Volume 3 Issue 2
IN THIS ISSUE
40M MAGNETIC LOOP ANTENNA P. 2
EASY & CHEAP CW KEY P. 7
WEDNESDAY NITE CONTROLLERS P. 8
MINUTES FOR THE ERC FEBRUARY MEETING P. 8
President: Al, VE3TET Vice-President: Johan, VA3JBO
Secretary: Bob, VE3IXX
Treasurer: Reg, VE3RVH
QSL Manager and Lighthouse: Bill, VE3WBJ
Repeater manager and maintenance person: Carl, VE3FEF
Website Admin: Johan, VA3JBO
It’s no wonder that Ken VE3KCY puts out such a power-ful signal from his high elevation vil-lage of Alma. Take particular note of his EH antenna on the top of the long tower. Thanks Ken for the photos.
Volume 3 Issue 2
A 40M MAGNETIC LOOP (IN THE GARAGE ATTIC)
By Bill Graham VE3ETK
A fter I had my stroke it was clear that my wife and I needed to move to a one floor house. The result was the lovely present QTH, but it came with antenna restrictions. Already interested in magnetic loops I bought an MFJ-1786 for 20-15-10M (with magnificent results – not a 5 element beam at 100’ – but great just the same) and built a 40M loop, which is what this article is about. First contact Cuba, second Germany, and after a while Turkey, so I’m happy with this one too.
I must give credit to K8CIT as that’s where I got my starting point – from his design as found on W2BRI’s web site. The concept is to fold the loop so it takes less space and will still give a decent Q
It is all made from .” copper pipe (inexpensive red grade) with every elbow and connection soldered. The electrical resistance of the antenna MUST be low (we’re talking fractions of an ohm) to get a good Q and a tiny bit of oxidation will make the resistance rise very quickly! The uprights are 60” long, as is the long bottom section. The top sections are 28” long each, and the bottom short sections are 27” long, although I trimmed one of them shorter to make the shortest possible connection to the tuning capacitors. I think Fig. 2 shows some of this more clearly, as well as the support base and some of the insulators I made using PVC plumbing ‘Ts’. They had a short PVC pipe section inserted and hot glued into a matching hole in the base. Hot glue also provided the adhesion needed to keep the pipe in place. Solder the elbows on first!
Fig. 1 shows both the structure of the loop and the resulting pattern when the bottom of the antenna is 10’ high (garage attic).
The points x/x are where the tuning ca-pacitor system connects (more on that later). The purple dot on the structure is the Gamma feed point (more later also).
Here you can clearly see the placement of the motorized tuning capacitor (some serious upgrades have been done here, which I will describe and show shortly) as well as some details of the Gamma match.
I had some great help getting the antenna in place from my good friend Rick Harrison (who also took the pictures) and Al Macdonald VA3TET (who has done my upgrades) – they will attest to the soldering.
Fig. 3 is looking up at the antenna top joints. The insulator/support is a nylon cutting board from the dollar store. You can also see how it is OK to fit the antenna around the roof struts.
In my original design I built a motor driven capacitor with glass insulation and copper plates in a wooden frame as seen in Fig. 2. It worked wonderfully, but after a couple of years it didn’t stand up to the cold and dampness.
Volume 3 Issue 2
I had also used some little R/G8 coax stubs as added capacitance. Let me caution you against using this ap-proach across a loop. It was found that putting 80 Watts or more into the loop then heats the coax capacitors that are in parallel with the loop, causing them to change capacitance and detune the loop. These were re-placed with a variable capacitor (about 15pf mid position that was still needed in the latest update) that reso-nates the loop to 40M. This capacitor has wide spaced plates to handle the voltage. A lot of “On Air” time has proven this approach is stable and reliable.
Fig. 4 shows the latest motorized tuning system. The motor driven capacitor was modified into a split stator capacitor so that there are no moving contacts to cause resistance that would lower the Q. The other ‘little’ capacitors were made from the leftovers from the conversion and limit the range of the main capacitor’s tun-ing, something you don’t need to do if you are happy with the motor’s speed and your needs.
Let’s get a better look at the Gamma matching.
I used the shield of RG8 coax to form the Gamma ‘rod’ and held it away from the antenna leg with ceramic insulators, not because the insulation was needed but because the length and mounting was right and simple.
I used the shield of RG8 coax to form the Gamma ‘rod’ and held it away from the antenna leg with ceramic insu-lators, not because the insulation was needed but because the length and mounting was right and simple.
Fig. 6 shows the antenna feedpoint coax, the Gamma match capacitor and it’s connection to the Gamma ‘rod’ (RG8 coax) by the white wire.
Fig. 7 shows the front of the control panel: Placement of the parts behind the panel is not critical, and this diagram should explain the rest:
THE GAMMA MATCH: In K8CIT’s design he fed the loop via a ceramic toroid. That intrigued me as I like working with toroids. However, the early stages of testing showed that, while broadband (K8CIT wanted to work both 80M and 40M with one antenna), the effi-ciency was poor. I tried a Gamma Match and improved efficiency about 20 fold. The computer software RF Tools has a section to make Gamma Match calculations. As my Gamma Rod would have to bend up one side of the loop I choose to use RG 8 coax with the outside braid as the conductor. Fig. 8 shows my calculations.
CONCLUSION: This was an inexpensive and rewarding project to build, and the “On Air” results have been equally rewarding, and I had a lovely chat with K8CIT, loop to loop, one evening. Tuning the antenna while operating is very simple, and it has opened up the 40M band for me from this QTH. I encourage anyone who would like to work 40M and has limited antenna space to build one.
A capacitor from my junk box com-pleted the Match. Although it has only 1/8” plate spacing this is a low impedance point, and hence a low voltage point. As I said, the ‘rod’ needed to bend up one of the verti-cal elements, and a point was found, and soldered, that produced 1:1.1 SWR at 7.2 MHz.. Fig. 2 shows the ‘rod’ held off the vertical element with ceramic insulators.
BUILD AN EASY AND CHEAP KEY FOR CW
A few years ago, I built myself a simple code oscillator to practice cw (I needed the practice then and still do!). It consisted of two items; a nine volt battery and a high pitched buzzer. But what I needed was a decent key. Rather than go out and purchase a commercial key, I saw an article in a magazine to build this simple, but rather attractive key- one that you’d be proud to show off. Unfortunately, I can no longer remember what radio magazine, or the author, so I cannot give proper credit. However, by show-ing the pictures of the finished prod-uct, you can see how easily it can be put together. All that is needed is a flat piece of wood, a tee hinge, a knob for a cabinet door, a small spring from an old ball point pen, two short pieces of wire and a few machine screws with accompanying nuts.
Figure 2 shows the side view. The ma-chine screw at A comes up from the bottom of the board at B. Slip on the spring from the ball point pen, then push the screw through the “V” part of the hinge. The nut at the top at A is used to adjust the gap at C.
Figure 3 shows the bottom side of the key. The wire is attached from C to one of the connectors at the front. I grooved out the wood along the wire so the board will lie flat. The two connectors on the front each have two nuts.
WEDNESDAY NITE CONTROLLERS
MARCH 6 – HARRY VE3EIX
MARCH 13 – WALLY VE3LCR
MARCH 20 – REG VE3RVH
MARCH 27 – M E E T I N G
APRIL 3 – DOUG VE3CXU
APRIL 10 – CARL VE3FEF
APRIL 17 – BILL VE3ETK
APRIL 24 – M E E T I N G
ERC Meeting February 27, 2013
Terry took a few minutes before the meeting to set up his display of the computer, a projector and the Don-gle.
The formal meeting then started at 19:37. There is a need for help to take care of Bing Harris’ estate. A volunteer will be required to take stock of Bing’s equipment.. Johan, Jim, Reg and Dennis volunteered for the market.
Reg gave his financial statement. There is $2401.68 in the kitty.
Bruce seconded to accept the report.
Al gave an update on a project for the software defined radio by Terry. (60 MHz to 2 MHz, no HF band.)
For the HF band for the Dongle, a 2 MHz to 102 MHz up converter is required. (Basically it is a Oscillator and a Mixer). 50 Ohms in and 50 Ohms out. The basic version should be $15 or thereabouts, the full blown version will be close to $30.
There will be further technical discussion on this project by Al.
Terry gave his presentation on
SDR or software defined radio. Will need to ask Terry if the presentation can be made available for the meet-ing report.
The software is freeware and the dongle is $16 and of course there will be several upgrades possible.
Al adjourned the meeting at 21:10.
A special note of thanks to Johan, VA3JBO for taking the minutes in my absence.
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