Just made my first transmitting loop
Even though I've been a member for a bit, I haven't tried my hand
at making my own transmitting loop until now.
I just completed my first one, and wanted to share my info and ask
a question or three. Sorry in advance for the long posting. I hope
that someone might find it interesting! ;-)
I made a single-turn loop using 5/8" copper pipe from a roll I
bought at Home Depot. It helped that the roll was already formed
into a coil that was about 18 inches in diameter, so it was a
simple matter to uncoil the roll into a larger loop without any
serious distortion or kinks.
I had previously bought a very nice Hammarlund dual-gang
capacitor from eBay that works very nicely for this purpose.
Since the two 8-220pF sections are electrically separated, I
don't have to make a connection to the rotor, so this is the
ideal capacitor. I wish I had a few more of them. Anyway, in
this split configuration, I've got an effective capacitance range
of about 4 to 110pF.
I mounted the loop with tie wraps on a piece of wood that's .75
by 1.5 inches. The capacitor is also securely mounted to the
wood with tie wraps, and soldered to the top of the loop with
very short pieces of 10 gauge solid copper wire that I hammered
flat--just to make it easier to shape and bend them. So from a
distance, the overall design looks like a big lollipop, so to speak.
For the feed, I used the conventional RG-58 loop 1/5 (actually a bit
closer to 1/6--about 6 inches dia.) the diameter of the main loop.
The center conductor is soldered to the shield at the half-way point,
with the shields soldered together at the bottom. This smaller loop
is then tie-wrapped securely to the bottom of the main loop, with
no electrical or metallic bond between them.
Oh, and I forgot to mention that the main loop diameter is 39 inches.
It is one continuous circle with no breaks or solder points other than
where the tuning capacitor attaches at the top. This was just a
convenient size for me to shoot for--1 meter in diameter.
I've checked the expected 2:1 SWR bandwidth at 10.1 and 14.2 MHz,
and those numbers agree very closely with the predicted values that
the loop design software suggests, so this tells me that I've got decent
loop "Q". Good.
So, I'm VERY pleased with its functionality so far. I know this might
sound like a cliche, but my very first two QSO's I tried yesterday
late afternoon on 17 meters netted me some great DX! Both stations
that I called heard me on my first call. I was using an Elecraft K2
transceiver running on its internal battery, using about 10 watts on
SSB. The wooden boom for the loop was pushed into soft, wet grass,
so the bottom of the loop was about 12 inches above the grass.
I'm near San Jose, California, and I first called a station in Hawaii at
the end of a QSO, and got a 5-7 report. He was running about 400
watts, and I also gave him a 5-7! Ten minutes later, I called a
station in Maryland, and got a 5-5. He was running 900 watts, and
I gave him a 5-8. Both stations were very surprised that they
could hear me as well as they could, considering my low power and
small antenna. (By the way, the late-afternoon QSB on 17 meters
yesterday was tremendously long and deep--almost like 10 meters
during a sporadic E opening.)
So...I have a couple of observations, mostly about the feed
system, and I wonder if those of you with way more experience
than me could comment and offer suggestions and improvements.
With the first feed loop (described above), I was not very pleased
with the SWR I could get, even after fooling around with the loop
shape and amount of coupling with the main loop. I did not
attempt to increase or decrease the diameter of this loop, and I
know it was a bit smaller than the suggested 20% of the outer
loop's circumference. Had I fiddled with this for awhile, I'm
assuming that I could have gotten the SWR down. Best I could
get was about 1.6 or 1.7 to one. Not horrible, but everyone
else seems to report something approaching unity.
Instead of doing that, though, I decided to try the "other" type
of feed I've seen suggested, so I changed it to the type where
you ground the coax shield to the bottom of the main loop
with a stainless hose clamp, then solder the coax center
conductor to about 14 inches of 10 gauge wire that follows the
inner curvature of the main loop, but about 2 inches to the
inside of it--out to about 10 inches, where another pipe clamp
shorts it to the main loop. This is just like the feed that W2BRI
shows on the "build it yourself" square loop article on his web
site. I experimented with the length and proximity of this feed,
and was easily able to get my SWR down to 1.1:1 pretty much
anywhere from about 10.1 to 21.45 MHz.
What I didn't realize until today is that this nice, low SWR is not
going to be realized continuously from the lowest to the highest
frequency that the loop can cover without changing the length of
this new feed system. For instance, I'm only able to get about 2:1
SWR at 28.1 MHz unless I shorten the coupling loop wire length,
but then my SWR at the loop's lowest frequency increases to about
1.7:1 at best. Am I doing something wrong here, or is this to be
expected? Am I just looking for a compromise value, or should
I really expect something close to unity SWR from top to bottom?
Also, is the first feed system I tried better than what I'm using now
for broader-band use, or are they pretty much the same expected
performance? Why use one over the other, then?
The reason I'm asking about this is that if I end up putting a motor on
the tuning capacitor for remote, continuous tuning, I can so far only
get good SWR for the lower half of the loop's frequency range, or for
the upper half, but not for the complete range that my tuning cap
will cover. I'd obviously like to be able to get a nice, low SWR across
the entire frequency range that the loop can cover.
Last question: I note that on one or two ham websites, there is a
warning that the tuning or shape of the coupling loop will change
when you move the loop around. One even suggested that you
really haven't "tuned" your loop until you get it into final position,
and only then can you make your final adjustments. Great, but that's
kinda difficult to do when you have it hoisted out of reach. My limited
observations so far seem to suggest that the actual frequency that the
outer loop is tuned to changes a bit, of course, but the SWR seems to
change much more noticeably than the tuning when going from 12"
above ground to 12 feet.
For instance, I get 1.2:1 SWR mounted near ground level at 14.060 MHz,
but then when I move the loop so that it is supported by a fiberglas
ladder at a height of about 8 feet, the SWR goes up to maybe 1.9:1,
but the resonant frequency only shifts down (at least in my case) by
maybe 20 or 30 kHz or so. I can live with the frequency shift, and it's
simple to retouch the tuning cap, but to change the SWR, I've got to fiddle
with the coupling loop length or position--not nearly as easy to do.
Any suggestions are very welcome, and thanks in advance for your
kind thoughts or encouragement. I'm getting a real kick out of this,
especially because my local CC&R's will never allow me to put up
even a simple dipole. I expect to park my loop in my small attic
eventually, and am looking forward to having a semi-decent stealth
- The wooden boom for the loop was pushed into soft, wet grass,
so the bottom of the loop was about 12 inches above the grass.
from what I have read in the literature magnetic loops seem to work well closer to ground than farther away--so what you have is good as indicated by the excellent contacts you have reported
yet I am wondering if 12" is too close
However I have a photo of a German Military loop and it seems to be near yours being between 14" and 20" of the ground ground from what I can scale from the picture
it also has a more elliptical shape with the long axis parallel the ground
I am wondering if perhaps 12" is "too close" and is affecting the SWR minimun and by increasing the spacing somewhat you might be able to get a better minimun
and also and perhaps more importantly to SWR,I have a paper from Switzerland showing the optimal shape for a transmitting loop is more like a rectangle with well rounded corners hence the nice shape of the German loop which is like a flattened loop
it could be easy for you to raise the height of the loop to say 24" and maybe also to force flatten the circular loop you have and see if SWR minimum is affected
[Non-text portions of this message have been removed]
- Hi Dave,
Sounds like you have your new loop working pretty well using the Gamma match
type feed. That seems to work best for me also. It is to be expected the SWR
will change when trying to use the loop over so broad a frequency range with
one set length for the Gamma match.
I would recommend building two loops - one for the high range 15-10 meters
and build a larger one for the lower range 30-20 meters. A larger loop will
give you better efficiency on the lower bands also. If you use only one loop to
cover that wide a range it will have to be a compromise setting for lowest
Best to adjust the SWR of the loop at least 6 feet off the ground. If you
can get it 8 feet off the ground adjust it there. The SWR should hold pretty
close from there if you hoist it up higher. Or if you put it in an attic then
set the SWR there.
I would recommend looking into building a better tuning cap also like the
trombone-style cap made out of short lengths of copper pipe and using Teflon
insulation between the pipes. A regular 'bread-slicer' variable cap is not
considered efficient enough for a transmitting loop unless the plates are welded
together but many people seem to report good results using them. You wouldn't
notice any problems using them at low power but the loss might still be
there. Any bolt-on or screw-on connections to a capacitor should be avoided. A
trombone cap is soldered directly onto the ends of the loop so it is very low
Also beware of any connecting wires between the loop and the capacitor. Just
a few short inches of #10 wire has more RF resistance than the entire loop
conductor. Using flexible braid is reported as being very lossy also. If you
have to make any connections between the loop and a capacitor use only flat
copper strip at least 1 inch wide like copper flashing.
Band conditions are so variable that it might be hard to notice if a loop
was really running at top efficiency or not depending on skip. Best way to
really tell is if you have someone local within 10-30 miles that can hear you on
groundwave. Then any changes will show up on his S-meter that you can rely on.
73 - Todd WD4NGG
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- Todd (and Paul):
Thanks for your thoughts!
I just bought some 2-inch wide copper flashing from HRO, and I'm going to try using a
narrowed strip of it on the tuning cap connections instead of what I have now. You make
a very good point about the losses in even a short piece of 10 ga. wire. I hadn't thought
seriously about that, but it makes sense.
I'm also going to try adjusting the loop in a fixed position about 6 feet high and see what
results I get. I'm also planning on running steel wool around the copper pipe to clean off
any oxidation and then spray it with urethane or something similar.
I'll share my results here when I get the time to experiment more. I actually took a day off
from work just to build the loop! Now I've got to find more time to tweak and poke at the
basic design. I'm very encouraged by what I've seen so far, though.
I'd like to try a 40 meter loop next, maybe an octagonal shape using straight pieces of 1
inch diameter copper pipe that's maybe 1.5 to 2 meters in diameter.
It's very instructive to feed these various dimensions in to the loop design software and
see what the results are--also very gratifying to find out that I can build something that
comes pretty close to the predicted values!
Thanks and 73,
--- In firstname.lastname@example.org, toddroberts2001@... wrote:
> Hi Dave,
> Sounds like you have your new loop working pretty well using the Gamma match
> type feed. That seems to work best for me also. It is to be expected the SWR
> will change when trying to use the loop over so broad a frequency range with
> one set length for the Gamma match.
> I would recommend building two loops - one for the high range 15-10 meters
> and build a larger one for the lower range 30-20 meters. A larger loop will
> give you better efficiency on the lower bands also. If you use only one loop to
> cover that wide a range it will have to be a compromise setting for lowest
> Best to adjust the SWR of the loop at least 6 feet off the ground. If you
> can get it 8 feet off the ground adjust it there. The SWR should hold pretty
> close from there if you hoist it up higher. Or if you put it in an attic then
> set the SWR there.
> I would recommend looking into building a better tuning cap also like the
> trombone-style cap made out of short lengths of copper pipe and using Teflon
> insulation between the pipes. A regular 'bread-slicer' variable cap is not
> considered efficient enough for a transmitting loop unless the plates are welded
> together but many people seem to report good results using them. You wouldn't
> notice any problems using them at low power but the loss might still be
> there. Any bolt-on or screw-on connections to a capacitor should be avoided. A
> trombone cap is soldered directly onto the ends of the loop so it is very low
> Also beware of any connecting wires between the loop and the capacitor. Just
> a few short inches of #10 wire has more RF resistance than the entire loop
> conductor. Using flexible braid is reported as being very lossy also. If you
> have to make any connections between the loop and a capacitor use only flat
> copper strip at least 1 inch wide like copper flashing.
> Band conditions are so variable that it might be hard to notice if a loop
> was really running at top efficiency or not depending on skip. Best way to
> really tell is if you have someone local within 10-30 miles that can hear you on
> groundwave. Then any changes will show up on his S-meter that you can rely on.
> 73 - Todd WD4NGG
> **************New MapQuest Local shows what's happening at your destination.
> Dining, Movies, Events, News & more. Try it out!