Twin Lead Wire for DTV and HDTV

Orrymain

, Blogger: Orry's Orations
#1
Okay -- now you have me curious. My antenna is a roof top antenna that's been there forever. I don't know -- it's gotta be 40 years old and maybe older. I can't control it at all; it's just the antenna that's been on top of the house. Now the antenna has a flat lead in wire. The flats are somehow hooked into an A/B switch because at the moment, I can still watch either cable or antenna by switching inputs. Am I going to need an adapter I haven't even thought about? Hmmm -- maybe I need to see if I can tell how the flat lead is hooked up. It's been years that I've this set up.
 

1inxs

DTVUSA Member
#2
You'll want to replace the twin lead with coaxial cable for the digital signal.
Okay -- now you have me curious. My antenna is a roof top antenna that's been there forever. I don't know -- it's gotta be 40 years old and maybe older. I can't control it at all; it's just the antenna that's been on top of the house. Now the antenna has a flat lead in wire. The flats are somehow hooked into an A/B switch because at the moment, I can still watch either cable or antenna by switching inputs. Am I going to need an adapter I haven't even thought about? Hmmm -- maybe I need to see if I can tell how the flat lead is hooked up. It's been years that I've this set up.
 

Orrymain

, Blogger: Orry's Orations
#3
Ut oh. My flat lead is currently connected straight to the VCR. It has the two prongs that attach to the back, and the box that plugs in a thingee (that's the UHF I think). Then my VCR has a coax that goes from the VCR to the A/B switch.

I have the DTV Pal PLus which is going to attach to this VCR.

You said I need to switch to coax for the lead in. Not to sound stupid, but how do I do that? Is that something I'm going to have to hire done (in which case, I'm just plain out of luck since I can't afford it).
 

Piggie

Super Moderator
#4
Orrymain,

If you can't change the flatlead from the antenna to inside the house because of not being able to do it physically or money, you can buy an reverse balun. You may already have one.

Any balun will work either way but by reverse I mean it has screw terminals to attach flatlead, then outputs into a coax plug normally.

I am a little confused because in one post you said the flatlead goes to an AB Switch and the other you said it goes straight to the VCR.

You might want to start over and explain your set up. Starting from the flat lead from the antenna, and the cable that brings in the cable signal.

=============

Then as an aside: The reason I don't suggest twin lead for anything any more is it must be installed perfectly or you have problems. It must be kept several inches away from any metal (remember the old stand offs for twin lead for a mast?). It can't take a corner with a radius any tighter than a 1/4 wavelength at the highest frequency its carrying. Extra can't be piled behind the TV, it has to be draped smoothly and to the right length. You can't just run the stuff through the wall unless it's away from metal studs, metal walls, other wires in the wall that are parallel to it or get hung in the wall where you can't see and get twisted or wadded up.

Ok that said. There is nothing stopping anyone from using flatlead for digital. (again I tell people to avoid it as it's so tricky to install properly)

Good quality flatlead has less loss than coax. Twin and flat lead are used all the time commercially. Most AM radio stations, the tower is feed with some form of flat lead, not coax.

Remember that ATSC signals come to us OTA on the same analog medium that NTSC did called Radio Waves that don't care what is modulated onto them, as far as propagation, loose in transmission mediums.

Know that wire that connects the elements in a standard 2, 4 or 8 bay collinear? That is a form of twin or flat lead in open wires. The wires that connect the many active elements in a combo antenna like a 769xP series are open wire line.

Just about all broadcast arrays that have muliti elements are phased with open wire line.

Heck just about all TV antennas are 300 ohm open wire line till they go in the balun.

Open wire, flat or twin lead are all balanced transmission lines that work on the same principle.

Flat lead is also much more tolerant to SWR or also known as antenna or receiver mismatch.

I found a quote from Antenna Engineering Handbook, for 100':


Channel 4 9 24 44 64

Twin lead 0.9 1.6 3.3 3.8 4.2
RG-59 2.4 4.1 7.2 8.5 8.8
RG-6 1.7 3.0 5.5 6.1 6.8

(my note here. RG6 made today is better see my interpolations from a chart below.)
* RG6 now 1.3 2.5 4.9 5.5 6.0


Balun insertion loss:

*about the same today

Channel 4 9 24 44 64

0.8 0.6 1.1 1.2 1.7

-------------


This is given properly installed flat lead above.

So at channel 24 say a flat lead system to an old TV with flat lead input would have only 3.3 db of loss.

Same setup with a balun at each end and 100 ft of modern RG6 would be

1.1 + 1.1 + 4.9 = 7.1 db loss for RG 6 system.

That is 3.8 db more loss in the coax. In percentage terms that means coax and 2 baluns loose 58% more signal than twinlead at 100 ft.

It amounts to 3db more loss in RG6 over twinlead at only 50 ft. 3b is 50% signal loss.

This doesn't even take into account the fact that most antenna being broadband are not perfect resistive 300 ohms across the TV bands. Stated above twin lead is much more tolerant to small mis matches up to about a SWR of about 2 to 2.5 without loosing little to no signal on a receive antenna system. Coax, with that kind of mis match which is there on some channels is another db or so.

So considering there is about as much chance the channel you are watching is not perfectly matched at the antenna as it is, you can easily add another db of loss to the calculations above for coax (RG6). One db of loss is about 20 percent loose.

I remember trying coax on my dad's TV back in the early 70's and the twin lead gave a better picture. But back then all I had was RG59, which meant on his system, I was loosing about about 6 db on VHF which were our channels. I put the twin lead back.

--------

But the sheer convenience of coax far outweighs the benefits of twinlead because it can be snaked through about any thing or corner as long as the turn is not so tight the center wire migrates through the insulation.

But this also explains the popularity of mast mounted amps. They overcome the loss in a coax. [Remember most of the improvement in short coax runs, under 20 ft, is not because the signal is amplified, but the amp has a lower noise floor than the receiver, another topic.]

Lets take the above system now with RG6 and an amp.

We have about 1.1 db loss in the balun on the antenna and a short coax to the amp. Maybe lets say, 1.5 db total between the antenna and the amp.

We add just a 12 db mast amp. Now we have negative 10.5 loss (but not more signal, as you can never have more signal than the antenna receives itself).

100 ft of RG6 looses -4.9 more. So at the end of the coax you still have negative 5.6 db loss. Most modern TVs have a 75 ohm input stage so no more loose.

Remember double over, we have not created an extra 5.6 db gain. But the fact the system loss between the amp and the TV is still anything in the negative range, means we have the same signal at the rear of the TV we had at the input of the amp.

So the TV sees about 80% of the signal picked up by the antenna with the only real loss of signal in the balun and coax to the amp from the antenna.

So now we have 100 ft of cable, RG6 on Ch 24 and only have 1.5 db of loss. This beats the twin lead hands down but only because it was amplified at the antenna.

For that matter with that system above you could have used some old RG59 from the amp to the TV and still ended up with negative loss between the amp and the TV.

And this in fact is part of why amps became so popular, as good RG6 until 10 years ago was a lot more expensive than RG59. Now so much RG6 is used in everything, cable, satellite, OTA, sometimes you find at the store, either no RG59 or it's the same price nearly as RG6. But back in the day RG59 was cheaper and RG6 even at a time was hard to find (remember local sources only or mostly as there was no internet and few 800 number catalogs even).

Most of the older amps only had 12 to 20 db of gain, which even today is enough for most systems.

But important note in the channel 24 example above. If you added a 4 way splitter to 100 ft of RG6 with only a 12 db mast amp, you would end up then with a positive 1 to 2 db of loss after the splitter (4 way adds about 7 to 8 db).

Then they started using more gain as an all encompassing benefit in an amp which it really isn't, as you can have too bid an amp and over load, loosing signal, even worse on digital.

So you don't want to buy too big of an amp for a system due to overload, but in the example above on ch24, 100 ft of RG6 and a 4 way splitter, one needs a 20 db amp at the antenna.
 

Orrymain

, Blogger: Orry's Orations
#5
Now that's a lot of info! lol Thanks! Actually, I'm taking the easy way out -- I'm letting the Geek Squad come and hook it up for me. They'll switch it over to coax, and I won't have to worry about it. The confusion, though, might come from the fact that I have two TVs hooked up to cable. The main one has the twin lead and that adapter or plug in thing for UHF plugged into the VCR and then a coax from the VCR to the a/b switch. I haven't been able to figure out the other one yet. I can't see between all the wires just how it's hooked up.
 

divxhacker

DTVUSA Member
#6
Nice going, Piggie, you went all too technical, leaving poor Orry a whimpering mess.

Yeah, Geek Squad is the best way to go for replacing the flat lead to coaxial- Let them put an adapter at the antenna, pull and replace the flat twinlead for the coaxial cable up through the walls, and splitters put in, if you have the antenna shared.

Then, from the output of the DTV converter box, VHF/UHF splitters would be hooked up for the VCR, and other devices that have the flat antenna prong connectors.

If the signal needs an amplifier due to poor reception, then the signal needs to be amplified before being passed through the splitter before going down through the house.

That way if you have any issues, you have someone to complain to about it. But, yeah, you should pick up more channels with the change to coax alone.
 
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Piggie

Super Moderator
#7
Now that's a lot of info! lol Thanks! Actually, I'm taking the easy way out -- I'm letting the Geek Squad come and hook it up for me. They'll switch it over to coax, and I won't have to worry about it. The confusion, though, might come from the fact that I have two TVs hooked up to cable. The main one has the twin lead and that adapter or plug in thing for UHF plugged into the VCR and then a coax from the VCR to the a/b switch. I haven't been able to figure out the other one yet. I can't see between all the wires just how it's hooked up.
Good as long as they can hook it up to your satisfaction Orrymain!

I probably should take what I posted above, copy it into it's own thread in some kind of resource thread. Think I will as Jay what to do. Yeah, it was a LOT of typing and a 30 minutes or so of research so I got the number within reason.

When I was a kid all the hams used ladder line (big twin lead), most of them made it themselves as coax back then was expensive and not that good. Plus they were more purists than today's hams and would not hang a dipole and not have a balun to feed the coax. Baluns for those power levels are expensive now and then, so they figured they could make 50 ft of ladder line in their shop for the cost wire and some Plexiglas for pieces of fiberglass.

Even then if you feed a large HF (shortwave) rhombic antenna, the best way to feed it even today would be a combination of ladder line and coax. A HF Rhombic is typically 50 feet high or more, so you bring the signal down on ladderline into an antenna tuner remotely in the yard, then coax into the ham shack.
 

Orrymain

, Blogger: Orry's Orations
#8
I appreciate all input, even if it is over my head! It's definitely going to be easier to let them do it; changing antenna wires is way beyond me. The rest I could probably handle. They're supposed to do it all and make sure you know how to work it, from what they said, so I'm happy to learn from them.
 

Don_M

DTVUSA Member
#11
Is there any way to reduce that remaining 20%, such as direct input into an amp sans balun, and coax connection coming out?
"Loss" like this is nothing to worry about at all as long as your broadcast stations never pause, freeze, tile or "blank out." In DTV reception, the addition of an amplifier can paradoxically worsen reception because digital tuners are more easily overloaded than analog tuners are.

If you have the time, please review some of our other discussion forums and threads, particularly this one about bit error rates and reception. To sum up, brute signal strength -- more or less the name of the game in analog TV -- has taken a third-row seat behind signal quality in digital TV broadcasting and reception.

To translate all this into non-Geekspeak: Digital tuners tend to tolerate very weak signals much better than those that are too "loud" or "noisy."
 

Piggie

Super Moderator
#12
:eek:



Is there any way to reduce that remaining 20%, such as direct input into an amp sans balun, and coax connection coming out?
In addition to what Don says, only having 80 percent of the signal means you have most of it. Since signal is rated log rhythmically and not linear such as a percentage 20% loss is less than you might think.

This math may go over you but if you only have 80 out of 100 units of signal in power available to your amp , this plugged into db power formula, where

db = 10 log (power1/power2) or 10 log (80/100). or 10 log 0.8 or a loss of 0.97 db (less than 1 db) which will never make or break reception except in very rare cases. The typical up and down fade of a TV signal can be 3 to 10 db from just atmospheric conditions. 1 db is not going to make the difference unless you are right at the edge of fading. This would most likely appear on the screen as tiny parts of the picture freezing (and I mean tiny).
 

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