that's a complete DIpole. That is then, fixed on top of a NON Radiating support Mast, or on a big pole, or your house chimney
The "Gain" of that is ZERO, (as a Dipole is usually used as a Reference, to compare other aerials "gain" against)
Zero gain means, no plus dB gain, or minus db loss
But at MW AM Frequencies, the wavelengths are so huge, that the Antenna needs to be very large as well, to work
(If you made a HALF wave for 648khz, it would be twice 360 odd feet (i.e. 720 foot odd)
So, for a Quarter Wave Antenna at 648, the tower is around 360 feet high, (as in the 5 mast array)
and then, the "missing" 360 feet below the mast, is replaced by the soil/ground earthing system (buried wires in the soil)
(Effectively It's like taking your Five foot VHF FM Aerial, digging a big hole, and burying the lower half of it in your garden)
(leaving the top half poking out as the antenna) you wouldn't expect it to work very well!
Trouble is, soil (earth) is not a good conductor, so depending on the MW AM site, (soil type, dry/wet area etc, and earthing)
quite a bit of power is wasted as heat, in the earthing/ground system, so lots of wires are buried, to improve this, costing lots
If you then, make the MW AM antenna, Even shorter than a Quarter Wave,
(Even with the same decent earthing system) even more power is wasted
(and additional coils/antenna tuners may be needed, to bring the short antenna to resonance)
This is Why the 648khz 60 meter omni (so called 200 foot) antenna, is said by Ofcom to be around 66% efficient total.
that means 34% of the power is wasted, and that's with a proper extensive earthing system, in an ideal wet area
Hope that explains the basic problems with Large antennas, Mike
Rod Watts
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