Using an Aurora as a pantry TV transmitter.

[Radio Wrangler]

Radio waves don't stop at boundaries, they just gently fade as they disperse over distance. How far they are detectable depends on what you're detecting them with. A good receiver with a high-gain antenna will find things that won't appear on broadcast sets.

Leaky feeder is just that. That cheap and nasty coax with little braid coverage is an example. It's what keeps your car radio running as you drive through better-class tunnels. Also, mine service tunnels are equipped for 2-way radio.

If the authorities do get involved, then their actions are steered by two considerations: Firstly, was the person involved aware that he was radiating a signal or interference? If he wasn't and it was a bit of faulty equipment, then he's asked to shut it down and either get it repaired or destroyed. If the individual was aware, and particularly if he was aware that he shouldn't be doing it, then they know they've got a sure-fire court case.

If the individual has ever held an amateur radio licence, then they can prove that he's sat an exam on the terms of the WT act, and there are no possible excuses.

These agencies like to get a few prosecutions in the press every year to show that they're doing their jobs and to get word to spread.

Because of the money involved in commercial pirate radio the fines and penalties have been ramped up over the past few decades to be enough to knock businesses for six, so any poor individual dragged into court for anything involving that act, however minor, is confronted with the possibility of unlimited fines and lots of years. Hopefully courts have a sense of proportion.

OK, with low power, it's unlikely to happen, but offsetting that, the consequences are rather nasty. I'm definitely on the wired-distribution side, because some mandarin could prove I'd passed an exam on W-T act conditions. Worse still, there are 140 'Spectrum management' vans chugging around Europe with pneumatic telescoping masts and genuinely functional antennae feeding into receivers with some of my design work in them. The British TV detector vans may be a joke to frighten the frightenable, but these are quite real. There's no way I can act innocent.

David

[terrykc]

The short answer to this is don't!

RF can do some funny things which can catch you unawares, even when you don't think it can go far. for example (quoted from another forum)

"I had a damaged coax on the output of my Aurora, and unbeknownst to me, it was "leaking" a fair lot of RF at band1 ch2.. Enough in fact, to give a receivable, weak and grainy but watchable signal at a distance of about 20 feet or so, but more surprising was the harmonic in the UHF which first alerted me to the problem.. The harmonic was actually causing serious "co-channel" type interference on the house telly (this was before DSO) some 100 feet or so away."

So, just because you can't resolve a good signal on a relatively insensitive broadband receiver more than a few feet away, what would happen if you used a very sensitive narrow band receiver? And did you check out all the harmonics?

Hands up if you knew that the third harmonic of Channel B1 vision is in the aircraft band ...

Oh, and it's not just OfCom you need to worry about!

From http://www.405-line.tv/phpBB3/viewtopic.php?f=27&t=967&p=11122&hilit=MOD#p11122 ...

"The development I referred to yesterday was that the MoD had informed Ofcom that for operational reasons they are not prepared to consider third party use of their allocations between 40.015 and 47 MHz. This outcome was elicited via a dry exchange of e-mails between Ofcom and the MoD, although Ofcom had emphasised to the MoD our charitable and educational aims."

So it's not just the knock on the door that you've got to be cautious of, tanks and crashing aircraft don't bother to knock!

What is the problem with cable distribution? A simple system capable of providing 32 outputs from an Aurora is shown below. CT100 type cable has a loss of only 4.5dB per 100m at Channel B1/45MHz so end to end runs of 25M will only introduce ~1dB loss and, as the losses shown are worst case, output levels could be even higher than indicated.

The Aurora output is +16dBmV (+76dBµV). The splitter and tap lose 19dB max between them; adding 1dB for cable makes it 20dB. Thus the output to each receiver is -4dBmV (630µV).

Using an equal way 3-way splitter (max. 24 outlets) improves the loss by 2dB, giving an output of -2dBmV (~800µV) and using a two way splitter for a 16 outlet system gives a further 2dB improvement to 0dBmV (1mV).

Finally, let's look at leaky feeder. In its simplest form, consider a coaxial cable with a solid outer screen. If half the screen is removed it becomes a leaky feeder but, and this is vitally important to remember, it is still a transmission line and must be correctly terminated!

When a leaky feeder system was installed experimentally over 40 years ago, the licensing of the system was contingent on there being no measurable leakage outside the building in which it was installed.

Preliminary test went well until the ends of the feeders (there were two) were approached. then enormous peaks and troughs occurred in the received (UHF) signal.

The cable had a solid copper inner and outer sheath. As it was designed to radiate anyway, why bother with connectors, they thought, and soldered the terminating resistor onto the end of the cable ...

After much head scratching, the terminations were removed and replaced by connectors onto which were mounted true coaxial terminations. The system performed perfectly after that ...

I am not advocating its use but if you must try it, place a good quality filter on the output of the Aurora with a cut off below the 2nd harmonic - although it is the odd harmonics of a square wave that are a problem, some Aurora users have reported 2nd harmonic interference to Band II (FM) reception. Only then should you connect your cable and leaky feeder which must be 75Ω impedance and correctly terminated.

Finally, using good quality test equipment, ensure that there is no external leakage at the fundamental or any harmonics. Obviously, if you do not have this equipment or access to it and know how to use it properly, there is no point in starting in the first place.

You have been warned!

[SwanseaSteve]

Sorry if this is a bit O.T. but I've a newbie round here! It's obvious what "Pantry Transmitter" means, but where did that phrase come from? Is it a phrase unique to this forum (plus maybe other vintage radio & TV restoration forums) or what?

A search on Google seems mostly to list articles from this forum.

[Brigham]

I've been waiting for someone to ask that!

[terrykc]

A pantry radio is a clandestine - and strictly illegal - radio transmitter, on Medium wave usually and typically used to broadcast recordings of old programmes, etc. for reception on restored vintage wireless sets.

I think the general assumption is that, so long as it doesn't radiate outside the building, the legal aspects can be ignored ...

As for the pantry? Possibly a suggested hiding place for such a device and may well stem from the need to hide receivers from the Nazis in occupied Europe during the war.

That is very much as guess but as I recall seeing pictures of one such one-valve receiver, complete with earphone and mains lead, built inside a Vim canister so that it could be stored openly on view on the kitchen shelf - it even had a small amount of Vim in the top, should anyone be suspicious as to its contents - I'm probably not far off the mark ...

[SwanseaSteve]

Originally Posted by Brigham

I've been waiting for someone to ask that!

Glad to be of service!

Originally Posted by terrykc

As for the pantry? Possibly a suggested hiding place for such a device and may well stem from the need to hide receivers from the Nazis in occupied Europe during the war.

That is very much as guess but as I recall seeing pictures of one such one-valve receiver....

Hmm, yes. Always a bit easier hiding a receiver than a transmitter! The giveaway tends to be the self-supporting 45m high mast in the garden with the two decks of Band I dipoles spaced half-a-wavelength apart vertically around the top, and the thick RG6 coax feeder from there disappearing into your pantry window....

Oh yeah, as if the unexplained appearance of a 405-line signal on Channel B2, obliterating the local ambulance service hadn't given the game away initially!

( Don't know about you, but here in Swansea we seem to have ambulances and fire-service mobile radio on Band I. I'm not 100% sure, but I'm suspecting they're on 48MHz plus or minus a bit. There's a harmonic audible on a car radio at 96.0MHz if you're in the hospital car park, but I don't have a comms receiver with band I coverage to be sure )

[dave walsh]

A chap from Denmark was understandably confused by the term "pantry". [Mervin Maxwell's thread Listening Pleasure 16/7/12]. I answered as best I could but then did suddenly realise that, coming from the North where we had pantries, I had never even wondered about the origin of the term before!
It's certainly not unique to the Forum and seemed a common place term in the pages of Chas Miller's Radiophile. I did mention a food storage/"Larder" area then but your photo sounds interesting Terry.
Dave W

[Jonster]

( Don't know about you, but here in Swansea we seem to have ambulances and fire-service mobile radio on Band I. I'm not 100% sure, but I'm suspecting they're on 48MHz plus or minus a bit. There's a harmonic audible on a car radio at 96.0MHz if you're in the hospital car park, but I don't have a comms receiver with band I coverage to be sure )

That wont be a harmonic from Band 1, it will be an image 21.4MHz below the frequency you are hearing it on. Virtually all FM receivers have an IF if 10.7MHz, with the oscillator running 10.7MHz below so a strong signal 21.4MHz below will break through. Low band PMR radio is in the range 66 - 88MHz so that would fit with this.

[Radio Wrangler]

Hospital radio services get assigned a frequency just a bit below 50MHz for use as an outside broadcast link, but the power is low.

The QRP gang (QRP-ARCI, G-QRP, OK-QRP etc) often publish articles on clandestine radios, hidden in cigarette packets, decks of cards, kid's suitcases. Johnny SM7UCZ in Sweden and G3ROO in the UK are exponents. There are also restorers of B2 and Whaddon WWII spy sets.

Another one of the gang worked part time for (ahem) during the late sixties and made occasional flights into Uganda to report on conditions there. He tells of the fun of his small plane being chased down the grass runway by Idi Amin's finest policemen, trying to shoot as they pedalled furiously after the accelerating plane.

The risks of operating a subliminal transmitter in your pantry are somewhat less.

David

[HamishBoxer]

Very grateful to all on this topic and glad it is allowed to go on.

[SwanseaSteve]

Originally Posted by Jonster

That wont be a harmonic from Band 1, it will be an image 21.4MHz below the frequency you are hearing it on. [...] Low band PMR radio is in the range 66 - 88MHz so that would fit with this.

Indeed - that would fit in with the observable facts: the aerials on the hospital roof look to be Band I, but you can't get close enough to guestimate their dimensions. Thanks, Jonster.

[Alex728]

In Ipswich the repeaters for amateur use are on the hospital roof (I think it is GB3PO and was once at Adastral Park but got shifted in the 80s). One of them is at 50 MHz so would look like a band I antenna (well I suppose it would be). For some reason only the 2M one seems to be regularly used...

http://www.sargroup.co.uk/south-anglia-repeaters/

I heard some RAYNET tests on them a couple of weeks ago, which were a lot more formal than the usual banter on there. I think there is an arrangement between the hospital, Eastamb and ST Johns to provide backup comms in case Airwave goes "Tango Uniform...".

In any case another good reason not to accidentaly introduce QRM to those frequencies...

[HamishBoxer]

The bottom line is use the Aurora as it was intended hard wired. Saves loads of grief.

[Sean Williams]

Agreed, and seeing that the matter has been discussed at length it is time to go QRT