
Antenna 'Traps' made from Coaxial Cable for a W3DZZlike dipole
My lowband DXAntennas are only up in winter, so during summer I need something simple to at least make a few contacts on 80 and 40m. There are plenty of websites on which one can read and learn about the theory and construction of coax traps. As I didn't want to use a traditional parallel tank circuit made from lumped components, I decided using simple RG58 on a grey waterpipe. All losses are converted into heat. A loss of 1dB equals 20.6% of power. Running 1kW into a trap of 1.0dB loss means the trap heats up with 205W. Thats quite a lot! It's widely known that coax traps, if carefully constructed, reduce gain by about 1.6dB. How to calculate the power in linear terms?
1kW = 60dBm
60dBm  1.6dB = 58.4dBm
10^{5.84}*0.001mW = 691W
1000W  691W = 309W
Just imagine boiling water (1cal = 4.184J), that means with 309W you can heat up 1ml of water by 74℃ within one second. Sounds amazing and scary at the same time. Tom, W8JI, investigated the loss of coaxtraps and how to reduce it. It's common practice to tune the tank direclty to the frequency of interest, but that means that a lot of energy flows inside the resonant circuit. High current flow and finite Q result in high loss. That means that the losses increase the closer the operating frequency comes to the resonant frequency of the trap. Moving the trapfrequency slightly out of band reduces the losses significantly! I tuned my traps to 6.57MHz and slightly adapted the length of the corresponding dipole elements.

40m Trap made from RG58 (click to enlarge)



Measuring the trap

S11 Curve (measured with my HP8753E)

