MFJ-1788 Loop

[Michael Madden-3]

I have this loop - it's the 40M-15M version with the
tuning box.  I have not had good luck with this very
expensive ($450) antenna.

Its high Q makes tuning a challenge.  Once you move
just couple kHz either way, the SWR is very bad and it
needs retuning.  It is hard to tune if there is
anything metalic near the antenna.  This antenna is
really only good if you plan to sit on a frequency and
call CQ.

However, if you're operating QRP, you're bound to have
more success by jumping around and replying to a CQ or
tail-ending a QSO.  This kind of operation is not
really compatible with the nature of this antenna.

On the 40M and 30M, this antenna is very inefficient,
to the point of being useless.  Once you get to 20M
and above, even a short length of wire with a tuner
will do at least as well as this antenna.  And it will
cost you far less.

For a temporary/stealth setup there are much better
and cheaper solutions.  If you're QRP, something I've
found to that work well is a cheap MFJ whip, Elecraft
T1 tuner, and a counterpoise.  Very portable and
economical.

73, Mike N9OHW

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[Thomas Beltran]

 
I just ordered the 40 -15 meter loop (it will be here on the 20th).  The
reviews looked surprisingly good (of course, if it looks too good to be
true, it probably is).  I've tried various indoor antennas (such as the
Buddipole), but I have +5 over 9 noise inside.  I've tried whips out the
window, but it didn't work well.  I built a 9' in diameter loop out of #6
copper wire, that I used inside with an SGC-231 tuner.  It tuned up well and
lowered the noise to about S1 - 3, but I was unable to make any contacts.
On SSB last weekend, I was able to copy DX stations on 20 meters, so I think
it wasn't that the loop was so attenuated that the noise just sounded lower.


So after reading the reviews, I bought the MFJ loop, and it is pricey.  I
live on the second floor of a two-story apartment, and I plan on mounting it
on the roof.  In fact, because other tenants have dishes up there, I am just
going to hire a dish installer to put it up to avoid any suspicion.  But
this is only possible because it looks similar to a dish, i.e., I'm not
buying it because I have some notion that I can compete with a SteppIr up
100 feet.   It is the only antenna that I have seen that could be mounted up
on the roof in the clear.   I'm going to try to use a rotator, but I don't
think I'll push it.  If I could put up a dipole or something better, I would
do so.    

Michael,  how do you have (or had) the loop installed?  I saw one ham who
had it sitting on a balcony, under a tarp and claimed that it worked great.
However, the date of the statement is important.  The good results might
have been at the sunspot peak.  

I'll report back when I get it set up.  Tom W6EIJ


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[Thomas Beltran]

I just received the MFJ-1788 loop today, from AES.  The package looked okay
except for a small area where it sustained a hit from a blunt object, but
nothing that looked really bad.  I opened the package, and there was no
packing - the loop was suspended by two cardboard holders on each end - the
loop looked undamaged.  When I took it out, I could hear something rolling
around inside the plastic shroud, and then, a gear left in the box.  The
damage inside the plastic case was significant, and I give all this detail
to say, the box wasn't in bad shape, so the damage was surprising.


I had read that the 1788 is built very well - having looked inside, I
disagree.  Inside the widest area of the plastic shroud, is a massive
capacitor - but the plates rub.  The literature talks about the polished
capacitor plates - these look like oxidized aluminum.  The ends of the
capacitor are pc board.  Others have remarked that the shroud is somewhat,
well....cheap.  I agree.  It is a two-piece flexible plastic shroud, that is
held together with sheetmetal screws.  I think it needs something, such as
plastic tape to strengthen and seal it.

The main damage is from the motor drive for the capacitor - the motor and
gears are contained in a black plastic cylinder, about two inches in
diameter and about 2.5 inches long. This, I think, is the weak link - the
plastic is very thin, and the small motor is at the end of the cylinder,
away from the capacitor, i.e., the weight is on the extreme end of the
cylinder. One would not expect that there is not enough weight to shear-off
cylinder from the end of the capacitor if the unit were dropped, except that
the plastic is very thin, but that is what happended.  The drive gears are
very small, some metal and some plastic.  When the cylinder broke, it
severed a wire.  In that this antenna is made to use outside, I would think
it would be better built.  The loop itself is in two pieces, a bit easier to
build, I think, then a full circle, but it is very sturdy.  

I'll report back, when I get a working one.  If anyone is interested in
pictures, my email is good on qrz.com.  Tom W6EIJ

   

     





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