Re: ANTUINO antenna analzer CERTAIN CLARIFICATIONS REG


Peter LB0K
 

Muthu - Checking antennas can be confusing.
An antenna only achieves its theoretical impedance and size when in free space, but we don't have long enough feeders for that to ever happen.
In real life we can get a close fit to the theoretical numbers when the dipole antenna is mounted at one half wavelength above ground, whatever 'ground' is. Also at one whole wavelength above ground, about 73 ohms.
One illustration is the graph of 'impedance variation at height over ground' given in the ARRL handbooks and antenna books.
In most of our real situations these sorts of spacing are very difficult to achieve, so we compromise and probably don't get them over a 1/4 wave of 10m or 5m, as in your case. Therefore the 40m section and probably the 20m sections won't achieve anything like 73ohm maybe 50ohm, or under.
These effects pull down the resonance frequency(s) but your antenna doesn't swing on 40m, nor on 20m, indicating that it is too short on both.
As G4NFR suggests make a new one say 5-10% too long, or add on short pieces to similar lengths.

As a rough starting point a 40m dipole will need about 20.5m of wire, a 20m one will need 10m.

The chosen lengths can then be adjusted by folding back the ends and taping them to the 'active portion', while you make more measurements, this way you get a good idea of how much can be easily removed before one cuts tooooo much off. Keep the lengths symmetrical.  I leave mine with a good bit folded back so that when I move the antenna it can be easily be lengthened!
One point not yet answered it that you seem to have an extra resonance in the middle, this is probably coming from one arm of the 40m dipole and an opposite arm of the 20m dipole resonating together.
But perhaps you can't get the projected length suspended above your property, don't despair, just fold the ends down or around.
If you have to cut the ends off, making it too short again, you can use an AMU (aka ATU) to achieve a match.

Now with your analyser you have an exciting time ahead seeing more of what really happens with antennas, and remember you'll see much more of the matching TO the antenna but only a little more of the matching OUT of the antenna into the æther.

Good luck!

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