Re: Folded Monopoles

Roy,

Concerning HF ribbon folded dipoles I do not think it makes muich
difference whether the short is at the end or a little ways in from the
end, however I believe that the BW of the antenna is enhanced, and the
impedance transfer is closer to 300 ohms -- but I have no real proof of
that. But I am a profectionist (as you know) and, for the folded monopole
case which I describe below, I like to see the asymmetric mode looking at
resonance at a shorted resonant stub.

My understanding about folded dipoles comes from the analysis by Uda and
Mushiake, "Yagi-Uda Antenna", Maruzen Co., Ltd, Tokyo, 1954, p.19; see also
Chen To Tai, "Dipoles and Monopoles", in Antenna Engineering Handbook
(editors Johnson and Jasik), McGraw Hill, 1984, pp. 4-13, 4-16 to 4-18].
This analysis and a reference to a work by King and Harrison, Thiele et.al.
and Richmond, are overviewed in the referenced chaper (by me) in The (IEE)
Handbook of Antenna Design.

Let us consider the two modes, as described by Uda and Mushiake, for the
case of a folded monopole since this is the case I have considered:

When the electrical length of the radiator is approximately a quarter of a
wavelength, the input impedance of the asymmetrical mode is very large
compared with the monopole impedance.

When the electrical length is very much less than a quarter wavelength, the
asymmetrical mode dominates, the radiation resistance is very small and the
antenna reactance is inductive. At frequencies between these limits, as
the frequency increases to the value where the symmetric mode becomes
dominant, the reactance changes from inductive to capacitive, and the input
impedance becomes very high. Thus the folded monopole does not behave at
all like a monopole for frequencies less than the frequency for which it is
quarter wavelength resonant (and for frequencies greater than --- which I
have not discussed).

The impedance vs frequency graph (computed by NEC-3) can be intuitively
interpreted in accord with the analysis of Uda and Mushiake.

Another antenna for which one uses transmission line velocity factor (v)
for one part of the antenna, and antenna factor (k) to dimension another
part of the antenna is the ribbon-J. You will say this is a different
antenna, but the quarter wavelength matching stub carries *both* in-phase
and out-of-phase currents, according to rigorous analysis, but out-of-phase
currents dominate; the upper-part of the antenna caries radiating
(in-phase) currents. This is an interesting antenna to model, as is the
folded dipole.

I have modelled folded dipoles as well as folded monopoles. Here is a
folded dipole I have modelled, if you would like to take a look: length 40
metres; spacing between wires 15 cm; #12 wire, source on centre of lower
wire. Resonant frequency 3.517 MHz. I put 3-segments on the shorting
stubs, and tapered segments on both wires so that the segment lengths at
the end of the wires was the same as the segment lengths on the shorting
wire.

The current distributions on the two wires are interesting. The current on
the upper wire increases from a small value at the end of the antenna to a
maximum value at the centre. The current on the lower wire first
decreases, as we move from the end of the element toward the centre, and
then increases reaching a maximum value at the centre (the source).
Nowhere is the current zero, as it is on a dipole.

This complication must clearly be due to the two competing modes.

Now change the frequency to 1.9 MHz. I will not try to interpret the
current distributions on INTERNET with no figures.

The ribbon-J is an interesting antenna to model.

Unfortunately I cannot model ribbon antennas using NEC-2 (EZNEC version).
I can only model air insulated wires.

73, Jack, VE2CV

------------------------you wrote----------------------------------------------

>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

John S. (Jack) Belrose, VE2CV
Director, Radio Sciences
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2308
FAX 613-998-4077
Received on Thu Feb 15 1996 - 16:24:00 EST