Hints on Small Loop Construction
The high interest in the small transmitting loop has caused a lot inquiries to come down the cable to us here at antenneX. So, since I am building one I will share with you what I have done so far. The biggest drawback folks have been concerned about is the capacitor needed to tune the loop. On the lowest frequency the loop will operate on legally, the voltage will be in the many kilovolt range. One of the versions I was looking at had over 20 kV on the capacitor. This is a problem, since many cannot afford the commercial vacuum variables that can handle the voltage and current found in the small transmitting loop. Clearly, an alternative is needed that is affordable and easily duplicated.
After checking the local plastic’s market and finding Teflon Shrink tubing that is costly at nearly 10 dollars per foot, an idea occurred to me. The Scotch brand electrical tape available over the counter has 9 kV breakdown and some even has much higher. So why not make a trombone capacitor and wrap the legs with 3 layers of the 9kV tape, then put shrink tubing over that? By using the clear heat shrink tubing over the electrical tape, a slicker surface will result than if you used the black shrink. Also by coating the shrink tubing, after it is in place, both the friction will be reduced and the breakdown voltage will be increased. It needs to be pointed out that heat shrink alone will not stand the voltage unless you are running very low power. So, take care in this regard.
Even at 5 watts using a 3 foot (.912 meter) loop on 30 meters, 2.375 kV will be on the antenna and moving to 28.5 MHz, the voltage drops to 1.2 kV. A ten foot (3.04 meter) loop will have 1.8 kV on it and moving to 40 meters only lowers the voltage to 1.24 kV. If you run 100 watts on a loop of that size, on 75 the voltage goes to 8.172 kV and 5.7 kV on 40. Going to 1,500 watts raises the voltage to 31 kV and on 40 it is going to be in the 21 kV range. These voltages are very high on the higher bands all the way to 10 meters, with approximately 21 kV being present on 10 meters. This was calculated using 1/2 (1.27 cm.) inch tubing
The only aspect that makes things easier is the capacitors are in series, and that makes the voltages divide between the two legs of the capacitor. So, if you wrap three layers of the 9 kV Scotch tape on each leg, then there will be 27 kV of insulation per leg plus the 600 volt rating of the shrink tubing. This method of construction is to be used for QRP operation only, at powers lower than 50 watts and preferably at power levels under 25 watts. This is due to the possibility of dielectric heating taking place at much higher power levels.
Some thought was given to using a length of plastic inside the smaller tubing that makes up the inside of the capacitor. But, this has not been explored because about the only advantage is it would allow you to completely remove the inner legs resulting in almost no capacity being present except what is between the two outer legs. Then the internal plastic rod will allow the inner legs to slide back into the outer legs as they would be aligned correctly.
The diagram, Figure L-1 shows the basic construction of a small loop and the manner in which the trombone capacitor fits together. There is no weather proof cover or remote motor tuning system shown. How you want to build a cover or remote tuning is up to you depending on what you have on hand to make these items. It is also possible to manually tune the loop by using any means that you can come up with, just do not do it when the transmitter is on. By finding each tuning point on the band where you operate, it will be possible to preset the tuning before you get on the air.
There will be more tips to follow later on the loop construction as my particular project is built and tested.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 9th March 2024