All the antennas you say you use are E-field antennas, they couple to
the E-component of the EM wave [which includes your horizontal loop].
Mag loops couple to the H-component. Away from the antenna [far-field],
the EM-wave has both E and H components orthogonal to each other, and it
doesn't matter how the wave was launched.
Antenna efficiency is complex. E-field antennas are strongly affected
by surroundings, imbalance, and the like because objects in their
environment have varying conductivities which contributes to their
inefficiencies. Mag loops are mostly insensitive to surroundings
because most of the objects [including me sitting under my Alexloop]
exhibit a permeability of 1.0 or very close [ferromagnetic metal
buildings, towers, and railroad tracks excepted].
The radiation resistance of most E-field antennas that hams use is
fairly high compared to the loss resistance in the wires. Radiation
resistance for mag loops is very low, usually measured in the milliohm
range, and in the general range of the loss resistance in the loop
conductor. Thus, loop resistance is a major factor in efficiency or the
lack of it.
In QRP field settings, my Alexloop [H-field] regularly outperforms the
E-field Buddipole I had in all the configurations I tried for the BP.
The resonant loop has a very high Q and suffers from very narrow
bandwidth [4-10 KHz depending on the band] and must be very carefully
adjusted after each QSY. On balance, my loop is quieter than the
end-fed I also use outside and it has a pronounced null orthogonal to
the loop plane which can be useful. The KAT2 takes care of matching the
end-fed, it's bypassed for the loop and tuning is critical.
As always, life is a series of trade-offs. I have not found any
correlation between the solar cycle and antenna choice. [:-)
You mentioned a KXPA100: Beware, mag loops exhibit some surprisingly
high voltages and circulating currents, even at 10-15 W. There's a
nifty calculator at
www.66pacific.com/calculators/small_tx_loop_calc.aspx
73,
Fred K6DGW
- Sparks NV DM09dn
- Northern California Contest Club
- CU in the Cal QSO Party 7-8 Oct 2017
- www.cqp.org
On 11/12/2016 11:23 AM, Terry Brown wrote:
I am looking for some feedback from those using magnetic loop antennas with
the current band conditions. I use dipoles or end fed random wires when I
am in the field with my KX3. When I am at my home, I use a horizontal loop
that is a close to a full wavelength on 80, and a 10-15-20 meter beam. In
the field I a running max 15w with my KX3 and at home 100W with my KXPA100.
I know that mag loops are not as efficient as a dipole and will never
receive or transmit as well because of their compact size as my field
antennas. I have been toying with the idea of building/buying one for times
when I need a compact antenna for 40-10 meters.
When I have looked at YouTube videos of folks using magnetic loops, they are
all from a time when band conditions were far better than they are now.
Can any of you current magnetic loop antenna users in the group tell me how
they are performing now and what is a realistic expectation as the cycle
continues to drop. I should also say that I am almost exclusively a CW
operator. I can envision using a mag loop from hotel rooms or other travel
times when I can't get a long wire or a dipole in a tree.
Thanks,
Terry, N7TB
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