Quadrifilar Helix
aa...@gate.kc5aug.ampr.org
[Reply for: Ed Nowicki evn...@nmaa.org]
Hi Ed, sorry to take so long to reply, normal fubar here...
The small loop has to go inside the larger one; it won't go otherwise. I
used RG-58 all the way around, instead of rg8 + small wire. I cut the
large wire and small wire separately. I used a gdo to measure the resonant
frequency of the separate wires. I mounted the small loop first, making an
incision midway for connecting the other loop. I did the large loop the same
making the connections as in the article. The article was by far the
clearest illustation of any book I've seen with the QF. I made each loop
an extra inch longer intentionally, for pruning, and general errors due to
connecting, soldering, etc. The small loop was exactly centered inside the
large loop, as close as I can measure. I fed all eight end pieces inside,
not outside, each spreader. I cut away a middle section of each spreader to
feed the coax out to be spliced to the other loop. Both spreader cuts are
still inside the main boom. I mounted the SO-239 on the main boom just below
one of the spreaders. I wanted to leave one side of the boom long, for ease
of mounting. I had to cut if off to get inside, for splicing the two loops.
The extra inch brings the antenna in about 10 MHz low, until I trim it.
I do plan to build the 440 version next, using the same construction, for
receiving UO-22, etc. Later, I plan to build two other versions.
1. Four QF's mounted in a square, phased, for extra gain.
2. A QF 4 wavelengths long instead of 1, also for more gain, by extending
the boom length.(QuadraQuadrafilar)
How would a QF work with a relector dish?
I've done some spotty testing using the 137.5 satellite, and the pattern
seems to be about what it should be. I need to update my kepler set, but
it seems to run just above the horizon a couple of degrees before dropping
out.
TS
Bob Bruhns
aa...@gate.kc5aug.ampr.ORG wrote (re quadrifilar helix antennas:
: ... I plan to build two more
: 1. Four QF's mounted in a square, phased, for extra gain.
: extending the boom length.(QuadraQuadrafilar)
: How would a QF work with a relector dish?
TS, that first idea holds some promise if you are going to make a
variably-phased array of QFHs. The problem with the second idea is
that you will lose the uniform hemispherical coverage, and therefore
you will have to point the antenna with an az-el mount. If you're
going to do this, you might as well use a regular helix, or crossed
yagis, etc.
The whole point of the QFH, as I understand it, is that it provides
circular polarization over a rather uniform hemisperical pattern, with
a small and simple antenna. This allows you to transmit to / receive
from a tumbling, non-stationary satellite without having to reposition
your antenna, and without polarization fading. However, it necessarily
means low antenna gain.
Clever QFH designs sacrifice some high-angle gain for some extra
sensitivity at lower angles, to compensate for the higher path loss
at lower look angles. But if you go beyond these "equal-signal"
patterns, you will wind up with an antenna that only performs well
for horizontal look angles, or a beamed pattern that you have to
continually point at the satellite as it passes across the sky.
Wide angle coverage necessarily means low gain. As soon as you
stack or redesign QFHs for gain, you will narrow the directional
pattern. Of course, in the case of the adjustably phased array,
this is OK; you would dynamically vary the phasings to direct the
beam towards the moving target. But in the case of a fixed-pattern
long-boom QFH, you might as well use a helix, or crossed yagis, etc.
The reflector idea has the same problem. I think the QFH gets its
half-spherical pattern because of reflection from the surface below.
But if you try to go beyond this for more sensitivity, you will at
best narrow the beam, and then you're back to physical repositioning
for maximum signal, and the regular helix or crossed yagi designs
already offer good, proven gain configurations. Of course, you may
stumble upon a good unidirectional gain design based on QFH
techniques, if that's what you want to do.
Bob Bruhns, WA3WDR, bbr...@li.net
Stefan Hegnauer
aa...@gate.kc5aug.ampr.ORG wrote:
>
> The article was by far the
> clearest illustation of any book I've seen with the QF.
>
Sorry, i missed the start. Which article are you referring to?
--
thanks
Stefan
==============================
Stefan Hegnauer
dipl. Elektro-Ingenieur ETH
Spectrospin AG
Sensorik Entwicklung
Industriestrasse 26
CH-8117 Faellanden
tel. ++41 (0)1 825 93 68
fax ++41 (0)1 825 96 96
==============================
Rich Griffiths
Sorry I missed the beginning of this thread (due to lots of business travel).
I'd like to offer a few comments.
Bob Bruhns wrote:
>
> aa...@gate.kc5aug.ampr.ORG wrote (re quadrifilar helix antennas:
> : ... I plan to build two more
> : 1. Four QF's mounted in a square, phased, for extra gain.
> : 2. A QF 4 wavelengths long instead of 1, also for more gain, by
> : extending the boom length.(QuadraQuadrafilar)
> : How would a QF work with a relector dish?
> : TS
>
> TS, that first idea holds some promise if you are going to make a
> variably-phased array of QFHs.
How would you do this? It's not obvious to me what "phased" would mean
for an array of QFH antennas. If you do make such an array, I'd love to
see the results of some careful measurements that compare the array and
a single QFH.
> The problem with the second idea is
> that you will lose the uniform hemispherical coverage, and therefore
> you will have to point the antenna with an az-el mount. If you're
> going to do this, you might as well use a regular helix, or crossed
> yagis, etc.
I think I agree.
> The whole point of the QFH, as I understand it, is that it provides
> circular polarization over a rather uniform hemisperical pattern, with
> a small and simple antenna. This allows you to transmit to / receive
> from a tumbling, non-stationary satellite without having to reposition
> your antenna, and without polarization fading. However, it necessarily
> means low antenna gain.
I think the whole point is that the satellites (many of which use QFH
antennas) are transmitting circularly polarized signals. An improperly
polarized receiving antenna suffers a loss (theoretical) from 3dB to 20dB.
Amateur use of QFH antennas for satellites that are not transmitting
circularly polarized signals is new (to me, anyway). I've read somewhere
(Taggart?) that it takes both RHCP and LHCP antennas and an antenna switch
to get good coverage through a complete pass.
> Clever QFH designs sacrifice some high-angle gain for some extra
> sensitivity at lower angles, to compensate for the higher path loss
> at lower look angles. But if you go beyond these "equal-signal"
> patterns, you will wind up with an antenna that only performs well
> for horizontal look angles, or a beamed pattern that you have to
> continually point at the satellite as it passes across the sky.
>
> Wide angle coverage necessarily means low gain. As soon as you
> stack or redesign QFHs for gain, you will narrow the directional
> pattern. Of course, in the case of the adjustably phased array,
> this is OK; you would dynamically vary the phasings to direct the
> beam towards the moving target. But in the case of a fixed-pattern
> long-boom QFH, you might as well use a helix, or crossed yagis, etc.
I think Kilgore (Mr. Quadrifilar Helix) worked out the principles of QFH
length and diameter vs. pattern. I don't have the reference handy,
unfortunately.
> The reflector idea has the same problem. I think the QFH gets its
> half-spherical pattern because of reflection from the surface below.
> But if you try to go beyond this for more sensitivity, you will at
> best narrow the beam, and then you're back to physical repositioning
> for maximum signal, and the regular helix or crossed yagi designs
> already offer good, proven gain configurations. Of course, you may
> stumble upon a good unidirectional gain design based on QFH
> techniques, if that's what you want to do.
I believe the QFH gets its hemispherical pattern from its shape and
feedpoint location, rather than from reflections from below. This would
mean that one must be used "upside down" if it were to be the feed for
a dish. That is, the "up" direction of the antenna would have to be
pointing into the dish.
For vhf and low uhf, it will take a pretty big dish. At higher frequencies,
I think there are better ways to make a circularly polarized feed.
Rich Griffiths W2RG
Bob Bruhns
Regarding a variably phased array of QFHs, Rich Griffiths (ri...@one.net)
wrote:
: How would you do this? It's not obvious to me what "phased" would mean
: for an array of QFH antennas. If you do make such an array, I'd love to
: see the results of some careful measurements that compare the array and
: a single QFH.
Hi, Rich. Phased array technology is sometimes used for
electrically directable antennas. With a fixed array of antennas, the
received signals are combined (or the transmitter power is split) in
such a way that a certain direction is favored. This is the way AM
broadcast stations use multiple towers to produce a particular
directional pattern. The signal is more delayed for the component
antennas that are closer to the desired direction, and less delayed
for the component antennas that are further from the desired
direction. It is also possible to use unequal power splitting
to deliberately make nulls less deep, allowing designers to achieve
many possible directional patterns.
By using many antenna elements, it is possible to achieve
substantial gain and directivity. In fact, multi-element antennas
such as Yagis are all phased arrays. They are usually optimized for
forward gain, or slightly modified for optimum front-to-back, etc. In
these typical arrays, the antenna elements are ordinarily "parasitic",
meaning they are not directly driven. But in some cases, such as the
log periodic V, and Sterba curtains, etc, they are directly driven.
It is possible to optimize an array for any desired azimuth /
elevation and direct the beam as desired by arranging controllable
variable delays to each QFH in the array. Of course, AM broadcast
band stations use vertical antennas, not QFHs; their directional
patterns are meant to be along the horizontal plane. The vertical
antenna null at high radiation angles would compromise the performance
of an array of verticals if the desired beam were to be far above the
horizon. But with an array of QFH antennas, this problem is
eliminated.
By the way, I have also read about circular polarization reversing
sense as a satellite passes zenith. I don't work satellites, but I
think this is not the case with QFHs. I could be wrong, though. And
I always thought the early satellites (some of them at least) had
simpler antennas that produced polarization fading as the satellite
orbited. Could be wrong about that too.
Bob Bruhns, WA3WDR, bbr...@li.net
Rich Griffiths
Bob Bruhns wrote:
>
> Regarding a variably phased array of QFHs, Rich Griffiths (ri...@one.net)
> wrote:
> : How would you do this? It's not obvious to me what "phased" would mean
> : for an array of QFH antennas. If you do make such an array, I'd love to
> : see the results of some careful measurements that compare the array and
> : a single QFH.
>
<snip>
> It is possible to optimize an array for any desired azimuth /
> elevation and direct the beam as desired by arranging controllable
> variable delays to each QFH in the array. Of course, AM broadcast
> band stations use vertical antennas, not QFHs; their directional
> patterns are meant to be along the horizontal plane. The vertical
> antenna null at high radiation angles would compromise the performance
> of an array of verticals if the desired beam were to be far above the
> horizon. But with an array of QFH antennas, this problem is
> eliminated.
I'm familiar with the principles of phasing and interference. But here,
the devil is in the details. One of the finer features of the QFH is that
it should retain true circular polarization throughout the hemisphere into
which it is radiating. I wonder if this would be true with an array of
QFHs. Or would the polarization degenerate to elliptical off the axis and
approach linear polarization at some point? If the pattern becomes too
weird, the antenna offers no advantages over crossed yagis and is more
complicated.
However, I don't want to discourage anyone who is willing to experiment.
I just want to urge him to measure carefully, as he will be doing something
quite unusual. And *please* post the results; I'd love to see them.
> By the way, I have also read about circular polarization reversing
> sense as a satellite passes zenith. I don't work satellites, but I
> think this is not the case with QFHs.
It's not the case with the polar-orbiting weather satellites. They are
pretty much rock-solid from AOS to LOS with an RHCP QFH antenna.
> I could be wrong, though. And
> I always thought the early satellites (some of them at least) had
> simpler antennas that produced polarization fading as the satellite
> orbited. Could be wrong about that too.
Some (still) use linear polarization. Satellite tumbling and poor look
angles can be real problems.
Rich Griffiths W2RG
Bob Bruhns
Rich Griffiths (ri...@one.net) wrote:
: I'm familiar with the principles of phasing and interference. But
: here, the devil is in the details. One of the finer features of the
: QFH is that it should retain true circular polarization throughout
: the hemisphere into which it is radiating. I wonder if this would
: be true with an array of QFHs. Or would the polarization degenerate
: to elliptical off the axis and approach linear polarization at some
: point? If the pattern becomes too weird, the antenna offers no
: advantages over crossed yagis and is more complicated.
Good point, Rich. I'll have to think about that. And thanks for
the info on the polar-orbit weathersats and the tumbling issue.
I frequently need a sanity check!
Bob Bruhns, WA3WDR, bbr...@li.net
Alan Koblinski (W7XA)
I built the QFH in the August with very good results. After a little
more research, I found a chapter in W2DU's ARRL "reflections...." book
that talked about the QFH and a 1.25 turn version that looked very
interesting for satellite work. The pattern is squished down a little
with about 4dbi toward the horizon and a bit less off the front.
The half turn version was definitely directive with good RS15 satellite
return signals (with 15 watts) off the front and barely audible off the
sides. Since this antenna will require some pointing anyway, I think
your idea of an array would be pretty good. I am considering it
myself. I think the array of 4 would be easier to handle than a long
yagi of similar gain (and would look pretty neat too!). I don't know if
the reflector (behind the antenna) wouldn't screw up the pattern but it
might especially since a reflected signal would be opposite circular
polarization and indeed might kill the performance of the antenna. If
you used the QFH to excite a parabola (in front of the QFH) it would
probably be very good.
Just some thoughts.
73, Al