Post Number: 10238
|Posted on Friday, June 09, 2006 - 7:32 pm: ||
I created this Topic to help Andy / Tecnicoloco / CEF#117 ~ CVC#71 with an Antenna Question/Problem he has.
Post Number: 71
|Posted on Friday, June 09, 2006 - 7:42 pm: ||
Hi tech833 or any Tech:
I have a Cubex MAGNUM-2CB 11 Meter 2 Element Quad,that comes with a piece of RG-11 coax to match the resistance.
The Specification's are:
9+ dBi gain @ 40 FT.; F/B >23dB
1 MHz Bandwidth - 2:1 VSWR
Rugged fiberglass spreader arm construction
2" Aluminum Boom, 59" long
1/4w 75 ohm Matching Section included
All weather construction
Handles full Legal Limit Power
Small size, big performance; weighs just 8 lbs.
Turning radius - 7.2 ft
I call Cubex to get info on how to adjust the antenna for 27.500,and they didn't support the idea.
I have a 1.5 SWR on 27.405 and want to lower it on frecuencies above ch.40.
What will be the proper way to adjust the SWR on this antenna with a standard swr meter??
Thanks in advance for the help
Post Number: 1255
|Posted on Friday, June 09, 2006 - 8:52 pm: ||
You'd have to make the elements a bit smaller and trim the matching stub to the proper length. Mechanically I don't know how that would need to be done with this specific antenna.
Post Number: 674
|Posted on Saturday, June 10, 2006 - 12:54 pm: ||
Tecnicoloco, the specs show a 1 mHz bandwidth below 2:1. This is likely centered near the middle of 11 meters as it is cut to CB band. Admittedly this is a bit narrow, but you might be seeing very good gain set this way, all-be-it at the expense of some rejection to the back.
So this is close enough that it should work with an SWR well below 2:1, but maybe you want to work regular on your rig even higher in the band. Looks like to me you should be close to 26.705 - 27.700 bandwidth and this tells me that your curve from 27.405 to 27.500 is 1/2 of the difference out to the 2.1 edge of the band. What is wrong with that SWR?
Sometimes a beam's curve is not smooth or balanced above and below center for some reason, and that may be your problem. You will have to tell us more, so run a graph, SWR bandwith curve, and tell us the results, OK?
What kind of problem do you think this small SWR difference between 27.405 and 27.500 will cause?
The RG-11 device is a tuned element and that along with the wires on both elements will have to be shortened an approporiate amount to remain resonant and show a good match. If your knowledge regarding this task is in question, then you might just mess up this beam. Maybe the guy at Cubex didn't want to have to deal with a potential later on after it was messed up.
The mathematical difference is 1.1% shorter for the wires which equals about 4.5" to 4.75" shorter to resonance. This has to be a guess on my part, because I have no idea what the overall length of your driven element is and maybe you don't either at this point. What that length is, is very important to the antennas working correctly. T
he RG-11 tuner will also have to be tuned by triming and that too may be calculated just to get close, but there are other variables that work on the tuner and thus it is better to tune and trim. It might be necessary to cut and tune the wires also. This can be a lot of work and maybe you are not up to that.
A good antenna tuner and the knowledge to use it effectively will get you where you want to go and do a real good job as well. You are very close to where you want to be, why not just invest in a good tuner (not a CB model) and be done with it.
Post Number: 1379
|Posted on Monday, June 12, 2006 - 11:36 am: ||
Cubex makes an excellent antenna. Part of the performance they acheive is through a very high Q feed design. This translates into a VERY narrow bandwidth. Having a 1.5 SWR is perfectly fine! Don't touch a thing!!
If you do want to tune the antenna for higher frequencies, you need to trim the driven element and the RG-11 transformer. I strongly suggest you do NOT do this.
Unfortunately, Cubex does have very poor customer support, so I doubt you will get them to satisfactorily respond to your request. However, I am positive they will tell you that your SWR reading is certainly below the 'normal' range for this type of antenna. Just use it as-is and enjoy wonderful performance.
Post Number: 72
|Posted on Monday, June 12, 2006 - 6:32 pm: ||
Thanks to all that came to help me with suggestions and tips.
I was under the impresion that I could improve the performance of this Antenna,but I promess to live it alone.
The antenna works good on skip,but wanted it to achieve a better swr for higher frecuency.
With all your comments and the Autority on Antennas Tech833,I could not fine better support.
Post Number: 1380
|Posted on Monday, June 19, 2006 - 11:06 am: ||
Keep in mind- a 1.5:1 SWR is only 4% reflected power. That is NOTHING! Please do not worry until the SWR reaches 3:1 (that is 25% reflected power).