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Outdoor Loop
- Thread starter HomerBB
- Start date
Hello,
W5LZ is rigth.
There are 3el quads for 11 meters where a direct 50 ohms impedance can be seen.
A example can be found here:
Cubical_Quad_3_element
Bare in mind the wire used has a diameter of 2,5mm and is bare copperwire.
Other wire diameter will ask for different dimensions.
Often it is heard "the quad has a wide bandwidth" well in this case again it is shown that is not the case.
The 2:1 swr points are reached after about 500/600 Khz...
Kind regards,
henry HPSD
19SD348
W5LZ is rigth.
There are 3el quads for 11 meters where a direct 50 ohms impedance can be seen.
A example can be found here:
Cubical_Quad_3_element
Bare in mind the wire used has a diameter of 2,5mm and is bare copperwire.
Other wire diameter will ask for different dimensions.
Often it is heard "the quad has a wide bandwidth" well in this case again it is shown that is not the case.
The 2:1 swr points are reached after about 500/600 Khz...
Kind regards,
henry HPSD
19SD348
I disagree that your linked article has anything to do with your claim Robb, but here is a thread on QRZ that discusses your issue. http://forums.qrz.com/archive/index.php/t-189614.html
odd. even the hygain 4 el and the sig eng 4 and 6 el quads used a matching system. dont know what cubex does.
doc would you advise just feeding it directly with the 50 ohm or direct through a 1:1 balun?
Here is my Eznec5 model for the idea that 'Doc talks about here, but I don't think this direct match idea is simply due to having three elements. In the process of finally following the element length and spacing ratios that Henry shows us...I saw all kinds of terrible mismatches while didling around with the match.
Now it maybe that the three element quad may allow one to get close enough to allow a direct match, but it still takes some really fine tuning or else you might be working with SWR greater than >20.0:1. Plus I noticed that the range for a good direct tune is very small, maybe in the range of less than a 1/8" and there are a lot of iterations that can affect the match for good and for bad. Also notice in the source report, that my model is not resonant at 27.205 mhz, and the fix for that too may also be less than a 1/4" to the radiator wire.
View attachment Doc's 3 element quad idea..pdf
Last edited:
Guys,
As always it is the combination of element distance, element length. boomlength, element thickness, heigth above ground and material used that will determine if a antenna has a 50 ohms input or not.
Why manufacturers often use `matching` systems..
In the past it used to be cause yagi´s performed best below 50 ohms.
In some cases that still is the issue. A antenna migth have the specificiations you want but with a impedance you need to match.
These days that often is not the case anymore.
Infact the opposite is happening.
Take for example the hair pin match..It is already known that a hairpin match on a beam can severly screw up the pattern, this though most still see the hairpin as `best`.
And no mather how one puts it..there will be loss in a matching system (as small as it might be)
Personally, The reason why i think most still use matching systems is cause i more and more tend to get the impression it is to illimunate problems.
A matching system will provide some match to a `range` of different impredances.
So if the antenna doesnt have the designed impedance, for instance 25 ohms but now 40ohms the gamma/match will still handle it.
IF a manufacturer provides direct 50 ohm antennas he must be 100 procent certain it is 50 ohms.
Where can it go wrong..
Well the manufacturer can make a mistake, the costumer can make a mistake. Nearby objects will have influence.
So it is rather nice to have a `backup` in the form of matching unit which will take care of small litlle issues.
Kind regards,
Henry
As always it is the combination of element distance, element length. boomlength, element thickness, heigth above ground and material used that will determine if a antenna has a 50 ohms input or not.
Why manufacturers often use `matching` systems..
In the past it used to be cause yagi´s performed best below 50 ohms.
In some cases that still is the issue. A antenna migth have the specificiations you want but with a impedance you need to match.
These days that often is not the case anymore.
Infact the opposite is happening.
Take for example the hair pin match..It is already known that a hairpin match on a beam can severly screw up the pattern, this though most still see the hairpin as `best`.
And no mather how one puts it..there will be loss in a matching system (as small as it might be)
Personally, The reason why i think most still use matching systems is cause i more and more tend to get the impression it is to illimunate problems.
A matching system will provide some match to a `range` of different impredances.
So if the antenna doesnt have the designed impedance, for instance 25 ohms but now 40ohms the gamma/match will still handle it.
IF a manufacturer provides direct 50 ohm antennas he must be 100 procent certain it is 50 ohms.
Where can it go wrong..
Well the manufacturer can make a mistake, the costumer can make a mistake. Nearby objects will have influence.
So it is rather nice to have a `backup` in the form of matching unit which will take care of small litlle issues.
Kind regards,
Henry
I agree with Henry about this, so matching has issues, but it provides the user's the ability to control more vairables.
I know Henry is talking about the gamma handling a wide range of impedance mismatches.
And I know the answer to which matching system is best is always answered with it don't matter except with high power.
And I know that the above answer references an antenna being used across a single band when the discussion is about CB antennas.
Now what I wonder is whether the impedance matching abilities of one system over the other proves to be superior in a wider banded antenna, such as for 11 and 10 meters use, or wider.
Now this is where someone will step in and ho-hum the question and suggest using a tuner. Please note the question is not about a tuner, nor is it about a panacea. It is about what research/experience has shown, and if one or the other matching system is superior to others and in what way.
And I know the answer to which matching system is best is always answered with it don't matter except with high power.
And I know that the above answer references an antenna being used across a single band when the discussion is about CB antennas.
Now what I wonder is whether the impedance matching abilities of one system over the other proves to be superior in a wider banded antenna, such as for 11 and 10 meters use, or wider.
Now this is where someone will step in and ho-hum the question and suggest using a tuner. Please note the question is not about a tuner, nor is it about a panacea. It is about what research/experience has shown, and if one or the other matching system is superior to others and in what way.
It is not so straigth forward as im writing below, but it will get a "mind-set".
The problem is:
We can design wideband yagi's for example a 50 ohms impedance owa yagi
(5 elements on a 5,5 meter boomlength) will show about 9,5 dBi and 25 Fb over a 2Mhz bandwidth. So no matching is needed.
If we drop the impedance of the antenna the bandwidth tends (not always) to be smaller.
High gain antennas are often (not always) low impedance antennas and therefor have a small bandwidth.
Just imagine if the antenna is 10 ohms and we need 50 ohms you need to have a 1:5 matching system.
If the impedance drops below the design freq say 8 ohms we still have that 1:5 matching
the impedance seen will be 40 ohms. (5x8=40)
On the other hand if we have our owa yagi with 50 ohms and we drop a bit below frequency with 2 ohms...and the impedance becomes 48 ohms the SWR will remain rather good....hence better bandwidth.
The opposite is true aswell.
If we have a very high impedance antenna say a 1000 ohms and attach a 20:1 balun the impedance will be around 50 ohms...if we change the original impedance to 1002 ohms...noboby will see the difference.
This is one of the reasons those imax antennas are so wide in SWR coverage.
However it tells us nothing about efficiency
One can within reason say ...the larger the impedance change will be the more loss migth be expected.
Oke...so back to that gamma-match.
A gamma-match is used to change the impedance below 50 ohms.
Your question was however in aspect to bandwidth.
In that case i would say ...try the direct 50 ohms OWA yagi's
....it wont get any better than that.
Im only refering to "monoband" yagi's and not taking in mind antennas like log-periodics etc.
If you are thinking about a hairpin match...bare in mind the antenna is often moddeled with "straigth" elements. Afterwards a hairpin match is connected.
A hairpin match is physical quite large. (especially on vhf up)
The currents of the system wont be anymore on the antenna alone but also "at' the hairpin match. That will mess-up a entire pattern gain/front to back etc.
G0KSC justin wrote an article about that explaining the downsides of the hairpin match.
Not to "advertise" so i wont tell which antennas it concerns, im sure you know what im talking about..
Perhaps now you see the beauty in having a 50 ohms direct impedance, but with the bennefits of low impedace high gain yagi.
I hope it helped.
Kind regards,
Henry HPSD
19SD348
Homer, I imagine all really curious thinkers about antennas have asked the same question. Some elmer's have provided us their limited and arguably twisted views on the subject, but I don't see a simple straight forward answer.
It would seem to be a really easy and simple thing to study, test, and explain, but nope...even in spite of the fact that my mentor in science James Clerk Maxwell - Wikipedia, the free encyclopedia help nail down the speed of light in 1865, by claiming the speed was finite which got scientists to studying what was the speed.
It would seem to me that the determination of the speed of light would be much more of a problem than trying to figure out which matching system was best, but instead our modern understanding and technology just begs the question.
the problem with quads is thew wires are more band specific. my solution was to make a 3 el quad with the reflecter tuned for 11 and the directer tuned for 10. the match was a 11- 12 foot wire kept 2 inches from the 13 foot boom and running toward the reflector then folded 90 degrees around the boom as it came back to the so239 mounted just behind the driven el so i could reach the conecters with the coax when installed on the rotator. one match wire for verticle and one for horizontle conecting at the side and the bottom corner. i terminated each one to the middle of a so239 which was mounted to a L bracket then to the boom with a hose clamp so i could move it as i shortened each match wire until tuned. i had to make some plexi insulaters with edge screws to keep tenshion on the wire to hold the wire every 3 feet or so at 2 inches away from the boom. this gave me selectable verticle and horizontle with a single closed loop set to about the middle at 27.900. it worked real well. like a 2 el on each band with about 1.6:1 swr on ch38 and 28.400.
i used .15 dir and .175 ref spacing
i used .15 dir and .175 ref spacing
All interesting answers.
I follow Henry's train of thought pretty good.
And I experience Marconi's every time I look for materials on the subject.
Needle Bender, your answer is intriguing, but if you could provide a graphic illustration it would be helpful. I get part of it easily, but I'm really visual.
I follow Henry's train of thought pretty good.
And I experience Marconi's every time I look for materials on the subject.
Needle Bender, your answer is intriguing, but if you could provide a graphic illustration it would be helpful. I get part of it easily, but I'm really visual.
I think he's referring to the matching device's design. I know that I have a 'time' understanding a 'word' description where a 'picture' is much easier to understand. From your description, I can imagine two types of matching devices that are sort of 'close', and I can't be sure if that's what you are describing or not. A picture or drawing would make it much simpler.
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