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Here, in VU-land, 6M allocation is from 50 MHz through 54 MHz... which is a fair chunk of spectrum.

If I think about a half-dipole antenna; between 50/54 MHz it's length varies by over a foot ... and I can't imagine an antenna with a 4MHz bandwidth. Is it possible to work on 6M with a simple dipole? What frequency should it be centred upon?

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Yes, a dipole for 50 MHz is most definitely possible.

Your 6 meter band is 52 ± 2 MHz. 2/52 is just over 0.038. This is the frequency variance involved: how much, in terms of a center frequency, does the frequency vary the most while remaining within the band edges. You could make this worse by cutting the antenna for one band edge and using it at another, but it doesn't get that much worse even then.

For comparison, assume 80 meters is 3.5 to 3.8 MHz. That's 3.65 ± 0.15 MHz, where 0.15/3.65 is just over 0.041. If you were to cut an antenna for 3.7 MHz (good for 80 meters SSB) and use it toward the lower end of the band, this effect becomes even more pronounced.

In terms of frequency variance between the band edges, 80 meters is actually worse, despite being far narrower in absolute terms.

Or consider 23 cm; where I'm at, the allocation there is 1240 through 1300 MHz -- that's a good 60 MHz wide swath of spectrum. (60 MHz isn't even an absurdly wide signal in the microwave range.) However, 30/1270 is less than 0.024, so in terms of frequency variance, this band is actually far narrower than is 80 meters.

The antenna doesn't care about the absolute frequency of your transmission. Ignoring any nearby obstacles that could throw off the tuning, antenna matching depends entirely on the error in terms of wavelengths.

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    $\begingroup$ I made a chart of fractional bandwidths which might help visualize what you're talking about. $\endgroup$ – Kevin Reid AG6YO Jan 10 '16 at 19:10
  • $\begingroup$ @KevinReidAG6YO Interesting. That almost, but not quite, makes this question a duplicate... $\endgroup$ – user Jan 10 '16 at 19:39
  • $\begingroup$ I wouldn't say so. That question says nothing about the feasibility of single antennas. In fact, I think your answer here could be improved by discussing that in particular, e.g. typical antenna bandwidths. $\endgroup$ – Kevin Reid AG6YO Jan 10 '16 at 19:42
  • $\begingroup$ You're correct that it's the percentage bandwidth that matters. A dipole is only resonant at one particular frequency. Any other frequency will give you a higher SWR that will be harder for your transmitter to couple to. If you have an antenna tuner, you can compensate for this up to maybe 3:1 SWR without too much loss. Most transmitters will work OK up to about 2:1 SWR without a tuner. The other thing is to increase the antenna's bandwidth by lowering it's "Q". You can do this by using thicker conductors or by making a folded dipole, etc. $\endgroup$ – Martin Ewing AA6E Jan 18 '16 at 22:10

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