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'Gibson Girl' Crystal Radio
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Author:  Peter W. [ 08 Sep 2016, 07:10 ]
Post subject:  Re: 'Gibson Girl' Crystal Radio

Geek wrote:
I take it you've never built a FM colinear or J-pole? ;)


As it happens, several. But, primarily for transmitting purposes where antenna length, sections and orientation are critical. But the original statement still stands. If one cuts an antenna to the ideal proportions for the transmitted frequency, it will optimize reception on that frequency. But without amplification, the gain is limited such that "bigger" is not better.

Well.......... I should not write that as an absolute - if one has the means and space to do a full-wave optimization, there will be more gain, true. And that would be "Bigger" certainly. I have a 1/4-wave Dipole for cut for transmitting local FM stereo - and it does nicely, vertically oriented. Used as a receiving antenna (horizontal), it sucks in about anything at the low end of the band, and drops off seriously above about 94 uHz.

Author:  Geek [ 08 Sep 2016, 13:25 ]
Post subject:  Re: 'Gibson Girl' Crystal Radio

When we did antenna research at Antares Antennas back in the 80's, there was a point of diminishing returns (6 sections), but given I did not state a length, makes me wonder how long you thought I was thinking.

As long as you have a resonant multiple with proper phasing, you will attain 50 ohms +/- j0 at resonance, with 45-degree ground planes (36 ohms otherwise). The bandwidth will depend on Q factor and will be reduced as the resonant length increases (acting like multiple resonators), so you have to use a multi section (many go off to a Yagi here), or increse the diameter of the conductor. Windloading soars here, but we did community broadcast antennas out of 6" copper, simply because as the bandplan chaged for the congested area, their "community license" demanded they have the ability to QSY. (they never had to).

This isn't transmitting though, but receiving. A little more leeway is available, as most receivers are anything BUT 50 ohms input. Resonance is resonance and as an avid FM DX'er, there's more to life than a dipole.


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