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Message |
    
Stepchild
Intermediate Member
Username: Stepchild
Post Number: 176
Registered: 11-2002
| | Posted on Thursday, December 25, 2008 - 11:57 pm: | 
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Just a few questions I've had for a while.
How do you know were your antenna is resonating?
Do you have to have an analyzer to know?
Do you still use a 3ft jumper to check swr's at the feed point?
Stepchild
CEF-187
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Marconi
Advanced Member
Username: Marconi
Post Number: 798
Registered: 11-2001
| | Posted on Friday, December 26, 2008 - 4:01 am: |
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Stepchild, you really need a grid dip meter to best tell the true resonant frequency of any radiator. It is also important to know how to work such a meter and understand what it is doing and telling you in the results. Maybe this was not your real question however.
That aside, you can get close enough generally using an SWR meter. An analyzer with the typical 11 meter commercially made antennas will tell you more about a particular match, but it too can give some skewed results that need understanding. Both these devices are working with complex values at the feed point and the values for true resonance and the values for a good SWR are not necessarily synonymous. When a length of feed line is involved you may also see transformation factors that can affect results adversely.
With an analyzer for example, you might find a good SWR reading at some setting of the matching device in combination with different radiator lengths, while the best resistive match (50 ohms typical) occurs at one frequency (ex:channel 40) while the best reactive part of the match (resonance) is at another frequency (ex:channel 2) several channels away. This is not to suggest this particular system will or will not work well however, but true resonance is a very specific characteristic. What we see here with such results, using these meter, can be considered technical compromises.
Luck thing about 11 meter stuff is that it is designed and built to work only in or near the center of 11 meters, so at least we are always real close. When you get into trouble and need to really understanding is when you are building something from scratch or from some model you find or hear about.
Use your SWR meter to build a bandwidth graph and if the bandwidth produced shows a reasonable range, and the curve produced is relatively smooth and bowl shaped, then you should be pretty close to what you working center frequency is at the lowest SWR.
A 3 ft jumper will generally do fine if your antenna is tuned close to where you wish to work (generally the case), but understand that any length of feed line can reflect transformation qualities into the results from these meters. The more the mismatch that exists the more this coax transformation will influence the results, even with 3 feet. |
Stepchild
Intermediate Member
Username: Stepchild
Post Number: 177
Registered: 11-2002
| | Posted on Friday, December 26, 2008 - 6:35 pm: |
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Thanks Marconi,I usually spend a couple hours a day just reading post and some times It really gets you to thinking,maybe a little to much LOL ! Thanks again
Stepchild
CEF-187
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Bruce
Senior Member
Username: Bruce
Post Number: 4918
Registered: 9-2003
| | Posted on Friday, December 26, 2008 - 7:01 pm: |
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I use a noise bridge but have a millen 90672 antenna bridge to use with my grid dip meter also .... works for me .....
Old stuff but does the job
On 6 since 66
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Marconi
Advanced Member
Username: Marconi
Post Number: 799
Registered: 11-2001
| | Posted on Friday, December 26, 2008 - 7:45 pm: |
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That's OK Stepchild, I'll try to remember that. |
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