Re: Sleeve dipoles

Dave Platt <dplatt@coop.radagast.org> wrote:

In article <1r4yps7.tln6011jcvupsN%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
 

https://kv5r.com    > Ham Radio   > 2-meter sleeve dipole
>
>
My reasoning is as follows:
>
1)  The co-ax is inside the bottom dipole element and close to the
actual radiating conductor, so the sleeve of the co-ax must be at about
the same potential as the corresponding position on the bottom element.
There is no attempt to separate them as there would be with a
large-diameter bottom element or 45-degree  Discone rods.

 
For sufficiently different values of "about the same potential", I think.
 
There will be capacitive coupling between the coax sleeve, and the
radiating element outside it.  How much, depends on the distance
between the two, and the dielectric constant of whatever's in
between them (PVC and air, in this case).
 
I don't think it's justified to say that they will actually be
at the same potential, since they aren't actually shorted
together anywhere other than up at the central feedpoint.
 

2)  That dipole is floating on approximately 7ft of insulating plastic
pipe and it appears that the length of the supporting pipe is not
critical.

 
Agreed, and it shouldn't be.
 

3)  The feed co-ax comes vertically downwards, not at right-angles to
the dipole.  If the length of the mounting pole is not critical, it
follows that the length of the co-ax is not critical.

 
I'd say that the length of the coax is going to be relevant, but
perhaps not critical.
 

4)  Therefore the coax sleeve does not carry standing waves and must be
at earth potential up to the point where it enters the bottom element.

 
That, I believe, is not a logical conclusion, based on what the author
writes.  Also, it doesn't jibe with what I've read elsewhere about
RF-on-the-coax situations.
 

5) Therefore the bottom of the bottom element of the dipole is at earth
potential.

 
At DC, sure.  At RF, I truly do not believe so.
 
I fear that you're chaining together a series of somewhat unsupported
assumptions to come to this conclusion... too many "therefores" which
aren't really justified.
 
I think you'd actually need to build one, and perform some tests and
measurements, to see if those conclusions and arguments actually
hold up.
 
The tests I'd do would be two-fold:
 
-  Test to see if there's actually RF flowing on the coax when
   transmitting.
 
-  Measure the SWR (ideally, the actual complex impedance)
   as seen at the beginning of the feed-line at the base of the
   mast.  Then, try adding a quarter-wavelength of additional
   feedline, and see if that changes things.  Try
   cross-connecting the bottom of the lower tube to the outside
   of the braid, and see if that changes things (it won't _if_
   the two are actually at the same potential, but I believe that
   it probably will).  Try connecting the bottom of the lower
   tube to a metallic mast which is grounded at the bottom.
 

6)  If all the co-ax sleeve below the dipole is at earth potential,
there is no reason why it could not be encased in a metallic supporting
pole.
>
7)  If the bottom of the dipole is at earth potential, there is no
reason why it should not be electrically in contact with the earthed
supporting pole - or even form a continuation of it.

 
In both of those cases, I think your initial "if" isn't valid.  It
seems to be based on the assumption that the entire outside of the
coax shield is necessarily at earth potential.  That's going to
be true at DC, but I don't believe it's valid at RF.
 
Rather:  consider the current flows.  Inside the coax, you're
going to have balanced current flow... up the center conductor,
and back down the inside of the shield (or vice versa depending
on which half of the RF wave cycle you're in).
 
When the coax hits the feedpoints, the current coming up the center
conductor is going to go into the upper-half-of-the-antenna radiating
element (the upper foil tape) since that's the only place it can go.
 
The current coming up the inside of the braid has two possible places
to go - out and down the lower foil tube (balancing the current flow
into the upper foil tube), or down the outside of the coax braid
(the "RF on the coax" situation, leading to "RF in the shack").
How the current divides itself, is going to depend on the RF
impedances the current "sees" presented by these two conductive
paths.
 
The outer tube will present an impedance of about half of the
antenna total... somewhere in the 25-35 ohm range, most likely,
at resonance.
 
The outside of the coax will present an impedance of... well,
it's going to depend on the installation.  It will depend on
the length of the coax (back to the transmitter or another
ground point) as this will influence its radiation resistance.
It'll depend on whether it's close to an odd or even number
of quarter-wavelengths, or somewhere inbetween (and it's
not likely to be purely resistive).  It is _not_ going to
be either zero (a "pure short to ground") or infinitely
high (and thus unable to carry any current).
 
So, you're going to end up with some amount of RF flowing
on the coax braid.  How much, and how much it affects the
pattern, is going to depend on the specifics of the
installation.  If you're lucky, the braid impedance is going
to be high and/or reactive enough that it won't allow much
real current to flow, the coax won't radiate much, and
 

The end result could be regarded as a quarter-wave whip above a
folded-back ground plane of indeterminate size.

 
Kinda agree, but I think there are actually three elements in
play here:
 
-  The quarter-wave upper whip.
 
-  The slightly-less-than-quarter-wave lower element, which
   is behaving like a normal not-grounded-at-the-end dipole
   element.
 
-  The feedline braid, and any metallic mast to which it's attached.
   This is connected to the "down" side of the dipole, at the
   feedpoint, but not elsewhere.  It acts as a secondary pathway
   for currents on that side of the antenna, it _will_ radiate,
   and it will affect the SWR/impedance/pattern in somewhat-
   unpredictable ways.
 
My overall conclusion is that the feedline coming down the inside
of the lower tube isn't really an "isolator".  Its braid serves
as an auxiliary (rather-unwanted) radiator, which one hopes doesn't
radiate enough to mess up the antenna pattern too greatly.

Thanks for those thoughts and I see your points.  My theory would work
as long as the feed point correctly terminates the co-ax - which it may
not.  Also, in theory, the co-ax iside the lower element ought to behave
like a quarter-wave 'metallic insulator', but it sounds as though you
have your doubts about that too. 

The propagation velocities of the co-ax and the lower element may
differ.  Perhaps this isthe real purpose of the offset feed point, it
keeps the length of co-ax inside the lower element at a
quarter-wavelength whilst maintaining the correct overall length of the
dipole.

I can certainly see that the theory may not hold good over the entire
band - but how much would that matter in practice?


--
~ Liz Tuddenham ~
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www.poppyrecords.co.uk