Re: Folded Monopoles

On Tue, 13 Feb 1996, john.belrose_at_crc.doc.ca (John Belrose) wrote:

>. . .

>If the antenna is made out of 300-ohm ribbon, or coaxial cable, there is a
>significant difference between the electrical lengths for the transmission
>line mode (characterized by velocity of propagation) and the radiating
mode
>(characterized by an antenna factor). Certainly the short at the end of
>the monopole should be determined by the velocity of propagation factor
for
>the transmissionn line mode down the two conductors, and since v (as a
>length factor) is always less than k the antenna factor, the short is in
>from the end of the monopole.

I've never seen the importance of making the transmission line mode
reactance be zero. I've always shorted my folded dipoles at the ends for
mechanical convenience. The feedpoint reactance caused by the slightly long
stubs is easily cancelled by a small modification of the antenna length.
Away from resonance, the reactance change of the antenna mode is much more
rapid than that of the transmission line mode, at least for a 300-ohm
twinlead folded dipole at HF.

For years I've seen the recommendation that the short be made at the point
which will make the stub reactance zero (i.e., where the stubs are an
electrical quarter wavelength). Would someone please explain why this is
important?

Roy Lewallen
Received on Wed Feb 14 1996 - 19:34:00 EST