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8 bay TV Antenna question

Tesla23

Member
Assuming that the feed impedance of one bay is 300 ohms, then you would run 300 ohm line to where they join, and when you join them in phase, the impedance would be 150 ohm as you say. If you want 300 ohms at the junction (to use your standard balun), then what you could do is make the last quarter wavelength to be sqrt(300 x 600) = 424 ohms to transform the impedance to 600 ohms prior to joining. The 600 ohm from each side would then parallel up to make 300 ohms seen by the balun. You could do this by keeping the 1.5 in spacing and reducing the wire diameter to 0.09in (using: https://hamwaves.com/zc.circular/en/index.html,) or keep your wire diameter of 0.186 and increase the spacing to about 3.2in, like:
1590193365103.png

lambda/4 at 550MHz is around 5.4in.

If the assumption of a bowtie impedance of 300 ohms is correct then this will improve the signal combining, if the assumption is incorrect then it may not. Experiment! Also you don't need hard right angles as in the diagram - you can 'smear' out the transition a bit. 3.2 in is also getting a bit wide for a tx line at these frequencies, you could also try the thinner line option.
 
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gary350

Well-Known Member
I think you answered my question. Wire diameter and spacing makes 600 ohms on both sides. When the 2 sides are connected in parallel the connecting wires are 300 ohms so those connecting wires need to be sized and spaced to make 300 ohms. Now 300 ohm to 75 ohm balun can be connected to 75 ohm coax 60 mph hour wind is common in every storm..

Bow ties are 1.5" apart. Transmission line calculator says I need .023" diameter wire to = 600 ohms. I probably should have calculated transmission line spacing first then made bow ties fit the transmission lines. The very first thunder storm & high wind will break .023 wire. We live in tornado alley.

If I use 1/4" copper tubing for wire I can get 150 ohm transmission lines on both sides. I can use .250" copper tubing to parallel the 2 sides to get 75 ohms for the coax. No balun needed.

300 ohm transmission lines paralleled = 150 ohms this won't work unless there is a trick I don't know.

What I really needed to know was that ohms of connecting wires are 1/2 the ohms of both sides. I could not find that online or in books. Someone already mentioned it I just wanted to double check.
 
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Tesla23

Member
If the assumption of 300 ohms for the bowties is correct, then this is how you combine them:

1590201241847.png

All the transmission lines are 300 ohms (D/d about 6), except for the quarter wavelength either side of the balun connection at 'B'. If you make this line 424 ohm (D/d about 17), then that will transform the 300 ohms from each bay up to 600 ohms, so that when you parallel them at 'B' you get back to 300 ohms. In case you aren't familiar with the quarter wave transformer: https://en.wikipedia.org/wiki/Quarter-wave_impedance_transformer
 

gary350

Well-Known Member
What does NM(dB) mean on TV fool?

All stations have high number NM(dB) at the top of the list. As you go down the list NM(dB) numbers get smaller and smaller. Does this have something to do with the power of the transmitter?
 
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JimB

Super Moderator
Most Helpful Member
NM(dB) is Noise Margin.

You asked this same question less than a year ago.
Look here:

JimB
 

gary350

Well-Known Member
NM(dB) is Noise Margin.

You asked this same question less than a year ago.
Look here:

JimB
Be glad you still have a good memory. At 70 I can't remember anything. I can remember things 60 years ago better than what I did yesterday.

I think I need a low VHF antenna there are several good Low VHF channels that my antenna won't pick up. I have no problem with 3 low VHF channels they have 12 sub channels that are 51 NM(dB). There are 4 other low VHF channels with 15 sub channels that are 13 NM(dB). The 13 NM(dB) stations must be low power transmitters. All 7 channels = 27 sub channels are in the same location 35 miles from me. New antenna gives me 30 more channels but better low vhf will give me more total of about 100 channels.

Maybe I should buy this antenna.


100_6837.JPG
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Be glad you still have a good memory. At 70 I can't remember anything. I can remember things 60 years ago better than what I did yesterday.

I think I need a low VHF antenna there are several good Low VHF channels that my antenna won't pick up. I have no problem with 3 low VHF channels they have 12 sub channels that are 51 NM(dB). There are 4 other low VHF channels with 15 sub channels that are 13 NM(dB). The 13 NM(dB) stations must be low power transmitters. All 7 channels = 27 sub channels are in the same location 35 miles from me. New antenna gives me 30 more channels but better low vhf will give me more total of about 100 channels.

Maybe I should buy this antenna.

It will certainly far outperform a shirt back-fire array, but I'm fairly dubious (as are most of the TV trade) about the three yagi parts and the overall design - the general impression over here is that they are designed for visual appearance, and don't give much (if any?) extra gain. I've never had occasion to use one, while we used to sell aerials where I used to work we wouldn't stop so low as to sell chavvy aerials like those :D
 

shortbus=

Well-Known Member
Most Helpful Member
My dumb question in this is, what is being gained in all of this? Getting the same channels from a farther distance is better how? In the USA there are only so many over the air channels no matter how far away they are.
 

Ylli

Active Member
Annotation 2020-05-27 093752.png

I really have not paid attention to typical gain figures for TV antennas, but the spec'd gains here do sound a bit low for an antenna that advertised for a 60 mile range. And the beamwidths seem a bit wide. But then again, maybe these are typical, I don't know. The F/B ratios are quite good though.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
My dumb question in this is, what is being gained in all of this? Getting the same channels from a farther distance is better how? In the USA there are only so many over the air channels no matter how far away they are.
Don't you get different regional channels?.

But regardless, getting channels from further away is a goal in itself :D
 

nsaspook

Well-Known Member
Most Helpful Member
Don't you get different regional channels?.

But regardless, getting channels from further away is a goal in itself :D
Exactly. Not everyone lives in a big city with all channels at one location.

IMG_20190807_174152176.jpg
My OTA setup.
 

gary350

Well-Known Member
My dumb question in this is, what is being gained in all of this? Getting the same channels from a farther distance is better how? In the USA there are only so many over the air channels no matter how far away they are.
If I were trying to receive the same programing from 3 to 5 different transmitters there would be nothing gained. ABC, CBS, NBC, PBS, and a few more are transmitted by several towns in our area the closest are, 35 miles, 72 miles, another one 85 miles, another 95 miles, another 120 miles. I get 77 channels 35 miles away there is no reason to try get the same programs from farther away. There are other independent stations that transmit there own programing no one else has. Some of the towns have programs other towns don't have. Channel 6 is a local town owned station with 7 sub channels. Channel 34 & 35 are small town transmitters too with their own type of programs that people in those small towns like. The little towns have low power transmitters only designed for the local population. OTA TV guide shows me programming in my area there are about 100 channels. Somewhere I read channel 6 is 5000 watts it has good shows on TV guide no one else has. Channel 34 & 35 are small town to with programs no one else has. TV Guide is a good way to learn what each stations programming is. If I don't like what I see on TV guide I want want to receive their signal. Sometimes it is nice to have duplicate programs last week I missed a program then watched it 2 days later on a different time same channel. I don't watch much TV in nice weather we are usually outside or we are gone camping on lake, national parks or state parks. We don't go camping with a TV or antenna.
 
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shortbus=

Well-Known Member
Most Helpful Member
Don't you get different regional channels?.
No regional's in my area, just the 4 normal ones ABC, NBC, CBS and PBS.


There are other independent stations that transmit there own programing no one else has. Some of the towns have programs other towns don't have.
Independents? Like what?


Exactly. Not everyone lives in a big city with all channels at one location.
I bet your city/area is bigger than where I live.
 

gary350

Well-Known Member
No regional's in my area, just the 4 normal ones ABC, NBC, CBS and PBS.



Independents? Like what?
Small towns population 5000 people have a small local TV station for their town. Several movie channels.


I bet your city/area is bigger than where I live.
 
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nsaspook

Well-Known Member
Most Helpful Member
No regional's in my area, just the 4 normal ones ABC, NBC, CBS and PBS.




Independents? Like what?




I bet your city/area is bigger than where I live.
Likely, my city is 10K pop @ about 30 miles from the Portland towers.
 

gary350

Well-Known Member
I don't understand db measurement. Antenna 14 db is more gain than 12 db and 12 is more gain then 10. Transmitter 51 db is noise. Makes no sense.
 
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Ylli

Active Member
A 'dB' is a logarithmic ratio. For an antenna, if the power it produces at its feedpoint is 5 times what a simple dipole antenna would produce, then it has a power gain of G = 10log5 = 7 dB. Since it is a ratio, it always has to have a reference (some times explicitly stated, sometimes inferred). In this example, it is compared to a dipole antenna so the proper units would be gain = 7 dBd. If the reference antenna was a theoretical isotropic antenna, the it would be 7 dBi.

If that 2 way coupler has a loss of 3.5 dB, then we can calculate that -3.5 dB = 10log(Pout/Pin). Pout/Pin = 0.46, so the ouput at each port is a bit less than 1/2 the input power.

Not sure what the referrence to 51 M(dB) is without researching it a bit, perhaps it means the signal power at the receiver location is 51 dB avoe the noise floor.

And if you are dealing with voltage instead of power, the formula becomes voltage gain = 20log (Vout/Vin)
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I don't understand db measurement. Antenna 14 db is more gain than 12 db and 12 is more gain then 10. Transmitter 51 db is noise. Makes no sense.
We were told decades ago at college, only those who don't understand what they are doing use decibels! :D

As it's simply a ratio it's completely meaningless without giving the reference - some do, some don't, and advertisers can make them appear as good as they choose.
 

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