Feed point voltage – W5WSS 7′ dipole


W5WSS describes his antenna at (eHam 2014). It is essentially a shortened dipole with capacity hats for 20m.

The configuration appears from several postings to be this shortened dipole with a Balun Designs 1115du balun at the center and an adjacent LDG Pro 200 automatic ATU.

W5WSS suggests that it is good for 200W, but he intends using it at 20W as it is indoors,


KF6A posted the results of an NEC model of the antenna, he came up with a feed point impedance for the dipole of 4.9-j1090Ω.

If we take KF6A’s work to be indicative, then we can calculate the current for 200W at the feedpoint, it is simply I=(P/R)^0.5=(200/4.9)^0.5=6.4Arms. Now the differential voltage impressed on the balun will be I*|Z|rms=6.4*(4.9^2+1090^2)^0.5=6980Vrms=9900Vpk… WOW!

9900V is an extreme voltage to impress across either the balun or ATU. Even at 20W, the voltage is 32% of that or 3200V… still quite extreme.

An interesting exercise for the reader is to consider the impedance transformation in the Balun Designs 1115du balun which essentially has around 1m of probably RG316 coax in line, which would transform the modelled feed point Z of 4.9-j1090Ω to something quite different and challenging in efficiency terms for the ATU (even though there is significant loss in the balun).

W5WSS gives testament that it works “very nicely”. The thread runs to 5 pages at the time of writing, it makes an interesting read.

This antenna was not ‘designed’, but ‘dreamt up’, and it is a great example for study of the potential for high voltage (pun intended).

There is no magic to physically small antennas, they all have the potential for very high conductor losses and extreme voltages in matching and feed line systems. Good designs consider these issues and find solutions that are both practical and have good performance.