Alternative means of propagation

Everything technical about radio can be discussed here, whether it's transmitting or receiving. Guides, charts, diagrams, etc. are all welcome.
Post Reply
Blob
ne guy
ne guy
Posts: 1
Joined: Mon Apr 01, 2019 7:08 pm

Alternative means of propagation

Post by Blob » Tue Apr 02, 2019 10:51 am

Does anyone here have any experience with carrier current or leaky feeders?

Since I live about half a mile away from an automatic frequency monitoring station there is a good chance that vans with antennas will start appearing in front of my house as soon as i start broadcasting so i am thinking about innovative ways to broadcast.

My first idea is just plugging a coax cable from transmitter into a cable tv outlet. Cable tv in my area recently stopped broadcasting fm radio on selected frequencies probably with a plan to upgrade upload internet speeds in the future, so with some luck and shitty installations there are probably a lot of leaky cables that can function as antennas nearby. I live in a row of about 5 houses that have cable just pulled through our lofts and then from the first house presumably underground to the cable plant so if anyone comes knocking i am just as likely suspect as all of my neighbours. I am not sure if they are even subscibed to cable, since i am not, but i still have outlets from the previous tenant.

In the best case scenario that i can imagine, cable plant has some filters but they probably allow upstream traffic for the cable modems on my selected frequencies and even boost it and since my signal will presumably be stronger than it is expected from cable modems the filters that limit downstream on selected frequencies will let some through.

This link is pretty much the only thing that even remotely resembles my idea that i can find and in my country i guess it will probably be considered hacking so the worst case scenario is probably prison time for me.

But this does not deter me enough, so I have a couple of questions, if anyone can answer:
1. Is it possible to destroy my neighbours (connected on the same coax) tvs/cable modems/any other electronics that way?
2. Will my neighbours hear my broadcast whenever they turn their tvs on, no matter which frequency they use?
3. Will i limit upload speeds or even prevent upload for everyone connected on the same plant?
4. Will i fry the cable plant?
5. Considering there is at least 200 ft of cable from my transmitter to the cable plant, do i have to put my transmitter in some kind of faraday cage so the signal that leaks from my transmitter and transmitter to cable connection wont be stronger than the signal that leaks from the cable plant?
6. Is my idea insane?
7. Is there anything else i should consider?

My other ideas consist of using some kind of carrier current, but is that even possible with fm and outside your own house? Something like PLC? I should probably avoid using the phone line in any way right?

User avatar
thewisepranker
proppa neck!
proppa neck!
Posts: 568
Joined: Fri Aug 22, 2014 12:53 pm
Location: Brighton

Re: Alternative means of propagation

Post by thewisepranker » Tue Apr 02, 2019 12:55 pm

From a practical point of view, your main problem is that you will need a considerable amount of power to radiate even a very weak signal, because the cable network really isn't that leaky, especially down at Band II. As a result of the high power you're going to be putting in, you will blow up most of the equipment attached to the cable network. It's designed to handle nothing more than a few tens of mW at most.

Cable companies are investing huge sums of money in making their network as leak-free as possible. The reason for this is that leaks cause a massive range of problems that are not easy to tell apart from other issues that are not related to leaks. What happens is that people complain that their internet doesn't work, and invariably go on to Facebook etc. and rant that "xyz service provider is a load of shit" etc. when the real cause is a loose 6p Screwfix connector in their house, that they put there themselves, badly. So the cable company has to send an "engineer" round to change all the connectors at great expense to the cable company.
Cable companies are now aiming for -105 dB or better screening effectiveness up to 1 GHz on their entire network, and are providing idiot proof solutions in order to guarantee this, so you really are not going to radiate anything no matter how much power you try to put down the RG59 in your wall.

There are lots of other reasons why it's not such a good idea, but this one should be enough on its own.

User avatar
yellowbeard
tower block dreamin
tower block dreamin
Posts: 305
Joined: Wed Jul 08, 2015 5:40 am

Re: Alternative means of propagation

Post by yellowbeard » Tue Apr 02, 2019 3:26 pm

Mr. Pranker has the right of it there, plus you are more likely to generate complaints by dicking with someone else's gear. You'd be better off going on shortwave or medium wave, seems like they won't raid you if you are not causing interference.

Albert H
proppa neck!
proppa neck!
Posts: 2737
Joined: Tue Apr 05, 2016 1:23 am

Re: Alternative means of propagation

Post by Albert H » Tue Apr 02, 2019 4:57 pm

Way back, when there was an analogue cable network in Greenwich and Woolwich, their "Head End" was in Lewisham. We had great fun with a little ½ Watt battery-powered stereo rig and a bunch of pre-recorded programmes on cassettes. We just had to be stronger than Capital 95.8Mhz at their receiving aerial, and we could get our programmes distributed down the cable network on 95.8 MHz, replacing Crapital! Great fun, but a fairly limited audience. We just set the rig up in the alley behind their building, so our signal was huge at their receiver!

If you want to radiate to a large potential audience, you should consider medium wave. Most people still have AM-band radios, and if you get a fully modulated, reasonable power signal out, you can get a huge audience - especially if you're playing stuff they want to hear. As long as you don't cause interference, you probably won't suffer visits from the authorities for a long time, though if the local commercial stations start losing lots of listeners to your station, they'll howl "Foul!" to OFCOM, and they might then bother to try to track you down.

A Medium Wave transmitter isn't a complicated beast to build, and you can get significant amounts of power out of FETs that cost just £1 - 2. The easiest way to general a stable output frequency is with a crystal cut for your frequency (or a multiple of it), but it's better to use a simple PLL, which will allow you to choose whatever frequency you find is free in your area. Three cheap CMOS ICs and a reference crystal will generate your carrier frequency. You then amplify it with a FET amplifier, match it to the right output impedance, low pass filter it and you have your carrier. Modulating it is simply a case of varying the supply to the Final - most easily done with a "PentaWatt" IC amplifier chip. You can build an effective MW rig for £20 - 30.

If you want more power, you have to use bigger power supplies, bigger FETs and find a more efficient way of modulating (PWM is my preferred method).
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

User avatar
SundayGroover
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 49
Joined: Mon Feb 02, 2015 6:57 pm
Location: Sunny Suffolk

Re: Alternative means of propagation

Post by SundayGroover » Wed Apr 03, 2019 9:03 pm

Blob wrote: Tue Apr 02, 2019 10:51 am
My first idea is just plugging a coax cable from transmitter into a cable tv outlet.
That alone would likely result in a blown transmitter output stage or at the least a protection shutdown due to the high SWR!

Post Reply