Alan,
I think 72% is really optimistic for both the AEA Isoloop and the MFJ.
For a 3/4" copper tubing loop 3 ft in diameter, at 14 MHz, I come up with:
Antenna efficiency: 38% (-4.2 dB below 100%)
Antenna bandwidth: 31.5 kHz
Tuning Capacitance: 85 pF
Capacitor voltage: 2,445 volts RMS
Resonant circulating current: 18.2 A
Radiation resistance: 0.058 ohms
Loss Resistance: 0.093 ohms
Inductance: 1.53 microhenrys
Inductive Reactance: 134 ohms
Quality Factor (Q): 445
Distributed capacity: 8 pF
TX power was arbitrarily set at 100 W. The conductivity of Cu is about
59E10 Se vs 38E10 Se for Al and I once saw an analysis that showed
silver soldering lengths of copper pipe and 45 deg fittings into an
octagon raised the loss by about 20 % @ 14 MHz. The best configuration
seems to be a continuous copper strip [wider is better within reason].
There will be some additional loss in the connection to the capacitor.
Since the radiation resistance is in the tens of milliohms range, this
is a case where antenna DX wax might actually work.😉
I have an Alexloop I use with my K2. It works, sort of, best on 20 and
up. One caveat to any small mag loop: They are essentially resonant
transformers and the operative word is "resonant." Any ATU must be
bypassed and the loop tuned exactly to resonance [R+j0X]. The swr is a
function of the area ratio between the large loop and the small driven
loop. Letting the ATU "find the match" results in a very good
approximation to a dummy load.
The above calcs came from
www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx
which I like, there are a number of others out there. I distrust the BW
number, my Alexloop has about 60% of the calculated value before it
assumes the shape of a dummy load.
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
On 1/18/2021 2:05 PM, Alan Bloom wrote:
MFJ makes a pair of small, remotely-tuned loop antennas, the MFJ-1786
that covers 10-30 MHz and the MFJ-1788 that covers 7 to 21+ MHz. As
far as I can tell, the two antennas are identical except for the size
of the tuning capacitor. Each consists of a 3 foot (91 cm) diameter
loop made of aluminum tubing and a plastic housing that contains the
tuning capacitor, motor, and coupling loop. No control cable is
required since the control voltage is sent from the control box in the
shack to the motor in the antenna via the coaxial cable.
Before I purchase one of these I wanted to get an idea of the
efficiency of such a small loop. MFJ is silent on the subject so I
did my own calculations. The calculations and results are on a 1-page
document that I uploaded to Dropbox and can be downloaded here:
https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
My calculations are based on the assumption that the efficiency of the
MFJ antennas is similar to the (no longer manufactured) AEA Isoloop
(my reasoning for that is in the document) and that AEA's
specification of 72% efficiency at 14 MHz is correct. From that
number I can calculate the efficiency and gain on all the other bands.
If you don't want to download the document, here is a summary of the
results:
Freq Eff Gain with respect to a half-wave dipole
MHz dB dBd
7.0 -7.3 -7.7
10.1 -3.5 -3.9
14.0 -1.4 -1.8
18.068 -0.6 -1.0
21.0 -0.4 -0.8
24.89 -0.2 -0.6
28.0 -0.15 -0.5
I'd be interested in any comments people may have on the accuracy of
my assumptions and calculations in the document.
Alan N1AL
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