Re: NEC-LIST: J-pole modeling, also LPVAs - 2

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

>I'm also interested in whether anyone has been successful in modeling
>J-poles, as i'd like to try some experiments with a true dual-band
>(e.g. not merely tuned for 2M and 70cm, as its vertical pattern on
>70cm is not very good). I understand this may be an antenna that
>NEC2 (or NEC4) has trouble with.
>
> -- KD6PAG (Networking Old-Timer, RF newbie)

The J-Antenna, the Ribbon J and so-called Slim-Jim (radiator section folded)
--------------------------------------------------------------------------

I have numerically modelled (initially using MININEC, subsequently
NEC-2 and NEC-4); and measured impedance and pattern for various
versions of J-antennas, off-and-on during the past 2-decades.

The J-antenna itself has been around for 5-decades --- see Terman,
Radio Engineers' Handbook, 1943 edition --- and it is an antenna that
has been published and republished in the amateur literature --- and
every version is differently dimensioned!!!

It is an interesting antenna to study, if you like experimental and
numerical modelling.

The antenna is a coupled system. If the dimensions of the radiating
element are too long or too short, adjusting the stub length can in
effect "tune" the antenna to resonance --- but maximum bandwidth is
obtained when both dimensions (radiator and stub) are the right length
for resonance. The radiator is the antenna, and so the antenna factor
applies --- the stub is a transmission line and so the trasnsmission
line factor applies --- see my discussion on this in QST Technical
Correspondence, April 1995, pp. 74-75.

Most amateurs have found difficulty in resonating the antenna. The
reasons for this are several:

There are three dimensions to trim and adjust, the length of the 1/2
wave radiator; the length of the 1/4 wavelength stub; and the tap
point on the stub for a 50-ohm match. In addition, a major problem
for conventional coaxial feed, if no balun is used, the transmission
line becomes a part of the antenna system --- currents are induced to
flow on the outer surface of the shield of the coax.

I have also worried about feeding the antenna --- it should be fed by
a current-balun, e.g., ferrite beads over coax, since the input
impedance of the two parts of the antenna are very different --- so
different it perhaps seems worrysome. If you feed the antenna via two
sources, the impedance seen by one source can be negative!!! (see
below*)

My comments to amateurs in radio with respect to the HF version of
this type of antenna, the Zepp antenna, has generally been to scrap it
--- particularly if this end fed antenna is to be used as a multi-band
antenna. A transmission line having whatever length, fed at one end,
where one conductor is connected to nothing at the antenna end, and
the other conductor is connected to the antenna, presents to the balun
an absolutely wild problem --- since ideally one wants equal currents
into both conductors.

A problem that has concerned me is modelling the Ribbon-J --- since
the presence of the floating wire, a part of the Ribbon-J, and the gap
width change the resonant frequency of the antenna.

In spite of this, and comments above, I have devised a 2 metre
Ribbon-J that works very well indeed, c.f. my comments in Pat Hawker,
G3VA's Technical Topics column, Radio Communication, May 1995,
pp. 61-62.

Now concerning your comment about a J-antenna used as a 2-band antenna
(2m and 70 cm). My remarks on this antenna system were published in
the above referenced TT column. While a J-antenna dimensioned for 2m
may have an acceptable SWR on the 70 cm band, the radiator length for
this band is 1 1/2 wavelengths, and so the major lobe is no longer
directed toward the horizon; in fact the lobe directed toward the
horizon is a minor lobe.

There are also problems with phasing, for the extended version of
J-antennas. It is easy to find that the major lobe is lifted off the
horizon.

A Happy New Year and have fun modelling.

73, Jack, VE2CV

---------------------------------------

* As an example, let us look at the 10 m J antenna that I previously
modelled (QST April 1995). If fed by two sources, frequency 19.4 MHz,
Z1 (referenced to the base of the long element) = 15.85 - j 14.72; Z2
(referenced to the base of the short element) = - 1.4 + j 14.97, and
Zant = Z1 + Z2 = 14.45 + j 0.25 ohms, Gain, on a 6 metre mast over
average ground 3.13 dBi.

If fed by a single source placed at the centre of the element that
shorts the 1/4 wavelength stub, Zant = 14.84 + j 0.34 ohms, Gain =
2.88 dBi.

John S. (Jack) Belrose, PhD Cantab, VE2CV
Senior Radio Scientist
Radio Sciences Branch
3701 Carling Avenue
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2308
FAX 613-998-4077
e-mail john.belrose_at_crc.doc.ca
Received on Tue Jan 06 1998 - 09:36:37 EST