Posted by
k6dgw on
Jan 18, 2021; 11:39pm
URL: http://elecraft.85.s1.nabble.com/Efficiency-of-MFJ-remotely-tuned-loop-antennas-tp7667578p7667584.html
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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