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Rocketman
New member
Username: Rocketman
Post Number: 1
Registered: 2-2005
| | Posted on Saturday, February 12, 2005 - 7:23 pm: | 
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I'm building a two element quad loop for 11 meter.
Does anyone know the most economical way to match the 100 ohm's back down to 50 ohms? |
Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Saturday, February 12, 2005 - 8:54 pm: |
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The match should be pretty good with direct feeding of the driven element, but if you want to fine tune it you can do so by adjusting the spacing of the elements. Remember to use a choke coil in the coax to prevent feedline radiation which would ruin the pattern of the antenna.
Are you using a reflector or a director as your second element? |
The ELCO guy (Unregistered Guest)
Unregistered guest
| | Posted on Saturday, February 12, 2005 - 11:14 pm: |
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Easy....
Get some 70 ohm coax and cut a piece that is 1/4 wave length long: First compute one wave length in feet as being 984 divided by the frequency in mhz you wish the antenn a to function times the velosity factor--((984 / F mhz)* V). If the insulation between the center conductor and the shield is that solid plastic stuff then use .66 as the velosity factor. If the insulation is the foamey squishey stuff use .88 as the velosity factor. Divide this one wave length by 4 to get a quarter wave length and cut a piece of 70 ohm coax this 1/4 wavelengh long (pluse add 4 0r 5 inches for making connections). Skin off carefully half of said 4 or 5 extra added inches of outer jacket off each end and separate the shielding from the center conductor. This will form your quarter wave matching section of 70 ohm coax with a short 2 inches or so of separated shield, conductor at each end for connection purposes.
Once you have made your matching section connect one end of the matching section to 50 ohm coax-- connect the center conductor of matching section to center conductor of 50 ohm and shield of matching section to shield of 50 ohm. Connect the other end to the loop--center conductor to one end of the loop, shield to the other end of the loop. Secure the connections.
The antenna will NOT be a multi-band antenna but it will be broadbanded for the frequency band that it was constructed.
Note: You can tweak the SWR by adjusting the distance between the loaded loop and the reflector loop.
IF you truly want to get fancy you can just cut out the strate quarter wave length matching section and use proper connectors and double female splicers rather than the physical tag ends. It makes the job look much neater and easier to seal from the elements as well.
This matching quarter wave technique works very well when the loading loop in feet is determined as : 1005 divided by Frequency in Mhx. And the Reflector element in feet is 1030 divided by th frequency in mhz.
OHHH for those who noted that 1005 is longer than one full wave length and asking why. Other than the book saying this is the number to use, I figure that the larger size of the loop adds band width to the loop. In other words, the loop will maintain its tune over a larger frequency range.
Remember that the way you feed your loop determines its polarity. If you feed the loop at the center of the bottom of the loop (looking at the loop) then the loop will be horizontal orientated. If you feed your loop in the center of one of the vertical sides the loop will be vertical orientated.
What kind of 70 ohm coax to use? Just about any will do in a pinch. The best of course is the 70 ohm coax with copper braided shielding. BUT you can use, and I have used, 70 ohm cable TV coax if that is all you have. IF you use cable TV coax remember that the aluminum and coper shilding connection will react (corrode) over time due to different metal reaction when current is applid and as such will need to be taken appart and cleaned. You will know when it goes bad--the SWR will slowly climb upward. So I recommend you use the 70 ohm coax with the copper braided shielding.
Let us know how it turns out.
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Tecnicoloco
Member
Username: Tecnicoloco
Post Number: 65
Registered: 4-2003
| | Posted on Sunday, February 13, 2005 - 12:13 am: |
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Hi Elco Guy:
I have a comercial build 2 Element Quad made by Cubex,the antenna came with the 1/4 Wave 75 Ohms RG-11 Matching Stub.
My question is:The loops came with some wire aparenttly pre-cut,but in the manual said that any excces to be trim after all the loop wire is in position and tigh.
I want to lower my SWR on this quad,but your method stays that distance from Driver to director needed to be change,my antenna has a fix postion at both ends.
Thanks in advance for reply.
Tecnicoloco CEF-117
Puerto Penasco,Sonora,Mexico. |
Tecnicoloco
Member
Username: Tecnicoloco
Post Number: 66
Registered: 4-2003
| | Posted on Sunday, February 13, 2005 - 12:18 am: |
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Hi again:
I place a Loop made out of 10 Turns of RG-213 in a 6" inches diameter, after the connection to the matching stub,I hope this will not distort my signal pattern or my SWR???
Any reply will be apreciated,and thanks again.
Tecnicoloco
CEF-117
Puerto Penasco,Sonora,Mexico.
de vy 73 |
Ferd1605
Junior Member
Username: Ferd1605
Post Number: 11
Registered: 1-2005
| | Posted on Sunday, February 13, 2005 - 10:07 am: |
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The way i tuned a quad is to vary the leanght of the reflector . I always cut the reflector wire to the desighn freq. and at the bottom of the element speader i ran the wire up the fiberglass parallel to eachother then ran a short jumper wire between them and run it up or down to tune the antenna . This is on the reflector , NOT the driven elemnet . The quad has a big goof factor for element spacing . The 1/4 wave feed as others have said is correct , just try the tuning of the reflector .Just also remember to solder the jumpers once tuned .. |
The ELCO guy (Unregistered Guest)
Unregistered guest
| | Posted on Sunday, February 13, 2005 - 10:53 am: |
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Re Techie...
In re to your questions in reverse order.
Before I reply in full to the loops you made, a bit of background on coax. When you use coax the signal can flow in three places. Yes three places. It can flow in the center conductor--thats good. It can flow on the inside half of the shielding-thats also good. BUT it can also flow on the OUTSIDE half of the shielding and that is NOT good in as far as the return signal is re-radiated rather than being contained inside the coax. This reradiation may contribute to interference to other services.
OK back to your loop post Techie.
What you have made is called a coax balun. It keeps the signal off the outside half of the coax shilding, or so the books say ;-). As to your questin on radiation pattern and the loop coax balun you constructed, it will not effect the radiation pattern. It should not effect your SWR. Some Antenna Manufacturers rather than use a coax balun send a 1:1 inline balun as a substitute for the coax balun. The Coax balun is cheaper. The 1:1 commercial banun, depending where you buy it costs about 20 bucks.
A coax balun does not effect SWR when properly constructed.
Your commercial made antenna uses stub matching ey. Interesting! For those who do not know, stub matching is when the loops have the ends dangling longer with spacers inbetween them or some type of electrical equal physical setup. You then connect the feedline/tap the matching stub where the SWR is the lowest. On some of the older homebrew quad antennas the builders also put a similar matching stub on the reflector loop so they could tweak the antenna.
In re to the movement of the loaded elemet relative to the reflector. This is of course when you are making a home brew antenna and you can use a boom long enough to move the loaded element a little closer or farther from is reflector. The thing to remember is that a perfect 1:1 SWR on any antenna is an illusion except for one single frequency. So if you get a SWR of less than 2 OK. If you can get a quad down to below 2 over a large band width then you have a pretty good antenna. The advantage of the quad andenna is not in its ability to tweak down to a perfect match but rather to be workalble effective over a wide rage of frequencies. So a little SWR is to be anticipated.
Another thing to consider is that unless Cubex specificly made the antenna for CB frequencies there will be some SWR. You see IF Cubex designs their antenna for 10 meters then in order to convert it to CB frequencies you have to LENGTHEN the wire loops and probably adjust the spacings farther appart between the elements to achieve the original design specs on the CB Band. The CB band is close enough to be 'functinal' for the CB Band but not perfect for the CB Band.
I hope I have not confused you.
OHHH In case you are curious. Do a web search on the "Quagi" antenna. Its is an antenna that has a reflector and loaded element of quad design with reflectors of a yagi design. The gain on a quagi antenna is better than that of a pure yagi antenna.
That ol ELCO Guy
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Rocketman
New member
Username: Rocketman
Post Number: 3
Registered: 2-2005
| | Posted on Sunday, February 13, 2005 - 9:18 pm: |
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Thanks guys for all your info!!!
Rocketman |
Tech291
Moderator
Username: Tech291
Post Number: 81
Registered: 11-2004
| | Posted on Sunday, February 13, 2005 - 9:39 pm: |
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Rocketman,
going back to info i gathered from one of the arrl antenna books,I have constructed quads using a reflector and driven element that worked well.the matching method on them was to use an electrical half-wavelengh of 75ohm coax direct connected to the driven element.and the 75ohm was coiled to ack as the choke balun.
tech291
CEF#291
kc8zpj |
Rocketman
New member
Username: Rocketman
Post Number: 4
Registered: 2-2005
| | Posted on Sunday, February 13, 2005 - 9:43 pm: |
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Another question if I use #14 copper electrical wire should I use stranded or solid? Should I remove the coating or leave it on? |
Tech291
Moderator
Username: Tech291
Post Number: 82
Registered: 11-2004
| | Posted on Sunday, February 13, 2005 - 9:48 pm: |
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on wire selection,which ever you have available.since you will be tuning for resonance any of the choices will work out.
tech291
CEF#291
kc8zpj |
Rocketman
New member
Username: Rocketman
Post Number: 5
Registered: 2-2005
| | Posted on Sunday, February 13, 2005 - 9:49 pm: |
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Teck291 Any number on the coils or the diameter of each coil?
Thanks Rocketman
tech291
about 6"dia.coil about 8 to 10 turns |
Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Sunday, February 13, 2005 - 10:39 pm: |
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I just looked at a Bill Orr book "The Truth About CB Antennas" and in the construction details of both a 2 and 4 element quad he suggests direct feeding of the driven element with 50 ohm coax. The only quad he suggests using a matching transformer is a single element quad loop - and that's a 1/4 wavelength 70 ohm coax matching stub.
The element spacing will alter the impedance of the driven element, but it's possible to build one that can be fed with no matching transformer. |
Rocketman
New member
Username: Rocketman
Post Number: 6
Registered: 2-2005
| | Posted on Tuesday, February 15, 2005 - 6:40 pm: |
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How can you set this up for both polarization? |
The ELCO guy (Unregistered Guest)
Unregistered guest
| | Posted on Wednesday, February 16, 2005 - 2:02 am: |
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Re Rocket...
If you want to set the thing up for duel polarization like a PDL II the only way I can think of and still be practical would be to use a couple of relays in such a manner as to close one relay and open up another--in a nutshell short out one feedpoint while opening up the other feedpoint. HOWEVER if you decide to go this route you have to run power to both relaysalong with the coax. On top of that, to complicate matters, depending on how you feed the loop you might need to use the same relays switch between feed coax as well. Remember the PDL 2 which had both vertical and horizontal modes required two (2) coax runs--one for the horizontal/one for the vertical.
Its probably capable of being done. BUT it will probably be much more complicated than the strate forward feed and much more prone to breakdown. Antenna Gremlins love complicated antennas ;-)
Might I suggest that you select one mode of operations (horizontal or vertical), whichever you use the most, and stick with just one for simplicity.
IF you do a lot of distance talking I would use horizontal--not as much man made noise.
If you do nore local talking then I would use the vertical--more noise but also more people use vertical and mobile antennas are almost always verticals.
An alternative would be to configure your antenna so that it is horizontal in orientation--for distance. With a good omni vertical, bought from our good friends at copper, for the local chit chat.
Sorry I could not be of more help.
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Rocketman
New member
Username: Rocketman
Post Number: 7
Registered: 2-2005
| | Posted on Wednesday, February 16, 2005 - 6:27 pm: |
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Thanks for that Elco guy. Probably going to set up first on vertical just to see how they perform. But later maybe roll them horizontal. Thanks for all the imput guys!!
Rocketman |
Tecnicoloco
Member
Username: Tecnicoloco
Post Number: 67
Registered: 4-2003
| | Posted on Saturday, February 19, 2005 - 1:26 pm: |
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Hi:
Thanks to Elco Guy,Tech 291 and Racer for all your input and sharing expertise and tips with us, regarding this interesting subject about "Quad Antennas".
Mine is bellow 2.1 from 26.000 to 28.000,seems very Broadbanded to me comparing this readings against my old 3 Element Beam.
I set up mine Horizontal for long distance and seems to do a pretty good job for DX.
Once again I want to say thanks to you all who share the knowlegde and answer my questions.
(You been a "A+++" for help,keep the good work)
Tecnicoloco
CEF-117
Puerto Peņasco,Sonora,Mexico.
de vy 73 |
