Topband: Small loop info

[Tom Rauch]

> 10.1 MHz:   R = 40 milliohms (25 theory); Eff = 18%

That's similar efficiency to what the MFJ loop does on 10
MHz.

> Note that by 10.1 MHz, the efficiency of the 1-meter loop
is rising rapidly,
> as the loop begins to approach the limits of "electrically
small." At this
> frequency, the loop has "folded dipole" mode radiation
that is just 12 dB
> down from the "magnetic dipole" or loop mode.

The thing that drives radiation resistance low is that
radiation from any point on the loop is nearly out of phase
with radiation from the adjacent point. As the loop gets
larger radiation interference moves further away from 180
degrees, so the antenna radiates "easier".

I don't know I would call that a "folded dipole mode" since
the behavior is much different. In a folded dipole current
is in phase in each conductor. With very close conductor
spacing, radiation is also in phase from each conductor in a
folded dipole. Radiation from the close spaced conductors
doesn't "fight" or conflict at any point in space. That
makes radiation resistance the same as an ordinary dipole
the same physical size.

This just doesn't happen in a loop, so I think claiming it
has a "folded dipole mode" is a very poor choice of words.

What I found measuring loop antennas while working on the
MFJ design was the capacitor is often much poorer than
people think, and the Q of the capacitor increases rapidly
as the capacitance required decreases. This accents the
theoretical advantage an increase in loop size has, and the
disadvantage that occurs as the loop is made smaller.

I think Belrose VE2CV is working through disagreements with
the folded dipole mode claims, so it will be interesting to
watch the folded dipole claim unfold (pun intended) over the
next few years.

73 Tom