Loop Antenna review (long)


From: Scott Sminkey - Sustaining Eng Group (sas@opus.xyplex.com)
Date: Thu Jun 09 1994 - 16:12:29 EDT

Tom Jennings KV2X asked if anyone had built the compact loop antenna
featured in the May 1994 QST. Art NT1M and I recently finished
building two of them so I thought I'd give it a review on the list...

The executive summary is that the 30-12m compact loop antenna works
well and is a straightforward construction project. The antenna
performs well enough to serve as a primary antenna for where a
"normal" antenna cannot be used, such as an apartment or condo.
An antenna tuner must not be used with the compact loop and if built
properly is not needed anyway. The author of the QST article also
provides dimensions for a 40-20m version, but he apparently did not
build and test one. The one I built tunes and works ok on 40m, but
I'm having problems on 30m and 20m.

The May 1994 QST article is light on the theory or compact loop
antennas, and instead focuses on how to build a compact loop using
readily available materials and components. The Summer and Fall 1993
issues of Communication Quarterly contain a two-part article on the
theory and construction of a compact loop antenna. This article is
very good reading if you really want to know how a compact loop
antenna works.

Construction of the antenna is straightforward and will take a whole
afternoon to complete. PVC (plastic) pipe is used for the stand and
the boom. Copper refrigeration tubing is used for the loop. A small
coupling loop is made from a short length of RG8-sized coax. (I had
RG-213 so I used that.) A 100 pf variable capacitor is used for
resonating the loop. Some copper braid, like the braid left over
from making the coupling loop, a plastic box and some miscellaneous
hardware complete the parts list. Except for the variable capacitor,
you can get all the parts for about $25.

The QST article does not specify the type of PVC tubing to be used.
Three types are commonly available: a grey type specified for use
as electrical conduit, Schedule 40 (white) used for drain pipes, and
CPVC (beige) used for water supply lines. I used one-inch schedule 40
nearly everywhere, i.e., for the stand and boom, because it is the
most rigid of the three types. I had to use a CPVC 1/2 inch coupler
to connect the open ends of the copper loop because a 1/2 inch
schedule 40 coupler was too large.

The copper tubing is of the type used for refrigeration or gas piping.
It comes in coils and is often sold boxed in lengths like 10 or 20
feet. The 10 foot length is good for the smaller loop and 20 feet is
fine for the larger loop. Be sure to inspect the coil you buy for
kinks and dents. I rejected all but one box I checked at two stores
until I found a good one. The QST article specifies 5/8 inch *outside*
diameter tubing. If you have a caliper, bring it with you when you go
for tubing because it appears that tubing is not always labeled
clearly as to inside or outside diameter.

Work carefully when uncoiling the copper and apply any bending force
along a large radius. You could fill the tube with sand before bending
if you're very worried about kinking. It wasn't clear how to connect
the CPVC 1/2 coupler to the copper ends. I used five-minute epoxy
which held fine. Remember to rough up both the copper and the inside
of the CPVC coupler with rough sandpaper before applying epoxy and let
it dry thorougly.

Because the radiation resistance of a compact loop is on the order of
milliohms, the "ohmic losses" in the system are important to consider.
Any connection carrying RF should be big and attached using as low
resistance a method as possible. Welding is best and brazing is next
best, but having no equipment for either, I resorted to soldering with
a torch. Where the braid from the variable capacitor was to connect to
the loop, I soldered on 1/2 inch wide brass strips and did the same
where the coupling loop is attached. The brass braid should be soldered
and not screwed to the variable cap.

The variable cap was mounted in a plastic box. The top and the bottom
of the loop and the plastic box were attached to the PVC boom using a
"non-slip" type of nylon cord using in making fishing nets. The QST
article called for other methods like plastic wire ties and bolted-on
copper strap, but the nylon cord made a strong, no-movement connection
that was more attractive as well.

There is a lot of voltage developed across the variable capacitor --
over 4 kV for 100 watts -- so the capacitor will need good spacing to
run the typical 100 watts. The BASIC program in the Communications
Quarterly article is handy for determining the exact voltage rating
needed for the variable, as well as several other useful parameters
for a loop.

The tuning of the loop is very sharp! A vernier knob makes tuning
less of a chore. Preliminary tuning can be done by adjusting for a
noise or signal peak in the receiver. Fine tuning requires a watt
meter or SWR bridge. Of course, use low power and be careful when
making adjustments while transmitting so you don't touch the loop
and risk getting an RF burn. Your body and especially the arm you
are tuning with will detune the loop, so you'll have to do a "tune
and step back" dance to get spot on.

As expected, less capacitance is needed on the higher bands. If you
can find a variable cap whose minimum capacitance is zero or close
to it, you might make 10m on the loop. We didn't. Apparently, most
variable capacitors have some residual capacitance even when fully
open and that is more than is needed for 10m. The BASIC program can
show you the theoretical capacitance needed for a given frequency.
We needed less in every case on both the small and large loop.

The small loop for 30-12m loaded up just fine on all bands from 30
to 12 meters with a very low SWR possible. Adjusting the shape of
the coupling loop might be necessary to get the best possible match.
Do not be tempted to use an antenna tuner with the loop! The loop
has a very high Q, i.e., a narrow bandwidth, so even though the tuner
may allow your transmitter to deliver lots of power to the loop,
unless the loop itself is resonant, it will probably be quite deaf on
receive. The variable capacitor on the loop *is* the tuning method.

If you have trouble getting the loop to resonate anywhere, be sure
to check all connections to make sure they are sound. Replace any
and all screwed down connections with solder, brazed, or welded
ones. Try adjusting the shape of the coupling loop (squash it down or
stretch it out). Be sure the loop is away from everything, conducting
or not. Note that vertical orientation is preferred unless the loop
is 1/4 wavelength or more above the ground. As a last resort, the
length of the loop could be shortened, but extending it by sweat
soldering a copper coupler and more tubing is not recommended again
due to ohmic losses.

Art and I finished the small loop on Sunday, May 29, and put it on
the air during the CQ WPX CW contest. Art was working stations on
on 20 and 15 meters from several areas in the USA, Caribbean, and
(I think) Europe with about 25 watts, catching most of them on the
first or second reply. I suppose his call being NT1M helped to get
some attention! The loop is very directional when mounted vertically.
Contrary to our intuition, the main lobes are in the same plane as
the loop and the nulls are perpendicular to that plane!

I made the large loop on Monday. Conditions were pretty bad but I
worked a guy in Michigan on 40m and got a 58 report with 100 watts.
The large loop tuned up perfectly on 40m. On 30m, the best SWR I
could get was about 3:1. It would appear that even the residual
capacitance of the variable cap I used will be too much for 20m,
since the cap was open about 90 percent for 30m. Nevertheless, I
plan to continue debugging the large loop to hopefully get a
better match on 30m using all the hints I mentioned above.

In summary, the 30-12m loop can't compete with a yagi on a tower,
but it's a good performer that can certainly serve as a reasonable
primary antenna. It's ideal for apartment dwellers or covenant-
restricted neighborhoods and can easily be carried in a car for
portable setups. I think I'll build a mobile mount for it next!

   Scott WO1G
Scott Sminkey email: sasminkey@eng.xyplex.com
Software Sustaining Engineering voice: 508 952-4792
Xyplex, Inc. fax: 508 952-4887
295 Foster St. (Opinions, comments, etc. are mine,
Littleton, MA 01460 not Xyplex's...)

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