Re: Insulated Antenna Wire

On 7 Feb 1996, Dan A Bathker brought an interesting point to my attention.
His concern was with my statements about folded dipole designs and where
to connect the ends together - at the physical end, or at a closer
distance reflecting a less than unity velocity factor. Here are his
comments.

> My understanding is that in the normal transmission-line mode
> where currents in the twinlead or ladderline are *opposed*
> (+/- side-to-side) then the velocity factor/effective dielectric
> constant is active and the wave on the transmission line is
> indeed slowed by the surrounding medium as you said.
>
> But the folded dipole itself has currents in the *top* and
> *bottom* wires that are *not* opposed, rather they are both
> (alternatingly) in the *same* direction. If one thinks about
> that for a moment, one realizes it must be so, otherwise there
> would be no radiation (I like that simple view so I don't have
> to remember all that stuff about the 4:1 impedance transformation
> that also occurs due to the *connections*, shown in many books
> but I never did quite follow all that).

At first thought, I agree with Dan. However, after scratching my head a
while I am not so sure. There really must be something more to this
question than this and I think the impedance matching considerations must
be considered. I hope some of the real experts here will comment.

This issue is of practical significance with antennas like the Franklin
array constructed with sections of coaxial cable. However, for the
purpose of this discussion, allow me to postulate a simpler case.

Assume you have a half-wave dipole made from the outer conductor of a
piece of coaxial cable. At the center of this length, the inner conductor
is split and a balanced transmission line is attached. At two points
(somewhere near the ends of the dipole or at the ends) the inner conductor
is attached to the outer conductor. The result is obviously a form of
folded dipole, only one conductor is hidden within the other. The question
is whether the short between the inner and outer conductors should be at
the end or not.

Now if the skin depth of the outer conductor is less than its thickness,
the currents on the inside and outside of this conductor should not
interact. The inside is clearly a TEM transmission line and with all of
its fields contained within, it should certainly have a velocity factor
less than unity. The outside of the conductor would be the radiating
antenna. Neglecting end effects and with a large length to diameter
ratio, it would basically have to be one half the free space wavelength to
be resonant.

This is what poses the interesting question: Where should you connect
the inner conductor to the outer conductor? At the ends of the antenna,
the wave traveling on the inside of the cable has gone more than 1/4
wavelength. [My guess, and it is _only_ a guess, is that the connection
would need to be slightly past the 1/4 wave point of the inner
transmission line with the total antenna length shortened slightly to
achieve resonance.]

I suspect this problem has already been thoroughly covered in one of the
classic antenna texts, but the only situation that comes to mind is a
vague recollection of an analysis of a sleeve dipole (perhaps in Kraus or
maybe Jasik; my books are unfortunately at home).

The concept of connecting the lower wire of a folded dipole to the upper
wire not at the ends but at a closer point seems to be popular with Bill
Orr, W6SAI, in his many Radio Handbooks. To answer Dan's second note to
me, the overall length of the top conductor is still a half wavelength;
it is only the distances to the shorts that are lessened. In this
respect, the lower dipole may be viewed less as a radiator and more as an
impedance matching device (T-match or double-ended gamma match). Once
again, I cannot state this with authority. I hope others here reply.

[However, I place little faith in Bill Orr's antenna theory after reading
his article in Ham Radio Magazine about lightning protection. He
suggested that a long, thin tube would act as a waveguide beyond cutoff
and would not propagate lightning pulses down it's center. I agree with
this. However Orr then suggested that antenna lead-in cables, rotor
cables, power cables, etc. could then be run inside this tube and be
extended the same protection. While a Faraday cage completely covering
all cables would afford protection, Orr specifically brought up the
explanation of a waveguide beyond cutoff - not a complete Faraday cage.
The problem is that when wires are run inside the tube, you no longer
have a TE or TM transmission mode but a TEM one. The cutoff frequency of
a TEM transmission line is DC!]

> Anyway, I am enjoying the insulated wire thread, as we got
> some *real practitioners* participating--it's refreshing in a
> society where so many academics sometimes dominate.
>
> Regards,
> --------------------------------------
> Dan Bathker
> Spectrum Planning and Engineering
> Jet Propulsion Laboratory 303-401
> Pasadena, CA 91109 USA
> --------------------------------------
> (818)-354-3436, FAX (818)-393-1692
> --------------------------------------
> ** dab_at_jpl.nasa.gov **
> --------------------------------------

Once again, I agree. This has been a friendly and very educational group
and while I can rarely contribute in discussions on electromagnetics, I
can offer a little advice on polymers and materials. But then, how many
chemical engineers do you know besides myself who took the EE Antennas
and Transmission Lines course for the pure FUN of it?

                                Barry L. Ornitz WA4VZQ
                                Eastman Chemical Company Research
                                ornitz_at_eastman.com
Received on Wed Feb 07 1996 - 21:42:00 EST