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Dual band folded dipole (even lambdas)

 
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Uberhood
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Joined: 05 Oct 2014
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PostPosted: Sun Oct 05, 2014 8:49 am    Post subject: Dual band folded dipole (even lambdas) Reply with quote

Basic understanding of the dipole antenna, also explained by antenna theory in this video: https://www.youtube.com/watch?v=RF5r64fmFhU&t=7m
shows the standard dipole will radiate at odd multiples of 0.5 lambda.
I want to create an antenna which does the even multiples of lambda (0.5, 1.0, 2.0 Lambda etc.) which of course means moving the feed points out of the middle of the antenna.
To clarify I've drawn a simple picture:

With the center fed dipole you can receive 0.5 and 1.5 lambda signals (in green and blue), what I want to do is receive 0.5 and 1.0 lambda signals (in green and red). In the above video this was explained with an open dipole, what I want to do is the same but then for a folded dipole. Can I shift the center feed point in the folded dipole design just as in the open dipole design? Can I still use a 4:1 balun (due to the impedance of a folded dipole being 300 ohm)?

If anybody knows please help.
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admin
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Joined: 03 Jan 2007
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PostPosted: Sun Oct 05, 2014 6:19 pm    Post subject: Reply with quote

I'm not sure. The argument for the regular dipole antenna works because it is open circuited on the ends, so the current must be zero at the end, independent of the feed point.

For the folded dipole, which is also a loop antenna, the same principle doesn't apply. If the folded dipole loop was closer to a circle (as opposed to a thin rectangle), then moving the feed would do nothing.

You might have to simulate or mock it up to see
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jacson33
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PostPosted: Fri Mar 13, 2015 4:47 am    Post subject: Reply with quote

For the folded dipole, which is also a loop antenna, the same principle doesn't apply. If the folded dipole loop was closer to a circle (as opposed to a thin rectangle), then moving the feed would do nothing.

Pass4sure ccnp route
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E Kafeman
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Joined: 04 Feb 2013
Posts: 25
Location: Sweden

PostPosted: Fri Mar 13, 2015 7:16 am    Post subject: Reply with quote

Dipole which is resonant for both lambda 0.5 and 1 is possible.
It can be done several ways but most common is with serial coils in the middle of each monopole. Compare cellphone monopole antennas mounted at roof of cars, resonant at both GSM 900 and 1800 (EU).

For an uncrippled dipole antenna, feeding-point does not affect antenna resonant frequency but correct impedance matching is more complicated if feed of center.

Often forgotten, but for an ideal 0.5 lambda dipole is its impedance 73 + j42 Ohm at center feed. Reactive part should also be taken care of.
Antenna structure is balanced, it cant be feed directly with an unbalanced coaxial cable without affecting radiation pattern.
Ideal dipole impedance at 1.0 and 1.5 lambda is not same as for 0.5 lambda. A folded dipole is no advantage in that matter.

Change dipole shape to an L or U and more resonant modes can be found. Compare fractal dipoles. It do however reduce main resonant frequency antenna efficiency.

A simpler way to make an dipole multiresonant is to use reradiators, feed or passive,
An example is a logperiodic antenna, which can have stable impedance over a wide frequency range.
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