Have you ever thought of making a small loop but been discouraged by the thought of vacuum variable capacitors and plumbing copper pipe!
Well if you can you scrounge a few meters of half inch foam heliax and a largish ferrite toroid (ui 100 to 200) give this idea a try – it’s simple and you won’t be disappointed by the performance.
This idea makes use of the coaxial cable’s capacity to resonate with its own inductance – similar in a way to a conventional coaxial trap.
See schematic herehttp://i620.photobucket.com/albums/tt283/mwmikep/Loopschematic.jpg
The antenna described is for 40m however the same idea can be used to make a loop for any frequency.
For the 40m band loop start with a 5m length of coax (foam dielectric) and at the exact centre open up the outer jacket and copper sheath as shown in the attached images. http://i620.photobucket.com/albums/tt283/mwmikep/Coaxprep1.jpghttp://i620.photobucket.com/albums/tt283/mwmikep/Coaxprep2.jpghttp://i620.photobucket.com/albums/tt283/mwmikep/Coaxprep3.jpg
Take care when cutting back the outer copper so as not to damage the foam dielectric beneath.
You need to peel back about 3,5cm of the coax outer – an old pair of side-cutters does the job.
Fold back any sharp copper edges to prevent corona.
Reinforce the opened area with a length of PVC tubing and seal with heat-shrink.
Make off each end of the cable exposing approximately 1cm of inner, 1cm of dielectric and 5cm of the outer.
Wind 14 turns of insulated hookup wire evenly around a suitable toroid and slide it on to one end of the coax. (36mm Philips 4C65 or T140-61)
Bring the two coax ends around to form a circular loop and temporary solder the outer jackets to a piece of blank PCB as shown in the following image.http://i620.photobucket.com/albums/tt283/mwmikep/Loop_couplingandtuning.jpg
Also solder a coaxial connector on to the PCB and connect the lightly twisted wires from the toroid. (To the connector inner and PCB ground)
Now it’s time to start with the trimming. Fasten the loop to a suitable mast that will allow the loop to be positioned a couple of meters above ground in a reasonably open area. The top section of the mast supporting the loop should be nonconductive material.
Connect up your RX and tune for the obvious noise peak that will most likely occur a bit below 7MHz.
You now need to cut back each of the coax ends slightly until resonance occurs about 100 kHz above the highest 40m band frequency you want to transmit on.
Once this has been achieved a small variable capacitor positioned between the ends of the coaxial inner is used to tune the loop lower in frequency.
The implementation of this tuning arrangement I’ll leave to you – you can see my rather crude homemade variable capacitor in the image of my loop.
Be aware though the voltage across the capacitor reaches 6kV at 100W input!
Being a very high Q antenna, small size and shape differences can have quite a big effect on the resonant frequency.
The final length of coax that gave me an upper resonant frequency of 7,2MHz was 4.86m with a 3.5cm wide top gap and 7.5cm spacing of the coax ends.
The home made capacitor that allows me to remotely tune the loop from 7,0MHz to 7,1MHz has a range of about 5pF and a minimum capacity of 3pF.
Whatever capacitor design you opt for be careful not to introduce additional loss into the circuit. Any additional loss will show up as an increase in the tuned SWR.
The transformer turns ratio of 14:1 should be correct and provide a tuned VSWR < 1.2:1 across the band for a well-made loop as described and supported at least 2m above ground and away from other conducting surfaces.
If you find you do not achieve this it means either your variable capacitor is introducing loss or the loop is coupling into the surroundings.
The transformer turns ratio indicates the sum of the resistive losses (Rl) added to the loop’s radiation resistance (Rr).
At 7MHz this 1,55m diameter loop has an Rr of about 0.04 ohms and Rl of 0.214 ohms giving a total of 0.254 ohms.
A 14:1 transformer turns ratio provides a 14^2 impedance transformation ratio or 196 times. 196 x 0.254 = 49.8 ohms.
The loops efficiency is therefore Rr/(Rl+Rr)*100 or 15.7%.
The unloaded Q of the loop is 830 giving a 1,6:1 SWR bandwidth of around 2,5 kHz!
Tuning needs to be precise and may test you ingenuity in implementing the variable capacitor.
If you have to use the loop closer to ground or near conducting surfaces, for example indoors, you will most likely have to reduce the transformer turns ratio to maintain a low SWR - this indicates some reduction in efficiency.
For example, if you have to reduce the transformer turns ratio to say 12:1 to get a good match this indicates that the (Rr+Rl) = (50/122) = 0.347 ohms. (0.347 – 0.254 = 0.093 ohms of additional loss has been added)
As the radiation resistance has not changed the efficiency falls to 0.04/0.347)*100 or 11.5%.
Self resonating coaxial loops can also be made for the other bands; as a starting point you can use 3.6m of coax for 30m, 2.7m for 20m and 2.2m for 17m. If you have the coax and space an 80m & 160m version should also work quite well.
Other coax lengths may be calculated using the formula y = 29.091x^-0.9071 where x is the frequency in MHz and y the length of coax required in meters. (Applicable for foam dielectric cables – velocity factor 0,81)
You can also use RG213 in place of the helix coaxial cable however the losses are considerably higher and there is also a chance of insulation voltage breakdown at 100W. For a 40m band loop efficiency falls to about 6%.
Finally an important consideration;
When transmitting at 100W the magnetic field close to the coaxial loop will greatly exceed the recommended limits of human exposure.
From what I can recall one needs to keep at least 5m away!