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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Mon 08, 2011 8:37 pm 
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ertzc2020 wrote:
In answer of which bal to unbal is best either is OK. The reason for the caps is because there were TV sets that one side of the set was connected the line before they had polarized plugs. The 75/300 is an average of impedance's of matching. They can be used either way. As far as to what I know, when they were used it was only on VHF for TV's, the UHF was a wire loop or a bow tie.

Thanks interesting, that is what the caps are for. Those are isolation caps to avoid being shocked by a hot chassis to ground. Interestingly I'm almost sure I got that with a pair of rabbit ears that came with a small solid state (digital tune) NSTC (analog/"cable ready") TV set. That TV set is 18 yrs old and dead now, I think a bad relay. I still have not thrown it away, because it planned on fixing it. :roll:

We all agree what a balun is, but it's confusing. The old TV's they stopped making in the 1970's had two screw terminals for antenna, some times two sets, one for UHF and VHF. Then TV's went to one 75 ohm F connector. This still requires a need for baluns, even more necessary, since most TV antennas are still 300 ohms (approximately) as always. However the normal source became VCR's, cable boxes with 75 ohms out. So TV to source is just coxial no balun (not withstanding antennas). By the way this balun works both ways, and an old TV needs one of these to connect a modern video source to it, like a cable box with the RF Chan 3 or 4 out.

The BALUN is for the antenna not the TV..... Obviously if you have a very old TV with screw terminals you don't need a BALUN, you can connect direct. The antennas are all 300 ohm impedance. If your TV has 75 ohm antenna jack however, as they all do now, and want to use rabbit ears for example, you need a balun like this, which are still sold and used even today, even on the latest digital TV's. They tend not to supply rabbit ears anymore since they assume you will use cable or satellite TV or provide your own antenna. However if you want to run your new flat screen on rabbit ears (as I have), you can and need one of these baluns. Here is proof of that and how well rabbit ears work in the "DIGITAL TV" age. You can clearly see the twin lead, BALUN and then coaxial, which went to my house cable distribution box and to the Sony Bravia flat screen TV.

Image

Even though this is a VHF rabbit ears (slightly broken) I got fairly high UHF signals just fine:

Image



First all analog TV's typically had two sets of 300 ohm antenna inputs (two screws) for VHF and UHF. Old radios had 300 ohm screw terminals but now most have BNC or F connectors. My Kaito KA1103 uses a 1/8" mini plug and is about 200-300 ohm input.

Later analog TV's tuners (NTSC) were (analog) cable ready. The 300 ohm screw terminals by this time went away by the 1980's. You now had one 75 ohm input for both VHF and UHF. You STILL needed a Balun if you used indoor rabbit ears for analog TV or bowtie for TV (uhf freqs), if not connecting to cable/satellite TV. These baluns I have are from TV's I bought in 1993 and 1999... that came with rabbit ears.

As far as UHF, most prime stations in the analog days were VHF. Still a VHF antenna could do OK on lower UHF frequencies, if you adjusted the rabbit ears right, but VHF was the focus and UHF the joke. You can see that a VHF antenna does fine on UHF above. HOWEVER if you wanted a dedicated UHF antenna you had a problem now with ONE 75 ohm input. If you wanted both UHF and VHF you had to buy a UHF-VHF-SIGNAL-JOINER (UVSJ). This is not just a splitter but a trap and balun in one some times. Some UVSJ's are made for 75 ohm in and output. Some have 300 ohm in and 75 ohm out and all variations you can imagine. The VHF input is a band pass channels 13 and less. UHF side is band pass 14 and greater. Channel 13 and 14 are not next to each other, 200 Mhz and 475 Mhz respectively. The UVSJ keeps the antennas from interacting and causing phasing, cancelling and multi path issues (in theory). Normally if the VHF and UHF antenna are directional and pointed in the same direction (as most all VHF/UHF antenna combos are), it was never an issue. If you start pointing multiple antennas in different directions then you might get RF weirdness.

I suppose you could just connect both the rabbit ears and the clip on bowtie to the same balun? Not great but might improve UHF reception. I think that is what some TV makers who supplied a little rabbit ear/bow tie combo expected you to do. Regardless these baluns were still sold with TV's that came with rabbit ears. Most flat screens don't come with antennas anymore, since they expect you will have cable, satellite or your own antenna. Of course old TV's were big, had room for an antenna to pop onto or sit on. New TV's are think and no place for an antenna. Most DIGITAL READY indoor antennas for TV have the balun built in. You can still go on eBay and buy rabbit ears with twin lead and one of these baluns on the end for a "HDTV antenna"! Most now have the BALUN inside and a UHF/VHF signal joiner and a coaxial feed to a F connector.

Image

Outside antennas have baluns, typically an in-line deal that has a short length of twin lead and lugs, stream lined sealed body with an F connector and rubber weather boot. The balun is still there, just out of sight. It looks like part of the coaxial.

Later TV's added ATSC tuners, and QAM "digital" decoders, while still retaining the NSTC tuners. So modern TV's sold in the last 10 to 5 years have all the tuners and can get analog OTA (if they still transmitted it), Cable TV and OTA DIGITAL and QAM. QAM allows digital signals over basic cable. You still need a balun if you want to use rabbit ears or some kind of UHF antenna. 300 ohm TV antenna terminals have been gone a long time and BALUNS are needed more than ever for OTA TV. I plan on buying a high quality in-line outdoor version of a 75/300 ohm transformer.

PS I tried testing my balun on the signal Gen and scope. As I thought my scope just starts to puke at it's band width. Actually anything over about 33% of a scope bandwidth is not accurate amplitude wise... still good for seeing traces and relative amplitudes. It really does not do any good to test a balun on gear that can only do 100 Mhz or so, when the balun needs to operate from 200 Mhz to 750 Mhz.

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Last edited by gmcjetpilot on Aug Tue 09, 2011 2:23 pm, edited 6 times in total.

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Mon 08, 2011 8:47 pm 
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Chas wrote:
Quote:
Does Raleigh NC allow "plastic" gas flex?

WOW! Scary! If diaphragm fails on regulator gas vents into attic!
I got polite warning from meter installer, AC unit to close to regulator and it is outdoors!!!

Ha ha you all are chicken... LIVE ON THE EDGE.... Really homes DO BLOW UP FROM NATURAL GAS... but rarely. The ones I have heard of are foundations settling, cracking and breaking gas mains that is under or comes into the house, iron pipes. Even if the furnace was in the basement or hot water heater in the gauge (which I have) and there is a gas leak, no ventilation, spark, boom.... HOWEVER the attic is vented out the ying yang... the eaves are totally vented as is the roof peek/ridge line. There are also dedicated vent lines. However I will keep my eye on the other 1000 homes in the area made the same way, and if any blow up, I might worry a little.. Also the flex line as been used to connect water heaters and stoves for decades. Chance is 100% if you have a gas house your appliance is connected through flex lines.

Mike Toon wrote:
Chas wrote:
WOW! Scary! If diaphragm fails on regulator gas vents into attic!

Or chewed on by a rat.


Ha ha ladies don't get your panties in a bunch. :lol: First it's stainless steel (not brass or aluminum) and it's sealed. That yellow cover is not edible or anything more than protective shield for the internal layers. The gas line to the house, meter and to the manifold is iron pipe as always. The flex lines just feed the appliance. Some say they should never pass through or into walls.

I am sure you can drill through it easier than iron pipe. You could be right, but the product has been on the market for over a decade, that I know of (age of my house). We might know how good or bad it is in 50 or 100 years, but guessing I will not care in 50 years (aka I'll be gone), unless I make The Today's Shows 100 yr older birthdays. I doubt it's more likely to leak than iron pipe and 20 connections and joints.... and !@#$% happens. If rats can eat through one of those yellow flex pipes, through all the outer layers, the stainless steel inner layer... that is a monster rat worthy of horror film stature.

Any time you have flammable gas in your house it might go boom. I have a gas fire place. I could leave it on and not ignite it (which is manual with a quartz igniter button). That is why they put the ROTTEN egg smell in the gas so you can detect it.

MYTHBUSTERS TO THE RESCUE

There is a move Borne Identity, where the main character Jason Borne is being chased. To create a diversion he breaks the gas line to the water heater, which is in a closet next to the kitchen in a small house. He then stuffs newspaper into a toaster, hits toast and runs out the building, only to have the bad guys show up just as it explodes BIG TIME! Well Mythbusters tried and tried to make it work, but it never went BOOM.... it flashed over at best. They had to pump MASSIVE quantities into this room and add other ignition and oxygen I recall to get a boom. Lesson: A small gas leak is so diluted by air it will not ignite. The right amount might cause a flash. I love natural gas and feel it's safe. I feel sorry for people who live in real cold climates, that deal with fuel oils. I had an all electric house, which is fine in the very temperate climate of Seattle area and electricity was not too expensive.

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Last edited by gmcjetpilot on Aug Tue 09, 2011 4:47 pm, edited 1 time in total.

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Mon 08, 2011 10:07 pm 
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Well Sister,

What that bow-tie needs is a reflector. Made out of 1" hardware cloth, about 4-5" to the rear, phasing bars to the front, should get 6db. improvement in forward direction. If mast is metal connect to mast...

I installed these some 40 y.a., they were $8.95 wholesale, I got $12.00 and installation costs...

Weren't all that directive, did a few with a rotor, even stacked them... The Channel Master Band Saw was the best, very directive. UHF is peculiar, unforgiving, line-of-sight. Best reception was with the rule of thumb, see the horizon 10 to 20 miles away and your good.

Oh, balun with caps is for current isolation, balun no caps is for delivering power to amp at antenna.

Some combination Yagi's used a linear balun, no insertion loss, made from phasing bars as part of antenna. Lowest loss to receiver is with foam twin lead. Twin lead is fine as long as there is no metal in close proximity or run through electrically noisy area... Then install balun indoors at set, gain maybe 6-12db. over coax...

Often, broad-band pre-amp was at antenna, weatherproof, powered via co-ax, made-up for co-ax losses.

I used a lot of Winegard stuff, got best wholesale, wife was accountant at the distributor :wink:

Channel-Master was a another distributer, couldn't a get decent discount, so didn't install no where near as often.

Did many professional TV antenna installations back then. Before the cable arrived...


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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Mon 08, 2011 10:44 pm 
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The one with the caps looks like the lightning discharge system for rooftop aerials.
Were there two resistors across each?

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Tue 09, 2011 1:52 pm 
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radiotechnician wrote:
The one with the caps looks like the lightning discharge system for rooftop aerials.
Were there two resistors across each?


I don't see any resistors, and the resistance (DC not impedance) across the terminals is zero, as expected, with the capacitors in there. One capacitor goes from terminal to center conductor (coaxial). The other capacitor goes from terminal to winding as I tried to show. The twin pair wire twist around the toroid side-by-side, one wire gray and one red for discussion.

Terminal 1 ------ Cap ----|---- gray wire ---- toroid----- Shield
..............................|
.......................(center pin)

Terminal 2 ----- Cap ---------- red wire ----- toroid ----- Shield

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Tue 09, 2011 2:17 pm 
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Chas wrote:
Oh, balun with caps is for current isolation, balun no caps is for delivering power to amp at antenna....Often, broad-band pre-amp was at antenna, weatherproof, powered via co-ax, made-up for co-ax losses.
Current isolation? Interesting, so the caps block DC current (not RF current of course), if you run DC through your coaxial. Strange, these baluns (not sure where I acquired them) likely came with back of the set rabbit ears. I suspect they just threw in what they could buy, or some one copied something.

The amps you talk about I am familiar with. They get power through the coaxial.

In my impromptu test of the two versions, each had pros and cons; the no cap binocular wound did a little better at higher Freqs I think. However the one with caps returned better SNR! It was only one count (per Sony TV signal diagnostic display). However the signal strength was less (but not always at lower Freqs). This shows you the compromises in a Balun, especially one that theoretically needs to work from 100 Mhz to 800 Mhz. It would be nice to have a spectrum analyzer... However testing it the way I did, with the antenna, feed line and TV as on system makes more sense, especially sans fancy UHF test gear....

Quote:
Some combination Yagi's used a linear balun, no insertion loss, made from phasing bars as part of antenna. Lowest loss to receiver is with foam twin lead. Twin lead is fine as long as there is no metal in close proximity or run through electrically noisy area... Then install balun indoors at set, gain maybe 6-12db. over coax...
Twin lead is awesome stuff, potential for very low loss as you say. The down side as you say is installation. Coaxial is low loss, shielded better. Theoretically twin lead is better, but for practical real world the coaxial now is it.


Quote:
Did many professional TV antenna installations back then. Before the cable arrived...
Interesting... The art of TV antenna installation is making a come back. However getting on steep roofs is not for me. When I was a kid most houses I lived in had shallow pitched roofs. Getting on the roof was fun. My current house is so steep it's impossible to climb with out some kind of safety gear. You start sliding I don't see how you could stop.

I just saw a classic "Lucy Show" bit where she and her side kick are on the roof trying to put up an TV antenna. It was pretty funny. OTA TV stations now with 2, 3 or 4 total channels, tend to use their SD sub-channels for reruns of very old TV shows. I guess they get them cheap.

The Lucy Show - She puts Up a TV Antenna to save money. This is two Parts and worth the time to watch I think. It's Lucy's 100th Birthday I understand.

Part two has the on the roof bit. Part one at 6:22 is at a TV shop and shows bunch of test gear and tube testing, and it's funny. At 7:55 she takes the antenna out the box. You can see a W on the mast bracket (Winegard?).
Part 1
http://www.youtube.com/watch?v=0OULMPdO7n0

Part 2 - This is the roof bit
http://www.youtube.com/watch?v=xTCPWLKCHJU

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Tue 09, 2011 7:47 pm 
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Quote:
You can see a W on the mast bracket (Winegard?).


Ha ha, great fun! I don't know how far back Winegard made antennas, sure looks like their Logo... That wasn't on any of the models I sold...

See the bars that run parallel to the boom on Lucy's antenna? Those are for matching the vhf high to the low sections. There is also a 1/2 turn loop involved too.

Too bad your so far away, I've got a medium fringe Radio-Shack antenna sitting in the shed, new, vhf/uhf/FM. FWIR the boom is about 10' long.

I took down a fringe model last Fall, I'm not going to replace it. That one has some age but is still in reasonable condition. Antennas like these and Lucy's do best with a rotor...

Chas


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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sat 13, 2011 4:15 am 
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Found two more baluns in the junk drawer and they are different from the previous two.... these two are pretty much the same with no capacitors (which I have no real explanation) and use much smaller wire (ceramic coated) and smaller ferrite; both are binocular.

Image

For example the first two I posted.
Image

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sat 13, 2011 4:55 am 
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fifties wrote:
....Some years back (about 30+), we had an OTA UHF station on channel 52 called "ON-TV", which was a subscription service at night, using a sinewave video scramble tied to the horizontal output frequency, and an audio subcarrrier frequency for sound. ....


I remember "ON TV". :lol:

We came out with a descrambler of sorts for the Heathkit TVs where we took the horizontal oscillator output test point and fed it via a variable phase network back into the IF's AGC line to sort of re-introduce the missing or scrambled sync pulse.
We built an audio decoder using (if I recall correctly) a Signetics 561 PLL chip.

It worked "after a fashion".....

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sat 13, 2011 4:59 am 
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gmcjetpilot wrote:
LOS is only about 70 miles typically, but very dependant on terrain, but earths curvature with typical antenna tower heights 65 miles max.


Most of our local TV stations broadcast from a 6500' mountain (Mt. Wilson)... That's quite a lot of "line of sight" <G>

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sat 13, 2011 5:17 pm 
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Unusualdesigner wrote:
gmcjetpilot wrote:
LOS is only about 70 miles typically, but very dependant on terrain, but earths curvature with typical antenna tower heights 65 miles max.


Most of our local TV stations broadcast from a 6500' mountain (Mt. Wilson)... That's quite a lot of "line of sight" <G>


Of course! Right on... the formula is based on flat (curved earth surface) and typical TV station antenna height of 2000 feet.

6500' feet is awesome.... increase range by a factor over 3, so 150 miles would not be out of the question if they have enough power. Of course good old FCC has cut digital station power back to keep range down....

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sat 13, 2011 5:19 pm 
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Does ANY ONE know how to make a 4 to 1 coaxial (RF6) balun?

That would be almost no loss.

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sat 13, 2011 9:36 pm 
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gmcjetpilot wrote:
....Does ANY ONE know how to make a 4 to 1 coaxial (RF6) balun?
That would be almost no loss.


There ain't no free lunch.

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sat 13, 2011 11:30 pm 
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Unusualdesigner wrote:
gmcjetpilot wrote:
....Does ANY ONE know how to make a 4 to 1 coaxial (RF6) balun?
That would be almost no loss.


There ain't no free lunch.


Albeit true some times, it's a totally useless right now. :D Do you want to elaborate your technical aspects of a UHF coaxial balun? I'd appreciate it.

A coaxial balun has the theoretical possibility of being much lower loss. Yes? No?

Image

B = 1/2 Lambda

Half wave length of the matching section of the 4:1 coax balun - equations are taken from the ARRL Antenna Book

B = 5904 * V/ F in MHz; V velocity factor, assume 0.66, Assume 500 Mhz

B = 5901 * 0.66/500 = 7.79 inches

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sun 14, 2011 12:30 am 
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George (I think your name is George) I'm going to re-iterate the frequency vs impedance conundrum.

The ferrite core arrangement is intended to provide a fairly flat 4:1 ratio across the band. And they really do that with loss on the order of 0.2-0.5 db within their freq confines. Although the antenna may only be "300 ohms" over a small specific frequency range, the transformer itself is flat. This is the same mindset as using a 9:1 xfmr with a random-wire antenna on shortwave, ie better match at some frequencies is better than no match at all frequencies .

The coax configuration for impedance transformation is frequency dependent, not wideband. And those coax transformation configurations are quite sharp in their frequency response. So from an 'overall system' perspective I think you could easily call it a more lossy arrangement than the broadband ferrite transformer.

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sun 14, 2011 12:59 am 
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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sun 14, 2011 3:11 am 
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gmcjetpilot wrote:
..... assume 0.66, Assume 500 Mhz ....


And you know what happens when you ASS*U*ME right? :)

Jack has it right when he states that coax balums are frequency selective, in fact you can almost call them filtering transformers or transforming filters.... I have made quite a few over the years.

Again, in PASSIVE components, there is NO free lunch. Trust me on that one.... :wink:

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sun 14, 2011 7:04 am 
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Jack Shirley wrote:
George (I think your name is George) I'm going to re-iterate the frequency vs impedance conundrum.


Are you really going to RE-ITERATE! Again! :shock: Jack, that's your name, Jack right? :lol: Thank you I know. Yes yes a thousand times, you are right. I know that Freq vs Impedance vary, antennas have limited bandwidth and are not exactly 300 ohms impedance, which you are found of repeating. We got that. WHAT ARE YOU GOING TO DO ABOUT IT? :D Seriously. I ask a question, does not mean I'm ignorant. Yes, I know of the quandary and conundrums. So? A balun lets you take balanced current and connect it to unbalanced coaxial while blocking common mode waves. Yippee. Nuff said there right? The free lunch cliche' is a good one but irrelevant. Repeating known limitations and laws of the universe is not an answer to making of (or purchase of) a good TV balun, for today's TV band, which mostly UHF now, at least in my parts. Not all TV baluns, aka matching transformers 75/300 ohms, are all the same! The answer could be as simple as buy model #XYZ from www baluns R us.


300 ohms a fundamental law of the universe

Jack think for a second about all the TV matching transformers, are all GENERIC 300/75 ohm deals. The 300 ohm end has end twin leads or two terminals. At the 75 ohm end is a F connector for coaxial. There are no custom BALUNS for each antenna. Every TV antenna sold is a 300 ohms, at least assumed (yes I know what assume means). One size fits all BALUNS! WHY? We can repeat over and over how Freq vs impedance vary, but we must go on from there. We must get over and accept the fact an antennas impedance is an idealized assumption from the ~77 ohms min loss impedance of air, rounded to 75 ohms, aka approx impedance of a monopole! A folded dipole is about four times that, 300 ohms. The 50-Ohm "standard" is a compromise, an average mean between 30 ohms (best power handling) and 77 ohms (lowest loss) or 48-53.5 ohms. 50 ohms is a compromise between power and signal loss for air dielectric, all ball parks. Unless you want to get into some serious math (and I don't), we use these ball park SWAGS. It does not answer my question, how do you test and/or pick the BEST TV BALUN? This is why making your own may be the best deal. It is not hard. I was hoped some one had done it. I guess I will blaze a trail. It may look like this! This is for cell phone folks. -3dB at 700 Mhz and -1 dB at 200 Mhz.... If I can even fine a place to buy one I'll give it a try... :wink:

Image



Next to the antenna, the balun is tied for first in importance. Everything you said is true, accurate and correct, but is not getting to the core question. I don't think off the shelf TV baluns are very good. You are welcome to waltz down to Walmart or Radio Shack and buy one for $7.00, but I think some TV baluns are made like they always have been, poorly, tuned more for VHF (55-211 Mhz) than UHF 475-700 Mhz. Are the ones sold by online TV suppliers with names like Magnavox, Philips, RCA, Winegard, Channel Master, Skywalker for $0.39 to $8.00 each any better? No way to tell. I could buy each one and A-B test them with an antenna, feed line and TV, using the TV's digital tuners signal meter to compare signal on a given station. With out spectrum analyzer that can go up to a Ghz micro-watt power meter, you can't test them. Most baluns at the store are generic. I should buy the no name brand ones locally, at ACE hardware, Walmart and Radio Shack. That way I can bring them back with out mail order shipping.

No doubt regardless of brand name they are all made in China, from the cheapest materials of dubious quality control. That is my point. Like it or not Jack the balun is important and can be optimized, even if it's a compromise. You have to shoot for something or you'll never hit any target. To have a nice antenna choked off by a crud balun is a shame. The China manufactures original design and QC may be OK, but then someone blindly substitutes the ferrite material? You might not know and struggle getting a signal.

The possibility is make one, DIY, roll your own transformer, to get a best match for your antenna and your Freqs of interest. Use the best materials and care. Despite my success with the junk drawer balun I still want to make one and even buy one or two "name brand" outdoor baluns, see if they make a difference. I suspect they will make a small positive difference. Who knows it might be worse? From what I see the outdoor ones are of higher quality? May be not.

"Your antenna system is only as good as your weakest component, and first device your signal runs through is the balun. Generally the baluns that come with antennas are not low noise, they can even add quite a bit of noise to your signal." Winegard


Quote:
The ferrite core arrangement is intended to provide a fairly flat 4:1 ratio across the band. And they really do that with loss on the order of 0.2-0.5 db within their freq confines.

How do you know? Have you seen some data? I just don't believe it. I think some of these TV baluns are garbage. I don't know where you get the insertion loss of 0.2-0.5 dB. That is excellent. How and where can I buy one of these make and models of Balun? Walmart or Radio Shack? You wrote earlier that there could be 10 dB loss above 500 Mhz. THAT'S A BIG DEAL IF YOU HAVE FRINGE STATIONS ON PHYSICAL CHANNEL 19, ~500 Mhz. Channel 34 is ~600 Mhz and Channel 51 is ~700 Mhz. Of the 17 stations, all but 3 are below channel 19. So most stations are above 500 Mhz for me. THIS IS THE NUMBER ONE BIG PROBLEM I HAVE WITH TV BALUNS, NO ONE PUBLISHES THE SPECS FOR THESE BALUNS? You don't know what you are getting. Taking apart four Baluns and "testing" two of them they do vary.

Here is an article of some one who did test some commercial TV matching transformers and found 80% loss in signal!
http://www.kyes.com/antenna/balun.html

In the old days VHF ruled TV. Present day UHF 450-700* Mhz rules TV, in most markets. This is a quantum shift in the TV band. Forget analog vs digital, the big news is shifting stations from low VHF to high UHF. However you must realize with digital TV, a small signal difference means the difference from all or nothing (drop out, pixelation)! So a poor BALUN made for a VHF antenna, is not ideal anymore and might make that little difference. I may have been crap for VHF to start with? A balun optimized for the UHF Freqs is a reasonable goal, regardless if you buy one or make it. A BAD BALUN CAN MEAN THE DIFFERENCE BETWEEN A RELIABLE SIGNAL AND NONE AT ALL.


Quote:
Although the antenna may only be "300 ohms" over a small specific frequency range, the transformer itself is flat.

Yes again I know. This is why they have VHF and UHF TV antennas, one for 55-211 Mhz and ones for 450-700 Mhz. I got that Jack. Most people don't need VHF any more. I think Las Vegas has 4 primary stations on VHF. In my case only one station of 18 is on VHF high (11). Of course I proved that VHF antennas (rabbit ear/dipole) can receive UHF Freqs well, and UHF antennas can receive VHF freq well. It's just a matter of gain. Obviously when you get out +20 miles from the station, a coat hanger is not going to work. Get out to 40 miles you need a directional antenna that is tuned more or less for that Freq. No kidding. I am not reinventing the wheel, just trying to make an efficient system. With digital TV a little can make the difference from a nice picture to NOTHING!

On HDTV forums or some folks are more nuts than I am. They have done spectrum analysis of antennas, using the balun the antenna came with I assume. There are discussions on the HDTV forums about this.

http://www.hdtvprimer.com
Image

*FCC is taking away the stations at the upper end of the current TV UHF spectrum, channels 60 to 69, for the new 700 Mhz band. They are also auctioning off 52 to 59 for mobile phone carriers. I have one station on 61, which will be going bye bye and moving I suppose.



Quote:
This is the same mindset as using a 9:1 xfmr with a random-wire antenna on shortwave, ie better match at some frequencies is better than no match at all frequencies .


Yes yes, I know, I know and totally agree Jack, but I still remember when I suggested how to make a random wire antenna and make a 9:1 balun. I got my backside chewed out on ARF on how useless a balun was and how 450 ohms was not a good estimate for a random wire impedance? Go figure. Please don't repeat how freq vs impedance varies, you can't do anything about it. Nothing is perfect and impedance changes with Freq. I am aware of all the compromises. As with TV antennas. I made my antenna to be great low/mid UHF, while not giving away too much at channel 51 (693 Mhz), yet still getting channel 11 (VHF HI). I have a channel 61 which I get fine dispite being 57 miles away, but the FCC sold that Freq off; it's going away someday. All stations will be 51 or less (under 700 Mhz).

Here is my DIY $6 antenna and junk drawer Balun. Below represents 17 networks and +50 channels.
Image

Note: any signal 76%/SNR 16 or more tends to be reliable on the Sony. All these signals are solid and quite proud of that fact. Right now the antenna is feeding one TV and one PC card through one splitter. I am going to have to add a CM3410 distribution amp (15 dB) to make up for splitter losses, when I drive all TV outlets in the house. Splitters are good for -3.5 dB loss each, plus extra feet of coaxial to get to each rooms cable outlet.




Quote:
The coax configuration for impedance transformation is frequency dependent, not wide-band. And those coax transformation configurations are quite sharp in their frequency response. So from an 'overall system' perspective I think you could easily call it a more lossy arrangement than the broadband ferrite transformer.

I agree... I never said it was broadband, but how narrow is it? Do you have some data? It can't hurt to make one. I might try it tomorrow with some coaxial I have. Only way to know is make it and test it. Here is a white paper on the topic and you can make broadband coaxial baluns... but this is not simple. http://www.polyfet.com/hfe0503_leong.pdf


Below are a few links on 4:1 dual core transformers. The one has nice response with type 43 material across a broad band up to 500 Mhz but dies. Using different ferrite material (I assume higher number, type 61, 65 or 75) will produce better high frequency performance. From Fair-Rite design guide for Broadband transformers they talk about using nickle zinc ferrite cores especially for +500 Mhz range. I'll buy a magnesium or nickle ferrite dual core and try winding them. It can't hurt. Dual cores do increase bandwidth, and lower number of turns makes winding resistance no longer an issue.

http://www.fair-rite.com/newfair/pdf/Broadband.pdf
http://www.home.earthlink.net/~christra ... 1Balun.pdf

May be I can make a SUPER TV Balun that is better.

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


Last edited by gmcjetpilot on Aug Mon 15, 2011 2:01 am, edited 5 times in total.

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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sun 14, 2011 6:12 pm 
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Joined: Jul Wed 22, 2009 8:32 am
Posts: 3807
Jack, a side note on radio MW/SW random wires. Yes, we can assume random and long wires are from 300-600 ohms, 450 ohms middle of the road, dispite it varying with freq. Again aim and shoot at something. Radios have locked onto the 50 ohm feed-line standard, not 75 ohms, so for a balun we have 9:1 balun works, 450/50 = 9 to 1. Freq vs impedance varies? Again no kidding who cares. You have to PICK something and work to it, unless you have an antenna analyzer or antenna tuner, which can cause more loss and complexity than it's worth, especially for SWL, these random wires are generic ball park numbers and WHY 9:1 baluns are made and sold. It is not an accident, it's a law of the universe. If you want something closer to perfect you really have to dig deeper and have some test gear like a MFJ-259B.

"You can read antenna SWR and Complex Impedance 1.8 to 170MHz, as series
resistance and reactance (R+jX) or as magnitude (Z) and phase (degrees). You can
determine velocity factor, coax cable loss in dB, length of coax and distance to a short
or open in feet. You can read SWR, return loss and reflection coefficient at any
frequency simultaneously at a single glance."


However getting your antenna away from "the shack" is golden (lower noise is better than more signal), thus we need feed-lines. If you have an old radio with 300 ohm terminals and want to route 50 ohm coaxial feed line, you need baluns at each end if you use coaxial, one to go from BAL to UNBAL (coaxial) and one balun at the radio to go from UNBAL to BAL radio input. There are insertion loss there. A RF amp at the antenna would be a handy solution, and you could make one. For old radios it is better to use RG6 which is 75 ohm coaxial and use a 6:1 balun at each end. I don't think there are any common coaxial lines that are higher than 75 ohms. Again a small RF amp to over come line-loss and insertion loss of the two baluns would be a killer low noise system for an old radio, MW or SW.


-----Wire-------+balun+amp+feed+balun---radio
..................|
..............Ground

The simple solution for old SW and AM radios for an outside antenna, is use 300 ohm twin lead feed, route that outside to your wire antenna and a ground. Twin lead installation has some precautions, like running it against metal, so you need stand offs. Some people do just connect a wire to the radio and "throw it out the window", but that's a poor noisy installation for SWL. I don't recommend it. For local MW AM BCB it is fine, but the built in loop antenna is usually best. Some console don't have a loop antenna which is unfortunate. As always a good ground, earth (in dirt) ground, is important for low noise and even safety.

For my random wire to a receiver with 50 ohm antenna input (coaxial of course, unbal current), I wound my own BALUN for a SWL random wire antenna, tuned it using the radios spectrum analyzer of the radio, by adding or subtracting turns on the primary or secondary of my transformer (balun) as needed. The "theoretical" turn ratio is 3:1 for a transformation of 9:1. I ended up with 30/9 not 30/10 turn ratios. I could see the affect across the band; it was a noticeable gain. I only have room for ~66 feet of wire anyway, but tuning the balun for max response of the antenna, which is about 7 Mhz worked well. Of course it's not ideal across the whole SW band, NO ANTENNA IS, but works for my main frequencies of interest 1-16 Mhz[/b]. With this antenna I have tuned in Europe West and East), New Zealand, Australia, South Africa, Middle East. With the wire, tuned balun and ground I went from so so performance to outstanding with much lower noise. The ground is key to this of course since you need to ground half of your "balanced" wire antenna, other wise you have monopole with no ground plane, very noisy.

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


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 Post subject: Re: Pics-TV Transformer 75/300 ohm which is better, how to t
PostPosted: Aug Sun 14, 2011 11:48 pm 
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Joined: Jul Wed 22, 2009 8:32 am
Posts: 3807
I tested two baluns, the one with capacitors and the one with the tiny wires as shown above (right one). I used rabbit ears outside, no amplifier and about 30' of coaxial. The stations are all the stations I could get, about 21 miles away. There was no stations I could receiver beyond that, which is to be expected for unamplified rabbit ears.

I did a better job of recording data. Clearly the balun with the capacitor works but it has loss across the board except in VHF channel 11. (I did not check the one with the large wire, double core because I'm using it in the attic. From my previous test it was better than the balun with the capacitors.)


Image

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


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