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Old 24-Jul-2014, 12:42 PM   #1
pips
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help with antenna ohm dropping

ok if this wrong section im sorry mods.

ok so my wiring backgroung is in car audio and in that form we would chain subwoofers and each one u wire in the ohm drops in half.so here my question.if i hook two antennas in parellel on a 300 ohm antenna then the ohm drops to 150 if i add a third in parellel then wouldnt it drop to 75 which i could splice and solder the coax and feed straight to tv elimiating the baulan completely?can anyone chime in on this ?i would use twin lead to hook them together.is this right or even possible?
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Old 24-Jul-2014, 3:18 PM   #2
tripelo
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Stack 4 Antennas: Twin-lead to Coax

Quote:
Originally Posted by pips View Post
...if i hook two antennas in parellel on a 300 ohm antenna then the ohm drops to 150 if i add a third in parellel then wouldnt it drop to 75...
The resistance would drop to 100 Ohms if three (300 Ohm) resistive lines were paralleled.

- Four in parallel would be 75 Ohms.

You are writing about an application of Ohm's law: parallel and series combinations. There are web calculators to help.

In some cases, the term 'impedance' can be substituted for resistance.

Quote:
...which i could splice and solder the coax and feed straight to tv elimiating the baulan completely?...
If one wishes to accept the consequences (loss, etc.), one can always eliminate a balun.

In this case, if you want the lowest loss, a balun is required. It could be a series of selected ferrite cores (beads) around the coaxial cable near the point of connection to the paralleled twin leads.

This type of balun is often referred to as; a 'ferrite choke balun'.

Use Google Images, "Ferrite choke balun".

Quote:
...i would use twin lead to hook them together.is this right or even possible?...
Four antennas (300 Ohms impedance) stacked with feed lines in parallel.

It is possible, several have done so. For some of reasons below, this form of stacking is probably not often used anymore.

Yes, twin lead is appropriate for all the connections to the 300 Ohm antenna terminals.

The twin lead lines should have equal lengths.

There are other constraints for best performance.

Examples,

-The twin lead transmission lines should be perpendicular

* For Yagis, vertical with respect to the antenna elements for some distance, before turning horizontally to meet the other lines.

* For panel arrays (8-bays and such), it is probably most practical to route twin lead rearward to behind the reflector before turning to meet other feed lines. Once behind a reflector, then orientation of twin lead with respect to antenna elements is not as critical.

-The twin lead lines should be kept away from any metal (i.e. mast) or other lines (can use standoff insulators).

- There are probably more.

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

Well done, this approach is among the lowest loss methods of combining a stack** of four antennas.

**Stack antennas: Can be two or more identical antennas arranged with respect to each other (horizontally, or vertically, or combination of both) and connected in phase to increase the gain.

.

Last edited by tripelo; 24-Jul-2014 at 5:11 PM. Reason: maybe clarify
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Old 25-Jul-2014, 12:27 AM   #3
pips
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Quote:
Originally Posted by tripelo View Post
The resistance would drop to 100 Ohms if three (300 Ohm) resistive lines were paralleled.

- Four in parallel would be 75 Ohms.

You are writing about an application of Ohm's law: parallel and series combinations. There are web calculators to help.

In some cases, the term 'impedance' can be substituted for resistance.



If one wishes to accept the consequences (loss, etc.), one can always eliminate a balun.

In this case, if you want the lowest loss, a balun is required. It could be a series of selected ferrite cores (beads) around the coaxial cable near the point of connection to the paralleled twin leads.

This type of balun is often referred to as; a 'ferrite choke balun'.

Use Google Images, "Ferrite choke balun".



Four antennas (300 Ohms impedance) stacked with feed lines in parallel.

It is possible, several have done so. For some of reasons below, this form of stacking is probably not often used anymore.

Yes, twin lead is appropriate for all the connections to the 300 Ohm antenna terminals.

The twin lead lines should have equal lengths.

There are other constraints for best performance.

Examples,

-The twin lead transmission lines should be perpendicular

* For Yagis, vertical with respect to the antenna elements for some distance, before turning horizontally to meet the other lines.

* For panel arrays (8-bays and such), it is probably most practical to route twin lead rearward to behind the reflector before turning to meet other feed lines. Once behind a reflector, then orientation of twin lead with respect to antenna elements is not as critical.

-The twin lead lines should be kept away from any metal (i.e. mast) or other lines (can use standoff insulators).

- There are probably more.

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

Well done, this approach is among the lowest loss methods of combining a stack** of four antennas.

**Stack antennas: Can be two or more identical antennas arranged with respect to each other (horizontally, or vertically, or combination of both) and connected in phase to increase the gain.

.
wow that is some great info.my intent is to build 4 sbgh with no reflector and stack vertical down the mast on top of a 50 foot tower.i will use pieces of pvc pipe to shield the twin line where possible.my question now is can i use and off the shelf baulen (4 to 1) or a (1 to 1)?
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Old 25-Jul-2014, 1:18 AM   #4
tripelo
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Balun: 4:1 and 1:1

Quote:
Originally Posted by pips View Post
...my intent is to build 4 sbgh with no reflector and stack vertical down the mast on top of a 50 foot tower.i will use pieces of pvc pipe to shield the twin line where possible.
You probably know that PVC only shields twin lead from the weather, it does not electrically shield it. PVC is a lossy dielectric at UHF frequencies, I would not use it continuously close to twin lead, probably OK if using it for spacers something like a standoff insulator, with no continuous exposure.

Quote:
...my question now is can i use and off the shelf baulen (4 to 1) or a (1 to 1)?
Off-the-shelf baluns are normally transformers and baluns combined. The 4 to 1 indicates a transformation ratio (i.e. 300 to 75 Ohms). If you use a parallel combination that yields 75 Ohms then you do not need a transformer, only a balun, a 1 to 1 balun.

Any commercial 4 to 1 balun that I have measured (many of them) will have substantial loss at UHF frequencies, ~1 dB or usually more. A ferrite choke balun can be made to have losses below a few tenths of a dB.

I do not know of any commercial 1:1 baluns. They can be made to look like a transformer, or they can be the 'ferrite choke' version.

If you should find a combination of antennas that yield 300 Ohms impedance, a 'half-wave loop' balun provides about the lowest loss.

If your transformation ratio is anything other than 4 to 1 or 1 to 1, you will need to do some more research. Odd ratio baluns or balun/transformers can be made, but most would not attempt. In such a case, probably the best approach would be to transform the non-standard impedance to either 75 or 300 Ohms, then use an appropriate balun. The transformation could be done with custom-made transmission lines and lengths.

----------------------------
Balun: A device that can convert (efficiently mate) a balanced transmission line (twin lead) to an unbalanced transmission line (coaxial cable).

.

Last edited by tripelo; 25-Jul-2014 at 1:42 AM. Reason: clarify
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Old 25-Jul-2014, 3:00 AM   #5
pips
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Quote:
Originally Posted by tripelo View Post
You probably know that PVC only shields twin lead from the weather, it does not electrically shield it. PVC is a lossy dielectric at UHF frequencies, I would not use it continuously close to twin lead, probably OK if using it for spacers something like a standoff insulator, with no continuous exposure.



Off-the-shelf baluns are normally transformers and baluns combined. The 4 to 1 indicates a transformation ratio (i.e. 300 to 75 Ohms). If you use a parallel combination that yields 75 Ohms then you do not need a transformer, only a balun, a 1 to 1 balun.

Any commercial 4 to 1 balun that I have measured (many of them) will have substantial loss at UHF frequencies, ~1 dB or usually more. A ferrite choke balun can be made to have losses below a few tenths of a dB.

I do not know of any commercial 1:1 baluns. They can be made to look like a transformer, or they can be the 'ferrite choke' version.

If you should find a combination of antennas that yield 300 Ohms impedance, a 'half-wave loop' balun provides about the lowest loss.

If your transformation ratio is anything other than 4 to 1 or 1 to 1, you will need to do some more research. Odd ratio baluns or balun/transformers can be made, but most would not attempt. In such a case, probably the best approach would be to transform the non-standard impedance to either 75 or 300 Ohms, then use an appropriate balun. The transformation could be done with custom-made transmission lines and lengths.

----------------------------
Balun: A device that can convert (efficiently mate) a balanced transmission line (twin lead) to an unbalanced transmission line (coaxial cable).

.
hmmm now u got me thinkin.i could go into 300 ohm wiring on antenna then twin lead to the tv and convert at that point or i could do balun and if i read it right i can do clamp on choke then to tv.so many options.i do believe stacking with the twin lead is the best way to start its just getting from tower to tv
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Old 25-Jul-2014, 5:21 AM   #6
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I agree with tripelo, coax such as RG-6 has proven itself superior to twin-lead in applications such as down-lead and distribution. Twin-lead must be installed with care to ensure it is not too close to metal that will alter it's characteristic impedance (the exact effect of which is hard to predict, but will always be negative in terms of overall signal quality). Also, a uniform twist of about 1 to 1.5 turn per foot needs to be maintained in order to reduce the tendency for twin-lead to act as it's own antenna (which will almost always result in the signal from the real antenna being interfered with).

And as tripelo already mentioned, to install twin lead correctly, you'll need stand-off insulators to ensure that the wire stays where it supposed to be.

I would only use it as a part of a phasing harness that ties two or four identical antennas together in a ganged/stacked array.

I'd use coax for the down-lead and distribution portions of my system given that it performs well in the UHF band and has much better self shielding properties, allowing it to be run in contact with metal mast, conduit, pipe and duct work if needed.
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Old 25-Jul-2014, 8:34 AM   #7
tripelo
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Stack Bay Antennas w/o Reflector, Twin Lead or Coax?

Quote:
Originally Posted by GroundUrMast View Post

Twin-lead...
... not too close to metal...
...1 to 1.5 turn per foot needs to be maintained...
...stand-off insulators to ensure that the wire stays...
...use it as a part of a phasing harness ...

...Coax for
...the down-lead and distribution...
...run in contact with metal mast, conduit, pipe and duct work...
Thanks GroundUrMast, all good points.

Quote:
Originally Posted by pips View Post
...my intent is to build 4 sbgh with no reflector and stack vertical down the mast on top of a 50 foot tower...
Pips, it seems you are trying to optimize performance, if so:

Without reflectors to help isolate active elements from effects of the transmission lines, it seems difficult (maybe impractical) to vertically stack four such antennas using either twin lead or coaxial cable.

Keep in mind, all four transmission lines (twin leads or coaxial cables) need to be the same length. If stacking four vertically, this means that the two center antennas will have to have leads/cables going downward and then turning upward and vice-versa.

Transmission lines should be gradually bent, no sharp turns. This means that at the turns, the transmission lines will be routed horizontally.

Additional caveats apply to twin lead; at all times it should be kept separate from other portions of itself, or any metal.

Horizontal twin leads or coaxial cable*, in front or behind, can disturb the pattern and the gain of a reflector-less bay antenna.

----------------------------
If one can accept less than optimal performance, then compromises can be made (Guidelines and rules-of thumb do not matter as much).
----------------------------


* Induced currents in the coax shields are the primary sources of degradation. One could load the cable with a few selected ferrite cores at the horizontal portions. The ferrite would inhibit the induced currents, thus mitigate most of the potential degradation.


.

Last edited by tripelo; 25-Jul-2014 at 9:21 AM. Reason: clarify
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Old 27-Jul-2014, 3:13 AM   #8
pips
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update guys picked up a 35 foot tower and 10 ft mast today we had to cut it into 3 pieces cause to many wires to drop it.i have a few other sections i will try to add to it.will need to be bolted and welded back together but free is always good!so i had a back to basics evening and went back to the original version of sbgh mad one with solid reflector based on the news channel style
http://www.centralmediaserver.com/WI...nnadrawing.pdf
the cost was 40 bucks i used 10g copper wire.the only diffrence is i glued on caps and used roofing screws with rubber washers to attach to the face of the caps.i dont like the element being behind the caps.so i set it out of a window pointing southeast into trees.i picked up 29 channels one being 11.1 which is 80 miles away.my thought now is that if i make another and point them 180 from each other and space them 3 foot on the mast i can use an a/b switch and get all the channels i need without the need for stacking.i am however trying to figure out a way to get the dbgh to 300 ohms then i would upgrade both to those.
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Old 27-Jul-2014, 9:36 AM   #9
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Quote:
Originally Posted by pips View Post
update guys picked up a 35 foot tower and 10 ft mast today we had to cut it into 3 pieces cause to many wires to drop it.i have a few other sections i will try to add to it.will need to be bolted and welded back together but free is always good!so i had a back to basics evening and went back to the original version of sbgh mad one with solid reflector based on the news channel style
http://www.centralmediaserver.com/WI...nnadrawing.pdf
the cost was 40 bucks i used 10g copper wire.the only diffrence is i glued on caps and used roofing screws with rubber washers to attach to the face of the caps.i dont like the element being behind the caps.so i set it out of a window pointing southeast into trees.i picked up 29 channels one being 11.1 which is 80 miles away.my thought now is that if i make another and point them 180 from each other and space them 3 foot on the mast i can use an a/b switch and get all the channels i need without the need for stacking.i am however trying to figure out a way to get the dbgh to 300 ohms then i would upgrade both to those.

Towers, can be a tricky animal.

First of all, you'll want to check for rust and pitting.
No big deal you say, it looks fine.
Yes, but you should check the I.D., too.

Secondly, it's probably not a good idea to do any welding, as the heating/ cooling, can affect the surrounding metal.

Finally, you should stage all sections on the ground, so no matter which order you hoist them up, you know you won't be wrestling w/ them when you're up in the air.

A nice method of cleaning I've used, is to wrap some 0000 steel wool around a drill bit and lightly polish the insides, while spraying w/ some lubricant.
You may want to polish the outsides, too.

Last edited by Stereocraig; 27-Jul-2014 at 9:37 AM. Reason: wording
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Old 27-Jul-2014, 2:40 PM   #10
pips
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Quote:
Originally Posted by Stereocraig View Post
Towers, can be a tricky animal.

First of all, you'll want to check for rust and pitting.
No big deal you say, it looks fine.
Yes, but you should check the I.D., too.

Secondly, it's probably not a good idea to do any welding, as the heating/ cooling, can affect the surrounding metal.

Finally, you should stage all sections on the ground, so no matter which order you hoist them up, you know you won't be wrestling w/ them when you're up in the air.

A nice method of cleaning I've used, is to wrap some 0000 steel wool around a drill bit and lightly polish the insides, while spraying w/ some lubricant.
You may want to polish the outsides, too.
we will be assembling on the ground.its a galvanized tower with the pipe crosses not the wire ones i have seen some small spots of rust before i put it up i will go over and polish and sand and prime those spots.i will be bolting and stich welding the seams.in real terms i prob wont have to climb it i have a buddy with bucket truck.
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Old 27-Jul-2014, 4:27 PM   #11
Stereocraig
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Oh, the things I could do w/ my towers, if I had a bucket
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Old 28-Jul-2014, 1:38 AM   #12
pips
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Quote:
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Oh, the things I could do w/ my towers, if I had a bucket
yep i also have used a scissor lift to get up a 40 ft antenna that was free standing.i rented it for half day 65 bucks.so much easier.

now since this still a ohm thread does anyone know how to wire the dbgh to get 300 ohm?
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