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Post by berzsenyilaszlo on Sept 3, 2009 9:54:42 GMT -5
From antenna textbooks and practical work I concluded, that the resonant frequency of a dipole (zero phase angle, zero reactance, max. return loss etc.) should coincide with the minimum SWR frequency. I've seen this on many of the AIM graphs I found on the internet, where the two frequencies were at least very close.
Unfortunately the scans I made for my 5 ele. 6 meter yagi and my W3DDZ trap dipole in an inverted Vee configuration show considerable distances between the two frequencies. The yagi is resonant on 49.68 MHz while the minimum SWR is at 51.2 MHz. The trap dipole is resonant at 5.19 MHz while it has minimum SWR points on 3,6 MHz and 7.0 MHz. (I am emailing the graphs to Bob)
I would be grateful if someone explained whether
- I am wrong in my antenna theory conclusions,
- I made mistakes in the measurement setup or at scanning,
- the difference comes from improper antenna adjustment or
- there is some other reason (e.g. the difference can be normal).
73,
Laszlo, HA5EA
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w0qe
Junior Member
Posts: 67
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Post by w0qe on Sept 5, 2009 10:21:36 GMT -5
Laszlo,
I hope you don't have an antenna text that states "minimun SWR occurs at the resonant frequency" because this statement is absolutely false. I have several texts that state exactly the opposite.
There is NO requirement that the minimum SWR occur at the same place where the antenna is resonant (reactive impedance = 0). If the antenna at resonance has the real part of the impedance near the Ref Zo (usually 50 ohms) then the resonant point will be either the minimum SWR or so close to it that you wouldn't know otherwise.
However as an example here is a table similar to a 3 element yagi without any matching:
14.0 MHz: 21 - j20 ohms (50 ohm SWR = 2.83:1)
14.1 MHz: 23 - j10 ohms (50 ohm SWR = 2.28:1)
14.2 MHz: 25 + j0 ohms (50 ohm SWR = 2.00:1)
14.3 MHz: 27 + j10 ohms (50 ohm SWR = 1.95:1)
14.4 MHz: 29 + j20 ohms (50 ohm SWR = 2.10:1)
This clearly shows the minimum SWR is at a slightly higher frequency than resonance. Changing the reference impedance used for calculating SWR will change the results but again there is no requirement that the minimum SWR and resonance occur at the same frequency.
I can't say whether you made any mistakes in your measurements but have you calibrated out the length of feedline to the antenna or are you seeing the result of the antenna plus feedline? Either way the statements above are still true.
73, Larry W0QE
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Post by smokchsr on Sept 6, 2009 5:18:04 GMT -5
Resonance is whenever the Theta line crosses zero. If at that point if the resistance of the antenna is equal to your reference impedance (Zo) then you will have your lowest SWR. For a series vertical element the first resonance usually comes at about 34 ohms. If you try to feed that point from a 50 Ohm source you will not have a good match. Thus the need for matching networks, to correct the antenna for the source impedance. Seldom will resonance occur at 50 Ohms without some assistance from some sort of matching network.
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Post by berzsenyilaszlo on Sept 7, 2009 9:49:44 GMT -5
Hi Larry,
Many thanks for the quick explanation. I did not say that my textbooks state that "minimun SWR occurs at the resonant frequency", just spoke about my conclusions which I doubted myself and which proved to be wrong now.
I did calibrate out the length of the feed-line, but as you pointed out, I missed to consider the fact that at the resonant frequency of my yagi the real part of the the Zload is only 12 ohms, which is quite far from the 50 Ohm value of Z0.
It is emotional, but will try to match the Zload on the minimum
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Post by berzsenyilaszlo on Sept 7, 2009 9:59:41 GMT -5
Hi Larry,
Many thanks for the quick explanation. I did not say that my textbooks would state that "minimun SWR occurs at the resonant frequency", only spoke about my conclusions which I doubted and which proved to be wrong. I did calibrate out the length of feedline, but missed to consider that the real part of the load impedance at the resonance frequeny of my yagi is only 12 Ohms, whic his quite far from the 50 Ohm value of Z0.
It is emotional, but will try to match the antenna on the minimum SWR frquency with a matching network as Bob has suggested it.
Will be happy to see a nice graph as the result of the new scan.
Thanks again for the help.
73,
Laszlo, HA5EA
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