owenduffy.net

Balun designs has a warning to users of baluns on a full wave dipole.

***ONE EXCEPTION!***

It must be pointed out that a 1:1 balun should never be used on the second harmonic of a half-wave center-fed dipole fed with coax (like an 80 meter dipole being used on 40 meters). The impedance can be as great as 10,000 ohms creating very high voltages which can bring about voltage breakdown and/or excessive heating.  This exception ONLY applies to Coax Fed HALF WAVE CENTER FED DIPOLES WHEN USING A 1:1 BALUN AT THE FEEDPOINT.

Whilst differential voltage can be an issue in antenna systems (Duffy 2011), the warning above is a bit dramatic for this case.

Firstly, it is very difficult to measure the impedance of a full wave centre fed dipole in the worst case, but modelling suggests it is unlikely to have an impedance at resonance greater than about 4200+j0Ω.

Lets suppose there is a balun located at the feed point of an 80m half wave dipole, and the antenna is fed with 25m (~80′) of RG58C/U feed line. Using TLLC, the transmission efficiency of that section of line at 7MHz with load of 4200+j0Ω is just 9.4%.

If we have a 100W transmitter, we might get 90W out of the ATU in this scenario, and 9.4% or 8.5W of that reaches the feed point.

It is a simple matter to calculate the RMS voltage as V=(P*R)^0.5=190V, or 270Vpk. This is not going to strain any balun!

They outcome here is due to the extreme loss on the coax under very high standing waves results in very little power reaching the balun anyway.

This is one of those cases that if the antenna was half as long, the system would be ten times as good!

References

• Duffy, O. 2001. RF Transmission Line Loss Calculator (TLLC). VK1OD.net (offline).
• ———. Jul 2011. Avoiding flashover in baluns and ATUs. VK1OD.net (offline).