlead going into the aerial junction box is configured to be entering it from below, I made sure he did that, but he knew already.

BTW, in the never ending saga of aerial connectors seemingly not being to some kind of standard (eg male and female sockets and plugs used willy nilly), I notice that the new Labgear disty unit has only F type connections. Yet look on the back of any modern tuner and it has standard coax for FM and F type for DAB. That means leads with different types of plugs at each end or the use of adapters. You can't have enough different cables, plugs and adapters these days! Even then, the one you want for the moment is always the one you haven't got!

[Nuvistor]

A terminator can be made with an aerial plug and solder a 68-75 Ohm non inductive resistor across inner and outer, very easy with a coax plug, may need to improvise with other connectors.
The resistor can be ¼ or ½ watt, whether it will make a difference is difficult to say, unterminated outlets can cause unpredictable problems, say suck out at a frequency. If that frequency is not required then there are no problems, if it falls within a passband of a station it can.

Always good practice.

Possibly commercial ones, I have never looked for them.

[stevehertz]

Cheers, will terminate in future!

[G8HQP Dave]

Has the amplifier PSU unit gone noisy? Swapping that might help.

Has a neighbour recently installed solar panels? They or their inverters can emit huge amounts of noise on Band 2 when the sun is shining. It may be that their noise plus the gain in the amplifier is pushing the tuner over the edge.

[vidjoman]

I’ve had bad electrolytic caps in the power supply cause various faults. Loss of some channels, low gain, noisy pictures etc. Simply replace and see what happens - you can always take them out if it doesn’t cure the fault.

[stevehertz]

Like I said above, I've got a later Labgear LDA204L that I will substitute for the older one. Can't do that yet though as I'm waiting for delivery of F type to coax adapters. Yes the earlier Labgear was all coax sockets (hence all my cables are the same) and the newer Labgear is all F type. Seems like plain old coax sockets are gradually falling out of favour.

[Nuvistor]

Belling Lee coax plugs and sockets were never the best for impedance matching but good enough for Bands 1 and 3, there wasn’t a cheap alternative when Bands 4 and 5 came into use so it was used with success on those bands.

The “IF” used to feed the Sat dish to the Tuner is too high in frequency for BL plugs and the F type was designed to be cheap yet suitable.

The BL has done good service over the years but the F type for impedance matching and loss is much better. No doubt the F type has its problems.

[Graham G3ZVT]

I am going to be even less kind to BL plugs than Frank.

A common issue is poor contact of the co-ax inner due perhaps to oxidation or under size wire gauge. This gives rise to the slightly counterintuitive effect of increasing attenuation as you go down in frequency. This was very evident back in the days of Band I television, and still is for those of us who still use it.
By the time you have got up to Band IV/V frequencies the reactance of the capacitor formed by the corroded contact is low enough so as to present less of a problem

The effect of a poor contact developing in BL connectors carrying a DC supply to a masthead amp can be a show stopper.

Some people advocate soldering the inner of a BL plug. The fact that it is probably a good idea for those who can carry it out without melting the insulator or cable dielectric only serves to reinforce my belief that they are not now fit for purpose, so I would advocate only using them when there is no alternative ie at the final connection to the radio or TV set itself.

Originally Posted by stevehertz

Like I said above, I've got a later Labgear LDA204L that I will substitute for the older one. Can't do that yet though as I'm waiting for delivery of F type to coax adapters.

Why use adapters? Why not replace the Belling Lee plugs with F-Type?

[stevehertz]

I've got the new Labgear distribution unit installed now and indeed it is much better than the old one. So it appears that the old one had gone noisy!

[stevehertz]

Originally Posted by rambo1152

Why use adapters? Why not replace the Belling Lee plugs with F-Type?

Because F type connectors are made to fit modern, thicker, low loss 'satellite' cable, not older, thinner diameter UHF/VHF coax that most people already have installed for FM use. I mean, if I was installing my domestic aerial cabling system now, I would use the latest low loss coax, but mine is all plastered into the walls now. It's not really worthwhile even considering ripping it all out to gain a few (unneeded) dB at VHF at the cost of massive upheaval and redecorating throughout the house.

[Scimitar]

Originally Posted by stevehertz

Because F type connectors are made to fit modern, thicker, low loss 'satellite' cable, not older, thinner diameter UHF/VHF coax that most people already have installed for FM use.

F types are available in WF100, RG59 and Mickey Mouse sizes (WF65). In the event that you only have WF100 sizes, pack the outer with a turn or two of insulation tape before pulling the braid back.

[Radio Wrangler]

The F connector uses the inner wire of the coax directly as its centre pin. Stranded centre wires are right out, and your single strand centre wire has only a small range of usable diameters. With everything using mostly air as dielectric (foam, cellular webs etc) this fixed the diameter of the outer.

Copper is a lousy contact material, so the F female grips the male inner with contacts like little knife blades (Ooooh nasty!).

Some F connectors for out of range cables have a separate centre pin, as do adaptors of course. This allows harder material and non-tarnishing plating, but the female contacts wreck that.

I know what the letters stand for in BNC, TNC, N, C connectors, but when someone asked me what F stood for in the context off the connector, I replied "Very bad" in a fit of uncharacteristic diplomacy.

David

[mhennessy]

You can always rely on Horowitz and Hill at times like these:

The so-called phono jack used in audio equipment is a nice lesson in bad design, because the inner conductor mates before the shield when you plug it in; furthermore, the design of the connector is such that both the shield and center conductor tend to make poor contact. You've undoubtedly heard the results! Not to be outdone, the television industry has responded with its own bad standard, the type F coax "connector", which uses the unsupported inner wire of the coas as the pin of the mail plug, and a shoddy arrangement to mate the shield.*


* Advocates of each would probably reply "This is our most modestly priced receptacle".

All said though, the F-type has proven to be surprisingly reliable provided it is done correctly initially and left alone - in my experience they are "fit and forget", and I much prefer them to Belling Lee for installations like RF amps, etc, but obviously they're not exactly convenient on the back of a receiver that has a Belling Lee socket. In that situation, I prefer to terminate the cable with an F-type plug going into an F-type to Belling Lee adaptor - much nicer than a Belling Lee plug. All the RF outlets around the house are F-type (using the old "F-type joiner mounted in blanking plates" trick).

It is important to have the correct size F-type plug for the cable in use. And decent quality cable with a good amount of braid is required - they certainly don't work well with some of the cheap cables available today.