AM outdoor aerial - feed into different rooms?

[Tyso_Bl]

In one of the more recent issues of SPRAT magazine there is a design for a "throw out" active aerial, along the lines suggested by G6Tanuki. I'll see if I can find it if no one else beats me to it.

[stevehertz]

Thank you kind sir.

[Tyso_Bl]

Ok here it is, its from SPRAT magazine No 135, by Des Kostryca M0AYF, from an original design by G8CQX. I've built this one using 2n3866 transistors with heatsinks, and using car brake pipe for the loop with the electronics in a plastics "tee" box. It works a lot better than the long wire active antenna that I used previously. N1 is a gas discharge spike suppressor from a modem in the original, and a neon in mine. The relay is to protect the transistors from EMP transients when not in use.

The output can be split with simple resistive dividers.

[Synchrodyne]

In case the original idea of using a longwire aerial and distributing from that is still in play, I have sketched out my earlier all-passive suggestion from post#13, attached.

Particularly if there is a lot of signal from the aerial, it might be possible to dispense with the splitters and simply parallel the coaxials. Or alternatively it might be possible to dispense with the step-up transformers (reversed LWBs) at the receiver connections, depending upon how the receivers (assumed to be of the typical domestic type with high impedance aerial inputs) behave when looking back into a low impedance coaxial aerial feed (some do not like it). The sketch is based upon doing it all “properly”, in much the same way as using a balun to connect an FM receiver with 300R balanced input to a 75R coaxial feeder.

But the LWBs and splitters are not exactly low cost. It might be worth checking to see if any of the major aerial suppliers still offer the classic antistatic AM aerial systems; I suspect that if so, they would be lower cost. The last reference I have to the Aerialite Mastatic system is circa 1980, so such may be relics of the past. But on the other hand, they may still be in production for use in apartment buildings, etc.

Cheers,

[G6Tanuki]

I'd try it first with just the one LWB between the antenna-wire and earth, feeding into the coax and using splitters to then 'fan out' the signal to the various locations. Use a separate coax from each port of the splitter to an outlet; don't 'daisy-chain' splitters! A good source of 4-way splitters is:

http://www.screwfix.com/p/labgear-4-...splitter/89218

though these are only guaranteed to work down to 5MHz. In practice they work OK on medium-wave.

One thing to beware of though is that a length of coax that runs to an outlet that has nothing connected to it can - depending on length - act as an open-circuit stub and appear as a dead-short to RF at certain frequencies! The technically-correct way to deal with this is to put a non-inductive "terminator" resistor on all unused outlets. In your case I'd try things without these resistors - it'll probably work just fine.

[turretslug]

Originally Posted by Ian - G4JQT

Anyway, drifting off topic for the original post.

Not really drifting- as far as I can see, a useful and relevant contribution from a professional user, pertaining to exactly the scenario that was being discussed! I.e. the broadband feeding of a number of receivers from one aerial. I assume it wasn't really in 1079, though

Of course, taking the 2N3866/2N5109 type route is rational, reliable, inexpensive and straightforward (and safe!)- but the EF91 approach has an appeal, after all, they do grow on trees.

[Synchrodyne]

Originally Posted by G6Tanuki

I'd try it first with just the one LWB between the antenna-wire and earth, feeding into the coax and using splitters to then 'fan out' the signal to the various locations. Use a separate coax from each port of the splitter to an outlet; don't 'daisy-chain' splitters!

Not daisy-chaining splitetrs is no doubt good advice in a general sense, but I am not sure that coupling a pair of two-way splitters in tandem is particularly deleterious. A two-way splitter contains a single hybrid and the signals on the output ports are typically 3.5 dB down on the input. A four-way splitter contains three hybrids, and the signal at any output port has passed through two of these, and is typically 7 dB down on the input. So the signal path through two two-way splitters is essentially the same as through one four-way splitter. One could argue perhaps that it is better not to do the splitting until really necessary, that is at an adjacent outlet in the chain, so as to maintain the highest possible signal level through the distribution system. That would favour “distributed” splitting rather than “lumped” splitting. Three-way splitters usually have one port at -3.5 dB and two at -7 dB.

Your mention of TV-type hybrid splitters reminds me that way back when, Grove in the USA (and probably others) used to sell them for HF purposes. Somewhere amongst the pile of papers I probably have a reference to just how low in frequency they would function – well below the nameplate numbers - but finding it is another matter. Anyway, definitely worth trying, as for example at Radio Shack prices they are a whole cheaper than the RF Systems SP1. Somewhere I have a couple of the latter as well as a few of the TV type, plus some Blinder-Tongue MF-VHF diplexers. Logically I would have compared the SP1 with the TV splitters back then, but funnily enough I cannot recall ever doing so.

Cheers,

[Ian - G4JQT]

I think the thread is revealing just how awkward this project is. Joining sets up with coax (even if there is a distribution amp) is going to loose signal because the coax is not matched at both ends. In effect you are putting a capacitor between aerial and earth when using coax into the wrong impedance of a unmatched radio aerial and earth input.

This is even worse if you have to use passive splitters!

There are wideband 'un-uns' that attempt to make the widely varying impedance of a random wire across different frequencies match 50 ohm coax, but it just reduces the variations a bit - and introduces losses, even when the coax is terminated with 50 ohms into a single receiver aerial input.

The project may be difficult, but may work better than expected, but probably only if there is a good distribution amp which is happy to work into who-knows-what impedance of multiple radios all connected to it.

Ian

[Synchrodyne]

The paradox is that what is difficult today was relatively easy back in the 1950s when it could be done with off-the-shelf items intended for domestic applications, as the attached items show.

I suppose that the chance of finding a vintage 1950s aerial system in good condition to feed a clutch of 1950s valve receivers is fairly remote...

Cheers,

[stevehertz]

So true! and we reckon we've moved on technology-wise?! You only have to compare the great 'new' DAB with 50s VHF to see that progress is not always 'progress'.

[Synchrodyne]

Further searching seems only to find that the something similar to the original question has been asked by others over the last decade or so, without any simple response pointing to a readily available solution. Lots to wade through, including much obiter dictum but here are a couple of references:

https://groups.google.com/forum/#!ms...o/ASnumRq1LfgJ

http://www.digitaltvbanter.co.uk/uk-...m-aerials.html


Regarding the classic antistatic type aerial, from further review it does look as if this disappeared from the scene quite some time back. It was mentioned in some text books from the 1970s, although without the detailed coverage found in 1950s and 1960s texts. Of the major UK aerial makers, Aerialite seems to have championed the cause later than the others. In its Hi Fi Year Book (HFYB) listings for FM aerials, 1971 through 1979, it included the note “Aerialite also manufacture a range of vertical rod-type aerials and anti-static systems for AM reception.” This note was missing from the HFYB 1980 entry, so perhaps Aerialite had given up by then. However, in both HFYB 1979 and HFYB 1980, the Vorta Axiomatic AM Skyrod antistatic system was listed, so it was as if, as it were, Vorta had picked up where Aerialite left off. HFYB stopped in 1980, so I have no easy way of knowing for how long after 1980 Vorta offered that product.

As I recall the first so-called (misnamed, really) longwire balun (LWB) was offered by RF Systems some time during the 1980s. Its primary purpose seemed to be to match longwire HF aerials to coaxial cable, thus allowing the use of screened feeder and thence direct connection to HF receivers that had 50-75R coaxial aerial inputs. Compared with antistatic systems, it used coaxial rather than screened twin feeder, and lacked the “bottom-end” step-up transformer to match typical domestic receiver input impedances. Also, as far as I know, the RF Systems LWB was non-isolating, which meant that the aerial return path to earth was via the coaxial screen, which can be disadvantageous. Classic antistatic systems made provisions for avoiding this. Wellbrook seems to have realized this potential problem; its LWB appears to be of the isolating type and it also offers a separate aerial isolating transformer.

Today, and as mentioned previously, one option appears to be to endeavour to reconstruct something like a classic antistatic aerial system using LWBs as step-down and step-up transformers. Referring to the attached excerpt from Molloy & Hawker, the fig. 4(d) arrangement is a logical target (for the time being not considering splitting the feed), albeit with coaxial rather than screened twin feeder. The aerial-end step-down transformer function is undertaken by an isolating LWB. The receiver-end step-up transformer, if needed, is undertaken by another LWB, which need not be of the isolating type. As an initial experiment, one could proceed incrementally, starting with one isolating LWB, the coaxial feeder, one other LWB, and trying each receiver one at a time both with and without the step-up LWB. This should determine case-by-case the necessity or otherwise for a step-up LWB for each receiver. Then insert a TV-type hybrid splitter – with unused port terminated – somewhere in the coax, to see whether the signal loss is unacceptably deleterious. If so, then some form of amplification before splitting would appear to be needed. But with a relatively long, longwire aerial, it seems that the chances of having an abundance of signal, at least at MF, are quite good.

As something of a sidebar, but not irrelevant here, at time during the 1970s both Fuba (HFYB 1978) and Hirschmann (HFYB 1971, 1972) offered combination FM-AM aerials, in which the AM side was taken care of by a vertical rod above that was transformer-coupled into the 75R coaxial feeder, with a splitter/transformer at the receiving end. I suspect that the UK listings for these products were intermittent, and that these were standard European products. (I am fairly sure that I have seen such aerials on houses in Belgium.) Anyway, their existence does validate the coupling of AM aerials into 75R coax using transformers at each end.

Cheers,

[Synchrodyne]

I have also since found example schematics for (TV-type) hybrid splitters, attached. These confirm that the 4-way type essentially puts two of the two-way type in tandem.

Cheers,

[Radio Wrangler]

Lossy splitters are quite tolerable if you have enough signal in the first place. So having an amplifier is a good start. Back in post 23 there's a loop and amplifier from Sprat.

I'm not so sure about EF91s growing on trees anymore. I once bought half a teachest-full of 'em from Jim Fish, but they all went in building oscilloscopes in the sixties. The 2N3866 transistors are what's now available in a flood. It's no coincidence that they were used in that article because several thousand of them were given to the G-QRP club, otherwise they were going to landfill. They got spread around the place, and parcels went off to other countries.

The 2N3866 and 2N5109 are excellent transistors for antenna amplifiers and distribution amplifiers. They were developed for just that use. They're also long enough in the tooth to be considered vintage items in their own right.

This is a very suitable application for transistors because you can put the amplifier right at the antenna and phantom power it up the antenna feeder.

You can do multi-way splitters in one go, but a tree of 2-way splitters is equivalent. Resistive splitters are also possible with plenty of signal.

It might be worth looking up the dummy antennae once used with signal generators.. they should be a ready-designed interface between the world of 50 Ohm coax and broadcast receivers.

I still have a few 2N3866 kicking around the shack if anyone needs a few to have a play with.

David

[stevehertz]

No disrespect to anyone - and I should be ashamed of myself as a qualified electronics engineer - but this thread has gone down a technical route that I can no longer make top nor tail of, what with so many differing views and well meant recommendations. I'm also not in a position to build something from scratch with no more than the mention of the valves or transistors that could be used in such a device.

I mean, out of all this, could someone please make a recommendation for a practical solution? TBH, given the difficulties and all problems mentioned, I am thinking about NOT distributing my AM feed to different sets, just feeding it to one main set, and generally using VHF sets in other rooms (where I have distributed VHF), or a DAC90A that has an internal (albeit not without its problems in these digitally noisy days) for occasional use.

Please don't anyone feel un-appreciated! I'm just left feeling confused midst all the claims, counter-claims and DIY suggestions that I am not understanding.

Thanks to all so far.

[Synchrodyne]

Possibly the simplest way would be for you to proceed with your originally proposed longwire aerial, and from the bottom of the downlead use an open-wire feed into whichever room you are going to place your single AM receiver. Also take an open-wire feed from the receiver to a suitable earthing point. In respect of the latter, I’d avoid conflating the functional (RF) and the protective earths, which might be difficult if you have an AC-mains receiver with a three-core mains lead and whose earth socket is connected directly to chassis, and so to the mains protective earth. That said, I am conditioned particularly by my USA experience in this regard and by NZ requirements; though I get the impression from the various UK literature that a less rigorous approach has been adopted there.

If it were me, I’d assume - based upon past experience - that the in-house interference field, picked up by the in-house parts of the aerial and earth feeders, was going to be a problem for AM reception and opt for a more complex approach, although one using off-the-shelf components and not requiring any circuit construction, etc. That would be to keep the vertical drop from the longwire away from the house, and connect it to a coaxial feeder using the Wellbrook isolating longwire balun with the aerial side (primary) connected to a suitable close-by earthing point. At the receiver end of the coaxial cable I’d use another longwire balun. The rationale here is that most domestic receivers from the valve era have high impedance aerial inputs. Also, the 9:1 stepup then offsets the 9:1 stepdown at the aerial end. Receivers (typically the HF communications type) with coaxial aerial inputs won’t need a second balun. Some domestic receivers and tuners will work with a low impedance coaxial feed, but others seem to suffer from detuning.

Beyond that – and at noticeably higher cost - the Wellbrook loop would probably be the next logical step.

Cheers,

[Synchrodyne]

Originally Posted by turretslug

How did they do this sort of thing in the case of , let's say, an appreciable number of ship's cabin receivers where the well-heeled would have expected good reception of the World Service, yet compromise would have had to be made as regards aerial size/number and that also had to play second-fiddle to the ship's main RX/TX requirements?

The attachments provide a couple of 1950s examples of shipboard RF distribution equipment; not much detail, though.

Cheers,

[turretslug]

A shame, really; those snippets serve to tantalise! I fear it is yet another field of knowledge that recedes into history. Nice to see the Electra, though. Visiting the aircraft carrier my brother served on a few years ago, he proudly showed me the latest investment in technology- an impressively fat loom of co-ax, feeding satellite TV all over the ship. Things ain't etc etc...

[Synchrodyne]

Well, the attached item on a Plessey HF multicoupler has a bit more information...

But this corner of technology, both valve and solid-state, seems to have escaped coverage in the various standard texts.

Cheers,

[Station X]

Two off topic posts deleted. If you want to discuss frame aerials please start a new thread.

[Synchrodyne]

It seems likely that the techniques used for industrial/marine/aviation active aerial splitters/multicouplers were also applied to domestic (e.g. apartment block) installations. For example, given that Belling & Lee was active in both the industrial and domestic markets, it probably used similar equipment across both.

Thus the attached information, although referring to aviation HF active splitters, is probably indicative of general practice, such as the use of cathode followers – in fact already mentioned by G8HQP Dave in post #10. A cathode follower for each output would appear to provide good isolation between receivers with negligible signal voltage loss. That is the incoming cable would be loaded by say a 75R resistor; each cathode follower, with naturally high impedance (and even higher if bootstrapped) would provide negligible extra loading on the input.

Cheers,