Certainly don't want to throw away power in wasted heat, but when I turn down
my 200W rig to 5W for QRP, it's still useful and that's 16 dB down.
With ant restrictions, I'm looking at building a small horizontal loop for 6m.
It'd be a little more than 4' in circumference, 16" dia, and the capacitor
could be be two 4 cm diameter plates ~2mm apart. That should be reasonably easy
to make relatively efficent as there are not a lot of plates that need to be
connected with very low resistance. I calculate ~85% with 1/2" copper. Should
be good for 200W. I'd orient it horizontally for horizontal polarization (weak
sigs) and local noise rejection.
I have a squalo made from a lawn chair, but technically that's not a small loop
and a little big to be stealth.
On Tuesday, January 19, 2021, 3:58:09 PM MST, Alan Bloom <n...@sonic.net>
wrote:
I've been convinced that AEA's specification of 72% efficiency for the
Isoloop at 14 MHz is too high, certainly too high for the MFJ antennas.
So I re-did the calculations using the 59% efficiency figure calculated
below. The new results can be downloaded here:
https://www.dropbox.com/s/ve1v49b3gjvmt64/MFJ-1786-1788_2.pdf?dl=0
If you don't want to download the (1-page) document with the pretty
graph, here's a synopsis of the results:
Freq Eff. Gain with respect to a 1/2-wavelength dipole
MHz dB dBd
7.0 -9.5 -9.9
10.1 -5.1 -5.5
14.0 -2.3 -2.7
18.068 -1.1 -1.5
21.0 -0.7 -1.1
24.89 -0.4 -.8
28.0 -0.26 -0.65
My basic conclusions still stand. With almost minus 10 dBd of gain on 7
MHz, the 40 meter coverage of the MFJ-1788 doesn't seem very useful.
That is confirmed by some of the reviews I have seen. I think you'd get
better results by just loading up the coax feedline as a random-wire
antenna with a tuner.
The 10 MHz performance is a little better. Good enough to at least
allow you to get on the 30 meter band.
On the higher bands, the gain is within less than 3 dB of a full-sized
dipole, which seems a useful trade-off for its small size and wide-band
continuous coverage.
Disclaimer: Again, I have never seen one of these things so this is all
based on theory and on the many reviews I have read. Even if my figures
are off a bit, at least this gives an idea of the relative performance
on the various bands.
Alan N1AL
On 1/18/2021 5:38 PM, Alan Bloom wrote:
> Well let's see...
>
> Radiation resistance of a small loop is 31,171 * (Area / wavelength^2)^2
>
> For a loop with a 91cm diameter at 14 MHz, I believe that comes out to
> 0.064 ohms.
>
> Assuming the loss is due to the RF resistance of the loop:
>
> From the internet I get the volume resistivity and skin depth for 6063
> aluminum is 0.03 microohms-meter and 23.3 micrometers respectively, so
> the surface resistivity is 0.03/23.3 = 0.0013 ohms per square. The
> outside circumference of the tubing is PI * 1.05" = 3.3" and the loop
> length is PI * 36" = 113" so the loss resistance is .0013 * 113/3.3 =
> 0.045 ohms.
>
> So I calculate an efficiency of 0.064 / (0.064 + 0.045) = 59%
>
> So worse than AEA claimed, but in the ballpark.
>
> Alan N1AL
>
>
>
>
> On 1/18/2021 3:39 PM, Wayne Burdick wrote:
>> Hi Alan,
>>
>> 72% sounds a bit high. Is this number based on loop size alone ("in
>> theory")? Or are they taking conductor geometry and other losses into
>> account?
>>
>> Wayne
>> N6KR
>>
>>
>>> On Jan 18, 2021, at 2:05 PM, Alan Bloom <n...@sonic.net> 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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