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What makes a TV antenna be 300 ohms? What makes coaxial cable be 75 ohms?

gary350

Well-Known Member
What makes a TV antenna be 300 ohms? What makes coaxial cable be 75 ohms?

I read online 300 ohms was chosen because there are fewer losses. That must mean the design of the antenna makes it 300 ohms?

I read online there are few loses in coax cable so 75 ohms works fine.

300 ohms is 4 times more than 75 why is there fewer looses in 4 times more resistance?

I don't understand what I read?
 
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Ylli

Active Member
What makes a TV antenna be 300 ohms?
The design of the antenna itself.
What makes coaxial cable be 75 ohms?
The ratio of the center conductor inductance to the center conductor to shield capacitance.

I read online 300 ohms was chosen because there are fewer losses. That must mean the design of the antenna makes it 300 ohms?
Early antennas used a folded dipole as the driven element. A folded dipole has a natural impedance of about 300 ohms. It became a 'standard'.

I read online there are few loses in coax cable so 75 ohms works fine.
'Open line' like 300 ohm twinlead does have lower losses than typical coax. But if you want the lowest loss, you would need to use 'ladder line' where you don't have the continuous insulation between conductors, but rather periodic low loss insulators that maintain line spacing. As for the coax, the minimal loss impedance is 93 ohms, and the highest power handling capability is 50 ohms. 75 ohms is a compromise. In addition 75 ohms is 1/4 of 300 making a balun easy to build.

300 ohms is 4 times more than 75 why is there fewer looses in 4 times more resistance?
The feedline impedance is not the 'resistance' of the line but rather the source and load impedances it is designed to couple.

I don't understand what I read?
Transmission line theory is one of the most complicated subjects you will come across.
 

gary350

Well-Known Member
This is what I have learned, I built a 4 bay antenna it worked good 40 miles from transmitters but small amount of bad weather, rain, fog, snow, no signal until weather got better. I copied channel master CM-4228 my antenna worked better than factor antenna so I sold factory antenna to get my money back. I continues to make improvements 1 by 1 signal strength meter told me when changes worked and how much better they worked. I read ARRL antenna book to learn what makes antennas work better. I decided to build an over kill antenna so signal was never lost in bad weather.

I built an 8 bay antenna, used a larger 4'x4' reflector screen with small 1/2" holes for better high Hz, I used #14 solid copper wire for connecting wire, solid bow ties doubles the reception, reflector screen 6" from bow ties, cross over wires 1" apart, buy several baluns test them all some work better than others, use 1 piece coax from antenna to TV, use T combiner and 2 18" coax to connect both 4 bays, no antenna in attic or inside the house, keep coax short as possible, make sure baluns are connected in phase. I can tune the antenna by changing 6" screen spacing, put antenna up high as I can for best signal 40 miles from transmitters but my antenna is only 8 ft high outside. Use TVFool & compass to aim antenna at station transmitters. Antenna was 20' high at the other house when we moved to smaller house i put antenna up quick just suck it in the yard and it works so well I never put it up 20' high like it was before.

I use to have signal loose when sun was low on the horizon every day but no problem anymore with over kill antenna. Antenna is not even standing up straight anymore. LOL It works great. I gave best of 2 antennas to my son this antenna can use 2 improvements but it works so well I am not motivated to change it.. You mention ladder lines I bet connecting wires on the antenna will work better with ladder lines.

100_9970.JPGAnt-01-1.jpgAnt-02-1.jpg
 
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Ylli

Active Member
If you want to do some antenna sim, google "4nec2". Haven't used it in ages, but when I did I thought it did a pretty good job of converting a physical model into electrical characteristics and radiation pattern.
 

crutschow

Well-Known Member
Most Helpful Member
The impedance of an antenna or transmission line is its "characteristic impedance" not its resistance.
It's resistance is typically much less.

If you put a step voltage into a long transmission line, the current will be equal to the the step voltage divided by the characteristic impedance.
Thus, for example, a 1V step into a 75 ohm coax, will generate a current pulse of 1/75 = 13.3mA traveling down the line.

There are several reasons that 300 ohm lines were used for early television sets, where fringe reception was often poor, and maximum receiver sensitivity was needed.
  • One is that the antennas were 300 ohm and thus the 300 ohm transmission line could be connected without any loss from a balun.
  • Another is that a 300 ohm balanced line twin-lead has a lower signal loss than coax, mainly due to less dielectric loss in the twin-lead.
  • Finally, for a given RF signal power captured by the antenna, the signal voltage is twice as large for a 300 ohm system as compared to a 75 ohm system. This tends to improve the signal-to-noise ratio at the tuner front end.
The main disadvantage of 300 ohm twin-lead is that it's not shielded from external EMI interference as coax is and is sensitive to any nearby metal, such as rain gutters or antenna masts, so standoffs must be used to route the twin-lead away from those.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Ylli

Interesting insight. You nailed 75, 93 and 300 ohm.

I think my antenna Winguard HD8600U is 50 ohms per side native, but you match it to 75 ohm.

Some 50 years ago at the introduction to UHF, coax wasn't common. TV;'s had two inputs, 1 for UHF and one for VHF. Ran foam and had to have 2 different amplifiers.

Coax is immune to interfereance. You can;t run twin-lead near metal.

See: https://en.wikipedia.org/wiki/Twin-lead

Twin lead is a balanced transmission line, or differential. Coax has one side grounded.

Any transmission line at high frequencies has to be terminated with it's characteristic impedance. Better yet, the transmission line needs to be purely resistive. There are reflections that occur on a transmission line.

This
is food for thought. An 10x oscilloscope probe is a divide by 10..
 

atferrari

Well-Known Member
When active as a ham, forced by the physical / social limitations I had to live with, I found the long wire antenna quite effective. The sole limitation (not a minor one) was the extreme directionality. When at home I had the Pacific and Japan available every night so to speak. Building an antenna tuner was a solution.

When on board, of course, it all depended of the ship's course. ;)

An exceptional experience was triggering a repeater near Rio de Janeiro, coming from the South, two days before reaching the place, that is, around 600++ sea miles away. Our array 2x6 Yagi - 2m band was at the bridge level, some 21 meters above the sea level.

We repeated that performance in many places.

The conclusion I made: for antennas the best ground is the sea.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
When active as a ham, forced by the physical / social limitations I had to live with, I found the long wire antenna quite effective. The sole limitation (not a minor one) was the extreme directionality. When at home I had the Pacific and Japan available every night so to speak. Building an antenna tuner was a solution.

When on board, of course, it all depended of the ship's course. ;)

An exceptional experience was triggering a repeater near Rio de Janeiro, coming from the South, two days before reaching the place, that is, around 600++ sea miles away. Our array 2x6 Yagi - 2m band was at the bridge level, some 21 meters above the sea level.

We repeated that performance in many places.

The conclusion I made: for antennas the best ground is the sea.
I used to be part of a local Radio Ham club, and we used to get occasional speakers come to tell us their experiences.

One such guy worked for the BBC World Service, running and maintaining a shortwave transmitter on a tropical island somewhere (I can't remember where, it's 40+ years ago now, and I don't think I've ever heard of it). Anyway, the island had a long valley, between (hopefully extinct) volcanos, and the transmitting aerial was a large 'V' running down the valley. The station didn't run 24 hours, so during it's downtime he retuned the transmitter to an amateur radio band, and used it for his amateur radio hobby.

By some fortunate coincidence the valley aimed directly towards Europe, and he could easily work amateurs over here with his high power transmitter and huge professional aerial system.

He was an interesting guy, but this is the only story I can remember.
 

atferrari

Well-Known Member
By some fortunate coincidence the valley aimed directly towards Europe, and he could easily work amateurs over here with his high power transmitter and huge professional aerial system.
Privileged person.

In my first 4-months long Antarctic campaign I had free access to the ship's radio station. During certain periods along the day I could use a transceiver of fame: the Collins 618T fitted with automatic antenna tuner. Working the ham bands with it was like stealing candies to kids.
 
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gary350

Well-Known Member
Antenna tuner gives me an idea. Is there an advantage to have a tuner for a TV antenna? If so how can I build a TV tuner?
 

Ylli

Active Member
Only if there is an impedance mismatch or you need help to reject an interfering signal.
 

crutschow

Well-Known Member
Most Helpful Member
I think my antenna Winguard HD8600U is 50 ohms per side native, but you match it to 75 ohm.
I think the antenna was more likely 300 ohms and that was matched to the 75 ohm coax with a balun.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Antenna tuner gives me an idea. Is there an advantage to have a tuner for a TV antenna? If so how can I build a TV tuner?
No, if want better than you've got then build a better aerial - your existing one is a very low gain aerial, intended to provide better anti-ghosting performance in high signal areas. We used to fit commercial ones years ago in specific areas with ghosting problems, and vert effective they were.

I'm not even sure that ghosting is an issue any more?, as the modulation used for DTT supposedly can use the reflected signals to increase signal strength (which is why it's different to Cable modulation or Satellite modulation).
 

gary350

Well-Known Member
I chose my antenna because it picks up all 44 channels 40 miles away without rotating the antenna. I started with factory antenna signal strength meter reading 74% small rain, fog, snow, blocked the signal. Then I buy yagi it has to be rotated to receive all channels 40 miles away meter reading was 79% small rain,, fog, snow, blocked the signal. I sold both factory antennas got my money back. First 8 bay antenna I built was better than factory antenna, I made improvements several times & each time signal got stronger. Larger reflector screen was a big improvement. larger screen with smaller holes works better at high Hz. Solid bow ties was the best improvement of all 98% field strength meter. I can rotate antenna by hand and pick up several more stations but not much programming is different that I want to actually watch. Factory antennas both lost signal for 30 minutes ever day when sun was low on the horizon every day. Sun on low horizon never a problem with my antenna. So far so good. At the moment I can't think of any more improvements that will make antenna better except home made balun might help. Reflector screen is 6" from bow ties if I change spacing it tunes in certain Hz better. Before improvements I was having trouble with real channel 5 and real channel 50. I check field strength meter on each channel to check signal strength of each frequency signal is 98% through the center, 94% real channel 5 and 97% real channel 50. Over kill antenna never signal lose no matter how hard it, rains, snows, fog, hale100_6308.JPG, wind, etc.
 

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