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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 28th Sep 2018, 3:02 pm   #1
G4YVM David
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Default Coaxial aerial feeder question

These days we wouldnt think of joining two pieces of coax with anything but the 'right' terminals, but when coax first came out and terminals were expensive and knowledge of the new fangled stuff scarce, surely the temptation to just solder the ends and tape it over must have been high.

Did it make much difference to the performance of the cable? Would it even be noticable today?

David
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Old 28th Sep 2018, 3:28 pm   #2
stevehertz
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Default Re: Coaxial aerial feeder question

I think the question is akin to, "how long is a piece of string?" Of course, theory will tell us that such a 'cobbled' joint is terribly wrong, but as discussed in another thread recently, if the signal level is good enough - and more to the point - if it works, it works. If i was, for whatever reason, making that joint I'd strip back a fair length of the sheathing, solder the centre conductors together in line, fold back the sheathing and carefully solder them together, then tape over to provide support and insulation. Not to the book by any means, but as said earlier, in many situations, good enough.
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Old 28th Sep 2018, 4:31 pm   #3
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Default Re: Coaxial aerial feeder question

If the joint length is less than a considerable fraction of the wavelength used (say 1/20th) it won't make a 'hapeth of difference. A lot of "proper" RF connectors (the PL/SO 259 series spring to mind) are awful both mechanically and electrically, still they are prevalent on HF equipment even today. I use N or BNC types dependant on cable size, clamp type Ns are waterproof too once connected.
 
Old 28th Sep 2018, 6:03 pm   #4
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Default Re: Coaxial aerial feeder question

Such poor connections cause ingress and egress of interference. Although you can probable "get away" with it on aerials, with cable TV distribution which often uses out of broadcast band signals interference could be caused to important services.
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Old 28th Sep 2018, 7:29 pm   #5
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Default Re: Coaxial aerial feeder question

It is perfectly possible to do a nice quality inline spliced joint. The key is in keeping the impedance change to a minimum.
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Old 28th Sep 2018, 7:55 pm   #6
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Default Re: Coaxial aerial feeder question

To answer this question various factors have to be considered - and I don't necessarily know all the answers to those:

1. When was coax introduced? It was definitely around - but rare - in the 1930s. Not sure whether it was invented earlier than that. As far as I know, the only people using it were the military radio people.

2. Given the early date of the first coax, what frequencies were then in use. If we go with my date of the 1930s, we know that people were struggling to get much above 30MHz. TV arrived 1937 (?) and was broadcast around 50MHz, which by modern standards is barely above DC, but back then was extremely exotic.

3. Given that botched connections introduce a VSWR that is significantly above 1:1, we then have to ask whether that matters in the context of the performance of the equipment at the time. There were many other shortcomings of equipment at the time - receivers had appalling noise figures. Local oscillators were so unstable, that IF bandwidths had to be artificially wide to accommodate signals that could not stay on channel. To put this into context, a 3:1 VSWR causes produces a return loss of 6dB. Sounds bad - but it means that roughly 1/4 of the incident power ends up being reflected back to source. At the same time 3/4 of the incident power reaches the load. I make the "mismatch loss" in this case about 1.25dB. Is this significant? I doubt it - particularly in a 1930s system with so many "non-ideal" things in place.
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Old 28th Sep 2018, 8:04 pm   #7
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Default Re: Coaxial aerial feeder question

In times-past my 'kludge' coax-feeder joint was to wiggle the individual strands of the two centre-cores into each-other for about an inch, solder, then fit over the joint a length of the dielectric-insulation, slitted along its length so it could be placed over the inner-conductor joint.

"Evo-Stik" over the edges of the dielectric and down the slit-along-the-side.

Let this dry for 24 hours, then fold the braids back over each-other and solder them together. Hint: use a *really* hot soldering-iron so you can make the joints before the underlying dielectric/insulation starts to soften and ooze-up between the braids.

Then - over the top, the length of adhesive-lined heatshrink sleeving you remembered to feed over one of the cables before you started. Blast it well with a heat-gun and trust to the good Mr. Raychem to deliver the waterproofing chemistry.

If you were truly paranoiac, a layer of "Sylglas" or "Denso" tape [evil coarse-canvas fabric loaded with evin-eviller grey-green slimy Tallow-based grease] smeared over the joint would provide extra waterproofing.

TDRing a joint like this at 200MHz showed a lot less impedance-discontinuity than the usual 'amateur' pair-of-PL259s-and-a-barrel-connector.
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Old 28th Sep 2018, 8:18 pm   #8
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Default Re: Coaxial aerial feeder question

An interesting subsidiary question would be to ask: "What was the first commercially available piece of equipment (not lab model) that used coax cable?"

It may have been a TV receiver (or transmitter) - and no doubt those into that kind of thing will tell us. I can venture an early military VHF transceiver - the Wireless Set No.17 - which was a militarised version of an amateur radio setup on about 50MHz, apparently dating back to around 1935. I have no idea whether amateurs back then had access to coax - though I doubt it. Balanced feeder seems to have been much more readily available - though its now hard to prove it.
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Old 28th Sep 2018, 8:32 pm   #9
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Default Re: Coaxial aerial feeder question

I'd bet on some of the first-generation AI/Gunlaying gear from the late-1930s.

From memory, coax was really not a big thing in the first-generation pre-WWII TV sets, which seemed to use flat- or twisted-twin cable for preference.
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Old 28th Sep 2018, 8:45 pm   #10
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Default Re: Coaxial aerial feeder question

Quote:
Originally Posted by G4YVM David View Post
These days we wouldnt think of joining two pieces of coax with anything but the 'right' terminals, but when coax first came out and terminals were expensive and knowledge of the new fangled stuff scarce, surely the temptation to just solder the ends and tape it over must have been high.

Did it make much difference to the performance of the cable? Would it even be noticable today?

David
I saw many taped joints, forget about the solder, just wrapped together, taped and an attempt to place it out of the weather.
This was the earlly 1960’s so there was enough knowledge to do it right. I have also seen were inner and braid were shorted together in the joint, deliberately, that didn’t work too well though.

Mind they were taping mains lead extensions as well, much more dangerous.
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Old 28th Sep 2018, 9:15 pm   #11
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Default Re: Coaxial aerial feeder question

Quote:
Originally Posted by G6Tanuki View Post
I'd bet on some of the first-generation AI/Gunlaying gear from the late-1930s.

From memory, coax was really not a big thing in the first-generation pre-WWII TV sets, which seemed to use flat- or twisted-twin cable for preference.
Yes, its possible. However AI Mk.IV - the first fully operational air intercept - was only being tested in July 1940. I'm not sure about the gunlaying radars - there is almost zero technical info on the early stuff, operating around 50MHz, so I have no idea quite when it first arrived or whether it actually used coax cable.

The WS17 I mentioned I think started production sometime in 1938 (can't remember which month off hand), so its going to be a close run thing.

To answer this question some detailed knowledge of the equipment is needed, since a lot of RF gear was using balanced feeder - sometimes screened balanced feeders (e.g. BABS), but I don't regard that as "coax".

Chain Home was definitely operating pre-WWII, but I have never seen enough detail about the system to know whether it actually used coax feeders or not.
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Old 28th Sep 2018, 9:34 pm   #12
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Default Re: Coaxial aerial feeder question

Though it's not an "aerial" feeder application, somewhere in my jumble of techie documentation I've got details of some of the first multiple-carrier stuff done by AT&T/Bell in the 'states, where they put 192 simultaneous full-duplex phone-calls down a single coax, covering many tens of miles without intervening amplifiers/repeaters. From memory the launch-amplifier used a bunch of 6AC7 (or was it 6CA7) octal-based valves as cathode-followers as the best way to drive the low-impedance line.

This would have been some time around 1937: they also said they could send several simultaneous TV signals over coax for the same sort of distance.

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Old 28th Sep 2018, 10:22 pm   #13
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Default Re: Coaxial aerial feeder question

I think Steve got it right in post 2*. I used to be pedantic about the "right" approach but often a "bodge" can seem superior. I'm talking aerial cable, amateur installation and often interior work though. It's different when providing a professional and exterior feed service [or should be].

My brother once bought a house with a shower feed that started out at a 30 amp cable from the board but in the roof space deteriorated to 15 amp and twisted connections [insulated with elastoplast] I kid you not and I don't think it was earthed either!

Dave
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Old 29th Sep 2018, 6:52 am   #14
M0AFJ, Tim
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Default Re: Coaxial aerial feeder question

We have examples of very early coax at the Marconi Wireless Station Museum at Bass Point on the Lizard dating from, I think the early 20’S. I’ll take some pics when I’m down there on Monday
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Old 29th Sep 2018, 8:44 am   #15
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Default Re: Coaxial aerial feeder question

Thanks all, some very comprehensive and interesting answers.

I was going to say, but Tim negated much of it, that IIRC the Marconi station had coax from 'his' time. I recall the underground feeder, in a trench really, to the big aerial was a sort of coax but probably stretching a point to call it such.

I like the point about soldered joints beating PL259s. I have always enjoyed a good relationship with 259 /239 connectors but I am.always amazed at how many times they actually self-unscrew. I can only put this down to hysteresis combined with vibrations. I ought to stop using them.

I'll be interested in the photos from Poldhu Tim

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Old 29th Sep 2018, 11:23 am   #16
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Default Re: Coaxial aerial feeder question

My sister once moved into tied accommodation in London in the early 1980s.
The land lord just said get a bit of aerial wire and connect it up.
The went up and found the entire roof covered with a spiders web of TV coax all twisted and taped into bodged junctions.
She only went ahead and connected the thing up while Corrie was on
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Old 29th Sep 2018, 12:03 pm   #17
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Default Re: Coaxial aerial feeder question

Things that screw together will always, given the chance, ie where there's very little friction, always default to the unscrewed, looser state. I have a theory that everything vibrates, and in doing so takes the path of least resistance; undone.
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Old 29th Sep 2018, 2:04 pm   #18
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Default Re: Coaxial aerial feeder question

Steve, I am sure you are right, that everything vibrates. But 259s do seem particularly prone.
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Old 29th Sep 2018, 3:05 pm   #19
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Default Re: Coaxial aerial feeder question

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Originally Posted by G4YVM David View Post
Steve, I am sure you are right, that everything vibrates. But 259s do seem particularly prone.
Tell me about it!
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Old 29th Sep 2018, 3:43 pm   #20
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Default Re: Coaxial aerial feeder question

Somewhere I have a matching cable which was made to convert 75R to 50R at 145MHz. It is a 'twelfth-wave' match, so splices two different pieces of coax together and then puts the 'wrong' plug on each end. The 50R cable has a Belling-Lee, the 75R cable has a 50R BNC (or was it PL259' - I can't remember). I did this to match a 75R Jaybeam Yagi to a 50R transceiver - I can't remember why I bought the 75R version of the antenna.

I joined the inners, covered them with a piece of dielectric cut from one of the cables, then interleaved the braids and soldered them. Finally covered the join with PVC insulating tape. Worked fine.
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