Topband: magnetic loop receiving antennas - summary (LONG)
Ted Roycraft
roycraft@research.att.com
Tue, 28 Nov 2000 14:25:04 -0500
Many thanks to all those who replied to my question about using
a ferrite core loop as a receiving antenna. The consensus was
that the ferrite core loop was an expensive inferior solution to
a small, cheap loop as in the ARRL Antenna Handbook. I've quoted
below from some of the responses that I've received (stripping off
names just in case anyone would like to remain anonymous).
Ted, W2ZK
Summary of responses ...
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I think an overlooked option for the real estate challenged topbander
is small vertical arrays. I built a 4-sq out of base loaded 32'
verticals and the RX performance is as good as a full size one. I give
up quite a bit on TX, but thats the way it is. If I had even less
space, I would try the mini 4-square from ON4UN's book. I would think
you could use almost anything conductive, for cosmetic purposes, and use
almost any height, as long as it was phased properly for directivity.
An RX only array wouldn't demand much of a ground system, and should
give you performance in 4 switchable directions equivalent to 1000'
beverages.
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I have not tried this type of RX antenna BUT I do have similar land locked
restrictions.
I have tried almost every type of RX antenna; Beverages; small rotatable
loops; snakes;
EWE. K9AY; K6STI ....and the winner for small lot; rocky soil is-> the FLAG
antenna as designed by Earl, K6SE. It takes up a minimum of space and its
performance is very close to my 585 ft. Beverage. [don't ask how I sneak a
Beverage in]. Oh, the cost is quite low also. If you wind the transformer
yourself it is less than $ 20.00. You can buy superb already made boxes from
K1FZ, Bruce Clark. George, K0FF makes good looking parts also. My parts are
homebrew. I have a friend who has good results using parts from both
sources listed above. I have seen their finished products and can vouch for
their quality. No cheap or under rated parts are used in either. I just
happened to make my transformer before either concern offered finished
product.
I use a pair of 1/4 wave phased inverted Ls, up in trees and spaced an 1/8
wave apart, for transmit. Rarely do the Ls have low noise as we know.
The FLAG and Pennant are chronicled in a QST article this year and there
have been several compendium web sites set up about them. The
Topband@contesting.com archive has all the emails. Contesting.com I believe
has links to the websites too.
The stuff works. After being active on 160 for 10 seasons, I have worked
203 countries.
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FWIW, I have one of the Palomar units from my days at Ham Radio Magazine. I
was never impressed with it's performance on 160 or the lower bands 300-500
KHz.
I'd build the W1FB loop from the Antenna Handbook or a K9AY EWE derivative
instead. Total cost less than $10.
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try a 20 Ft coax loop. It should tune broadly using a BC vari cap
and doesnt take up much space. As a shielded loop it is a quiet antenna
at my QTH. Depending on the rig it will need a preamp
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Ted, everything in life depends... If you have a large backyard, you could
think of a Ewe or a flag/pennant.... the problem here is that you need one
for each major direction you expect incoming signals...
A loop, ranging from a couple of feet across to fifteen or twenty feet
across may serve you better... The smaller loops can be rotated, either by
hand in the shack, or larger ones outside on a tv rotator... Ground mounted
is fine...
Temporary beverages can be put out at night and taken up in the morning -
run along sidewalks, in the street gutter, across the neighbors yard, etc...
<Caution: Some stealthy assembly may be required>
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Ted - I wouldnt waste my money on a ferrite rod antenna like that.. Try the
LOOP in the last few editions of ARRL Antenna Handbook OR one of the snake
antennas in the newest edition of ON4UN's book - kind of like a small (150')
beverage but it lies directly on the ground ... And you can make it endfire
from the feed point or away from the feedpoint so all you need to do is run
it on the ground in the directions of interest..
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How big is your suburban lot? My lot is 40 x 120, the 120 runs
east-west. I just installed a new rx antenna here using 7 slinkys and a
9:1 (450-ohm) matching xfmr. These 7 slinkys equals 454 ft of wire.
This thing is absolutely amazing on 80m without a preamp. It was built
to use on 160 with a I.C.E. 20db preamp, but I found it to work well on
80 also. There are others that uses this design. Joe K8JP has two of
them, John W8WEJ use it, and Sidney K3SX used this design for years.
The other night during the contest I had a S-9 noise level on 80 and was
trying to copy HZ1AB, there was NO way that I could hear him on my Inv-L,
when I switched to the Slinky Beverage I could hear him, the same with
YJ0PD the other morning.
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I noticed that you are located in about the same situation I am here
in Florida. I am presently using a TS 850 and my noise level was quite
high,usually over S 9. I first had to determine whether or not is was
actual surburban noise ,line noise OR VCO noise generated inside my
reciever. As it turned out it was a combination of all three. The worst
offender however was the VCO noise generated in the 850. This s a
resuld of many stupid things done by Kenwood in there design process.
Everything below 9 mcs is phase lock loop technology. Not even the good
stuff. This frequency control is so bad that in combination with the
wide open broad banded front end allows any rtadio station ,tv station
or other shortwave stations to be picked up throught the front end of
the radio. It sounds very much like lines noise or an elevated noise
floor in the reciever. I tried everything from seperate listening
antennas to extrermey high "Q" transmit receive antennas. Nothing
helped. Just more of the same to a lesser proportion. I got a MFJ 1026
and hooked it up to a 1/4 wave low "Q" sense antenaa and that helped a
great deal but I still had the elevated noise floor. Then I remember
what a friend of mine told me a couple of years ago. When I used to SWL
with a really poor reciever I would put a "PRESELECTOR" in front of the
receiver and this would take care of front end overload,IMD problems and
lower the noise floor increase the dynamic range of the poor reciever
making it pretty respectable. I guess what I am trying to tell you is
you might want to try on of those MFJ 1048 preselectors. I dropped
mynoise level from an S 9 to around an S 6 with out much loss on the top
end(maybe 6 to 8 db-maybe 1 s unit). This increased my dynamic range by
nearly 20 db giving me the ability to hear things that were in the noise
before. I am not an advocate of MFJ.There are several others out there
that are probably much better but the price was right and It work for
me. If you don't have a radio with like a 1000 d or an MP or something
with the newer DDS phase lock loop and IF DSP ,This is probably what the
doctor ordered. You are suffer with front end overload just as I was
from all the garbage coming in via the RF route to your radio. Hope I
have given you some Ideas. Good luck and 73's
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Remember, Dr. Beverages's grad assistants did the original research with a
wire on the ground running along side of a boardwalk at the seaside...
Beverages work laying on the ground... Even, brown/green, insulated, #22
hookup wire running through the grass... :)
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My opinion is that a ferrite rod loop is a waste of money if aim is DX.
The capture area is too small to have a decently efficient antenna.
Bulid instead a classic tubing loop (at least 7ft diameter) with an 1/5
diam. coupling loop (i.e. RG213).
A 7/8" coax works nicely as a "tubing".
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There is a lot of mythology surrounding "magnetic antennas". One
thing that's often overlooked is if the antenna has a dominant
magnetic induction field near the antenna, a small distance away it
becomes electric field dominant (about 1/10th wl or so distant). A
small electric field dominant antenna becomes magnetic field
dominant in about the same distance.
After about 1/2 wl or so away with a small antenna, you can't
possibly tell one antenna from another. An "electric-field probe" or
a "magnetic loop" would all look exactly the same so far as fields
go.
The same applies to noise sources. There is nothing that makes
noise sources dominant in electric induction field coupling or
magnetic induction field coupling near the source. It all depends on
what the noise radiates from (what the noise source's "antenna" is)
and how far you are from that noise source.
With all that in mind, it is just the luck of the draw if you happen to
improve things with a small loop because it is a so-called
"magnetic antenna". Outside of those induction field effects, which
only dominate very very close to the antenna (within say 100 feet or
so for a small 160 antenna), only the pattern will make a difference
in S/N ratio. Small rod antennas, even "large" small rod antennas,
are not very directive at all and likely won't help you at all unless
your noise is coming almost entirely from one very narrow direction
that the rod can null.
There's nothing wrong with experimenting, but unless your situation
is "just right" a small poorly-directional antenna won't do anything
for you.
Now......as for all that expensive ferrite...all it buys you is a bit
more sensitivity. You can probably get down into external noise
floor with a small single rod about a foot or less long and a really
good amplifier. You might be better off spending the money on a
really low noise amplifier, instead of all that ferrite.
Better yet, build a conventional loop. It'll do the same thing for a lot
less money, unless you plan on having it in the room with you and
need small size.
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Here we tried the "KC2TX Shielded Loop" with some success. See
www.qsl.net/kc2tx for a picture. His original 20 foot circumference was
too much to fit inside the shack so I cut it down to a total of 12
feet. Made it into an octagon (8 sides * 18 inches), used 1/2 inch PVC
irrigation pipe with 45 degree elbows at the 8 corners. The tuning
capacitance is more like 500mmf because of the smaller size. (I don't
have any business interest whatsoever).
Results are good, not spectacular. We are able to copy distant stations
down in the noise, not because the loop is positive directional, but
because it seems to null noise sources. It is better (once you know
where you're looking) than receiving on the Inverted L.
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Ted....have you considered an air core loop, say 3'-5' diameter or square? I
can send you the plans for one that I built for BCB DXing as well as LF
beacon DXing. If you have the room and a few simple tools you can build one
of these things for peanuts and it will work as good as your ferrite loop.
If you make it large enough, no preamp will be required either.
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Thanks again to all those who responded. It looks like a small
loop is the first thing to try.
73, Ted, W2ZK
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