[Elecraft] Efficiency of MFJ remotely-tuned loop antennas

[Fred Jensen]

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