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I an trying to get my brain around some aspects of vertical sleeve
dipoles (in particular for 2-metres wavelength).
I understand the principle that the feeder (assumed 75 or 50-ohm co-ax)
is threaded up through the bottom quarter-wave element The
quarter-wave piece of feeder ascts as an isolattion stub so that the
bottom of the element can be earthed and the feed point is 'half-hot',
with the top of the upper element 'fully-hot'.
A further refinement is to offset the feed point slightly lower than the exact physical centre of the dipole so as to allow for the different propagation velocity of the waves in the feeder from that in the dipole elements, thus achieveing a better match.
If the bottom of the sleeve dipole is standing on the ground or a ground plane, this makes sense - but what if it is mounted on top of a
conductive metal pole of unspecified length? Won't the pole act as a
number of other dipoles which, depending on its length, can distort the radiation pattern in various ways?
Worse still, what if the bottom element of the sleeve dipole is simply a continuation of the pole (or eletrically connected to it) and the co-ax
is continued down inside the supporting pole to the bottom? Does the
co-ax need to be bonded to the pole at the point where the bottom
element should end?
Is the pole length irerelevant because a pole diectly below a vertical
dipole is in the null zone, so anything below an earthed bonding point
will not be energised?
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
I an trying to get my brain around some aspects of vertical sleeve
dipoles (in particular for 2-metres wavelength).
I understand the principle that the feeder (assumed 75 or 50-ohm co-ax)
is threaded up through the bottom quarter-wave element The
quarter-wave piece of feeder ascts as an isolattion stub so that the
bottom of the element can be earthed and the feed point is 'half-hot',
with the top of the upper element 'fully-hot'.
A further refinement is to offset the feed point slightly lower than the exact physical centre of the dipole so as to allow for the different propagation velocity of the waves in the feeder from that in the dipole elements, thus achieveing a better match.
If the bottom of the sleeve dipole is standing on the ground or a ground plane, this makes sense - but what if it is mounted on top of a
conductive metal pole of unspecified length? Won't the pole act as a number of other dipoles which, depending on its length, can distort the radiation pattern in various ways?
Worse still, what if the bottom element of the sleeve dipole is simply a continuation of the pole (or eletrically connected to it) and the co-ax
is continued down inside the supporting pole to the bottom? Does the co-ax need to be bonded to the pole at the point where the bottom
element should end?
Is the pole length irerelevant because a pole diectly below a vertical dipole is in the null zone, so anything below an earthed bonding point
will not be energised?
I donΓÇÖt think I follow your description, sounds like using the coax outer has the lower vertical element?
The kind I know was made from a bunch of baked bean tins as the
sleeve/lower vertical element. Upper vertical radiator plugged into
connector on topmost can lid. Coax feed hung down inside the sleeve.
Support was insulating pole going up inside cans, so high voltage node at
the rim of lowest can was in the clear.
The kind I know was made from a bunch of baked bean tins as the
sleeve/lower vertical element. Upper vertical radiator plugged into
connector on topmost can lid. Coax feed hung down inside the sleeve.
Support was insulating pole going up inside cans, so high voltage node at
the rim of lowest can was in the clear.
That's more like a conventional dipole with the centre feed point
'cold'. In the design I had in mind, the co-ax ran up inside the lower
tube of the dipole, which was earthed at the bottom. The dipole got >progressively 'hotter' as you went up. The feed point in the middle
was 'half-hot', so the quarter wavelength of cable feeding it acted as
an isolator and there was no difference in potential between the tube
and the braid.
....
That's more like a conventional dipole with the centre feed point
'cold'. In the design I had in mind, the co-ax ran up inside the lower
tube of the dipole, which was earthed at the bottom. The dipole got >progressively 'hotter' as you went up. The feed point in the middle
was 'half-hot', so the quarter wavelength of cable feeding it acted as
an isolator and there was no difference in potential between the tube
and the braid.
Can you point me to a picture or article which shows this sort of construction?
Dave Platt <dplatt@coop.radagast.org> wrote:
[...]
Can you point me to a picture or article which shows this sort of
construction?
https://kv5r.com > Ham Radio > 2-meter sleeve dipole
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.
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.
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.
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.
5) Therefore the bottom of the bottom element of the dipole is at earth >potential.
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.
The end result could be regarded as a quarter-wave whip above a
folded-back ground plane of indeterminate size.
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.
I an trying to get my brain around some aspects of vertical sleevedipoles (in particular for 2-metres wavelength).I understand the principle that the feeder (assumed 75 or 50-ohm co-ax)is threaded up through the bottom quarter-wave element Thequarter-wave piece of feeder ascts as an isolattion stub so that thebottom of the element can be earthed and the feed point is 'half-hot',with the top of the upper element 'fully-hot'.A further refinement is to offset the feed point slightly lower than theexact
The bottom dipole half is never grounded. Another name is coaxial
antenna. Usually you find them in marine application because of
the counter poise.
Suppose the feed point had a 1:1 isolation transformer, there would be
no reason why the dipole couldn't be grounded at any point alog its
length. The same isolatio could be achieved by a quarter-wave line.
liz@poppyrecords.invalid.invalid (Liz Tuddenham) Wrote in message:rsleevedipoles (in particular for 2-metres wavelength).I understand the principle that the feeder (assumed 75 or 50-ohm co-ax)is threaded up
I an trying to get my brain around some aspects of vertical
The bottom dipole half is never grounded. Another name is coaxial
antenna. Usually you find them in marine application because of
the counter poise.
In article <1r5178a.10eyjis13gf62kN%liz@poppyrecords.invalid.invalid>, liz@poppyrecords.invalid.invalid says...
The bottom dipole half is never grounded. Another name is coaxial
antenna. Usually you find them in marine application because of
the counter poise.
Suppose the feed point had a 1:1 isolation transformer, there would be
no reason why the dipole couldn't be grounded at any point alog its
length. The same isolatio could be achieved by a quarter-wave line.
The bottom half can not be grounded no matter what except at the feed
point maybe.
Suppose the feed point had a 1:1 isolation transformer, there would be
no reason why the dipole couldn't be grounded at any point alog its
length.
The same isolatio could be achieved by a quarter-wave line.
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.
