According to another ham who recently posted here, he had to "tighten"
the plates on the MFJ capacitor to get it to work properly. That
doesn't sound like they are welded, and given the cost difference for
welded air variables I doubt MFJ used them.
I sincerely doubt that an actual practical small loop is only down 3 dB
from a full size antenna. That makes no sense to me at all. If that
were the case everyone would be using one, because they are not that
difficult to make ... at least for manually tuned ones.
But you seem determined to believe differently, and it's not my place to
convince you otherwise. You asked for inputs and I have made mine.
Hopefully you are right and I am wrong.
73,
Dave AB7E
On 1/18/2021 9:54 PM, Alan Bloom wrote:
> There is a reason why top quality variable capacitors often use
welded plates.
I believe they do weld the capacitor plates and also weld the loop to
the capacitor. (I don't have one, but that's what I've read.)
> Yours is a limited theoretical analysis ... not a practical one.
A number of reviews I have read (including the QST review of August
1994) have reported comparable performance to full-sized wire antennas
located on the same site. If the loop is down by, say, 3 dB, that's
only half an S unit, which would hardly be noticeable in the QSB of a
typical amateur band.
As I see it, the advantages of the MFJ-1786 10-30 MHz loop are:
- Continuous coverage on 6 amateur bands. A convenient way to cover
all the WARC bands.
- Small and light.
- Omni-directional (when mounted horizontally) so does not need a rotor.
- No control cable required - control voltage is fed through the coax.
- Narrow bandwidth provides excellent RF selectivity. Might be good
on Field Day to reduce inter-station QRM.
- Users have reported lower receiver noise compared to wire antennas.
No doubt that is because the isolated pickup loop prevents feedline
radiation/pickup.
And the disadvantages:
- Expensive ($500 list price)
- Less gain than a simple dipole (although you would theoretically
need 6 of them).
- Fiddly to tune. If you QSY too far you have to re-tune.
- MFJ quality control leaves something to be desired. (You may have
to open it up when you get it and make minor repairs.)
- You have to pay attention to the problem of entry of water and/or
bugs into the housing.
- The controller can be damaged by a DC short in the coax e.g. from an
shorting-type antenna switch. (I don't understand why MFJ didn't
include a fuse or some other way to protect the controller.)
I probably wouldn't buy the 7-21 MHz MFJ-1788 because of the poor
efficiency at 7 MHz. I think you'd have a better signal just using
the coax as a random end-fed wire (with a tuner).
Alan N1AL
On 1/18/2021 8:17 PM, David Gilbert wrote:
You are neglecting the losses in various connections in the system
... including possibly the construction of the capacitor itself. I
don't believe that they are insignificant. There is a reason why top
quality variable capacitors often use welded plates.
I would also guess that contact resistance is worse for dissimilar
materials, such as a copper wire to an aluminum tube.
Yours is a limited theoretical analysis ... not a practical one.
Dave AB7E
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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