Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: J pole from flat strips  (Read 3003 times)
RADMANCF
Member

Posts: 12




Ignore
« on: February 16, 2013, 08:22:35 AM »

All of the J pole designs I've seen use cylindrical elements. I've been wondering if using flat metal strips would require any alterations to the design, due to changes in capacitance between elements. My primary hobby is High Power Rocketry, and I am starting to use RF beacons for tracking. I'd like to use flat strip for antennae, as thry could be glued directly to the surface of the airframe, and would be extremely lightweight. Any thoughts?

King David 0 Union Frank Charles
Logged
G8HQP
Member

Posts: 120




Ignore
« Reply #1 on: February 16, 2013, 11:01:30 AM »

The J-pole needs to be well away from nearby items as it is essentially an end-fed halfwave dipole with a quarter-wave matching section. It might work if the airframe is non-conducting, but a simple dipole would be much better - simpler and lighter too!
Logged
RADMANCF
Member

Posts: 12




Ignore
« Reply #2 on: February 16, 2013, 11:26:52 AM »

Conductive airframes are exceedingly rare in hobby rocketry. On the lower end, cardboard tubes are very common. On the high end, alumnum is used occasionally, but fiberglass and carbon fiber is much more common. One problem with carbon fiber is it's opacity to RF, which is one of the reasons I'm interested in external antennae.
Logged
W4OP
Member

Posts: 392


WWW

Ignore
« Reply #3 on: February 16, 2013, 11:44:17 AM »

The simplest solution would be to end feed the half wave wire with  either a tapped L-C or an L network. That way, the antenna is 1/4 lambda shorter, lighter and you are not concerned with getting the radiator length, stub length and feedpoint all correct.
You can even make the TX PA output  a high impedance and do away with the matching network that an end fed half wave would otherwise require.

Be cautious though- just because  the rocket tube may be non-conductive, tells little about what it looks like to VHF or UHF RF.
Design the antenna and then temporarily stick it to the tube and see how the properties shift.

Dale w4OP
Logged
KB4QAA
Member

Posts: 2260




Ignore
« Reply #4 on: February 16, 2013, 03:26:00 PM »

A Jpole is a very poor antenna selection for a rocket.  Not that it is a particularly great antenna to begin with.  Wink

All the is needed for a tracker beacon is a small piece of wire.  Ideally it would be 1/4 wavelength long, but if the size or construction of the rocket limits it, any length is adequate.  They only transmit a few milliwatts and Impedance/VSWR matching is not in any ways critical.

One comparative example is the RC modelers.   All they use is a short piece of hookup wire, about 22AWG-24AWG, Multistranded.  That's all that is necessary. 

You can get wire at Radio Shack, from some flexible Cat5/6 Ethernet cable, or salvage from some dead electronics.  The wire antenna does not need to be external to the rocket as long as the rocket is not made with any shielding (metal) material.

Follow the manufacturers' recommendations or you can calculate the 1/4WL dimension as follows:

234/Freq in Mhz = Length in Feet

ex.   234/ 430.110 Mhz = 0.544 Ft x 12 inches = 6.53 inches.

« Last Edit: February 16, 2013, 03:31:20 PM by KB4QAA » Logged
WB6BYU
Member

Posts: 13010




Ignore
« Reply #5 on: February 16, 2013, 09:27:03 PM »

Flat metal strips, aluminum angle stock, and many other shapes will
work for a J-pole.  You always have to make some adjustments based
on the resonant frequency of the radiator (which changes with the
effective cross-sectional area) and the velocity factor and
characteristic impedance of the matching section (which is a function
of the size and spacing of the conductors and the dielectric constant
of the surrounding material.)

That last piece can cause unexpected problems - it is why a J-pole
will change SWR when it is slipped inside a length of PVC pipe even
though the pipe isn't conductive.  If the antenna is attached to
the side of a cardboard tube full of propellant, for example, the
cardboard might not make much difference, but if the propellent
has a higher dielectric constant the impedance of the matching
stub will change to some extent as the propellant burns.


While that might not be a problem:  design the antenna for best
match without the propellant, since that is when the transmitter
will be the furthest from you.  But in general I'd agree with the
others that a quarter wave of thin insulated, stranded wire is
probably a better choice as it would be less prone to damage
on impact, assuming you are using the beacon to recover the
remains of the rocket.  Another options would be to wrap a
spiral dipole around the rocket frame.
Logged
KB4QAA
Member

Posts: 2260




Ignore
« Reply #6 on: February 16, 2013, 09:51:39 PM »

One other point against using a Jpole:  Safety.

It's been a few decades since I launched rockets, but as I recall the Safety Code for rocketry had restrictions about using sheet metal, and in general limited any use of metal to the absolute minimum necessary.   I don't fancy the idea of a rocket coming down with a failed recovery system, loaded up with essentially what are essentially flying razor blades.  Smiley
Logged
RADMANCF
Member

Posts: 12




Ignore
« Reply #7 on: February 17, 2013, 10:23:13 AM »

With regards to safety, it's important to remember the distinction between model rocketry (MPR) and high power rocketry (HPR). There are seperate safety codes for each, which in turn are based on different NFPA codes, (1122 and 1127, respectively). The HPR safety codes allow for a greater use of metal than the MPR codes do, and there is some debate as to wheather the wording of the HPR codes may require the use of metal airframes in certain cases ( at higher supersonic speeds, fiberglass, carbon fiber, and other composites fail, as they can't take the heat). For reloadable engines, anodized aluminum is the standard material, so I doubt many people would question the use of thin aluminum strips glued to the exterior of a rocket. As to why I'm interested in using strips: they would be more aerodynamic than insulated wires, and easier to attatch using standard rocket construction methods, such as epoxy. Durability would be greater than a wire, also. A peice of wire would be hard to glue to the surface, and would create excessive drag, and be more prone to damage. It's important to remember that high power rockets aren't the model rockets you launched as a kid, with A through C engines. High power rocketry uses H through O engines, are capable of supersonic, and in some cases hypersonic speeds, and can easily accelerate from 0 to 300mph in under a second. What works well in an RC plane could easily fall apart in an HPR
KD0UFC
Logged
KCJ9091
Member

Posts: 0




Ignore
« Reply #8 on: February 17, 2013, 04:05:39 PM »

How big are the fins and what are they made of?
Logged
RADMANCF
Member

Posts: 12




Ignore
« Reply #9 on: February 17, 2013, 06:41:55 PM »

Fin size depends on a lot of things, but typically, the fin span is less than twice the airframe diameter. Chord varies widely. Fins are typically less than 0.125in thick. Common fin materials range from plywood, to G-10 fiberglass, to carbon fiber sheets. On rockets expected to reach higher speeds, it's common to wrap the tail section of the rocket, including the fins, with fiberglass or carbon fiber cloth. While it isn't common, it is possible to use balsa for HPR fins. It just needs to be reinforced with other materials. I built fins for one rocket by sandwiching layers of balsa, paper, and sheet styrene.
Logged
WB6BYU
Member

Posts: 13010




Ignore
« Reply #10 on: February 17, 2013, 06:52:53 PM »

Choice of designs will depend on the operating frequency and the
relative size of the rocket.  

I've looked all over and can't find my copy of an article on missile
antennas created by cutting a spiral slot in the shell.  This was
useful when the missile wasn't long enough to insulate the two
halves of the shell and use it as a dipole.

Other options have included winding the antenna wire in a spiral
inside the nosecone, cutting a notch in the base of a fin to load
it against the rest of the frame,  etc.  But I can't think of a
condition where I would choose to use a J-pole instead of a
single strip of conductor running in the same place, just not as
far.  Well, you might manage it if you used a series feed rather
than a tapped quarter wave stub, but a J-pole as an antenna
doesn't have enough advantage to justify the extra work.
Logged
N4CR
Member

Posts: 1650




Ignore
« Reply #11 on: February 17, 2013, 09:25:58 PM »

A J-Pole is 3/4 wavelength long, has a high impedance driven point has displays an array of bad habits. A dipole is 1/2 wavelength long, has a low impedance driven point and is better behaved antenna all around.

And they have equal gain.
Logged

73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
KCJ9091
Member

Posts: 0




Ignore
« Reply #12 on: February 18, 2013, 04:44:51 PM »

Fin size depends on a lot of things, but typically, the fin span is less than twice the airframe diameter. Chord varies widely. Fins are typically less than 0.125in thick. Common fin materials range from plywood, to G-10 fiberglass, to carbon fiber sheets. On rockets expected to reach higher speeds, it's common to wrap the tail section of the rocket, including the fins, with fiberglass or carbon fiber cloth. While it isn't common, it is possible to use balsa for HPR fins. It just needs to be reinforced with other materials. I built fins for one rocket by sandwiching layers of balsa, paper, and sheet styrene.

Could you not lay the wires for the dipole on the fin core before laying the fiberglass?  Or a quarter wave wire on the fin and use the metal rocket body for the ground plane?
« Last Edit: February 18, 2013, 04:49:08 PM by KCJ9091 » Logged
W5WSS
Member

Posts: 1631




Ignore
« Reply #13 on: February 19, 2013, 08:50:03 AM »

Some thoughts on attaching an antenna on a model rocket.

The quarter wave vertical with sloping radials would occupy less total vertical space.

Would the utility of the 1/4 wave version preclude it relative to the 5/8 wave?
The difference in ERP is in pattern development where some additional manifested power expressed as gain is really a function of Where that power manifests and at what angles etc. At that height above ground an analysis would be needed to prove otherwise.

Relative to the 5/8 wave:

The 1/4 wave will be lighter in weight when all of the parameters are held equal.
The 1/4 wave will be easier to match for transmitting purposes.

The tail fins could? allow the radials to be held symmetrical and routed at an angle that will help raise the feed point for matching purposes.
They can be installed on a non conductive material without the need to worry about the added weight of insulation.

The Aerodynamic changes could be evaluated prior to launch or done empirically.
Logged
G8HQP
Member

Posts: 120




Ignore
« Reply #14 on: February 19, 2013, 10:02:28 AM »

For a 1/4 wave or 5/8 wave you have to ask what the antenna will use for a ground, as these are both 'ground plane' type antennas. A 1/2 wave does not have this problem. Note that a decent antenna may be more important for the transmitter than the receiver (e.g. model control). You may need to keep stray RF away from the rest of the circuit, so a well-behaved antenna may help.

If you have enough metal you can either use it as the antenna, or cut a slot in it and use the slot as an antenna. Otherwise a 1/2 wave fed via a simple balun will be best. If necessary you can fold it to reduce length, but this will change its feed impedance and reduce bandwidth.
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!