"Szczepan Bialek" wrote in message
...

"mr1956" wrote
...
On May 24, 4:26 am, Owen Duffy wrote:
mr1956 wrote in news:943a9bbd-214b-43b2-ac31-
:

I am looking for some help developing a properly tuned piano wire
antenna for an all metal experimental rocket.
This particular vehicle currently uses a Digi International 9Xtream
100 mw transmitter which operates using Frequency Hopping Spread
Spectrum from 910 to 918 MHz.

The first flight used a .062" diameter length of piano wire cut so
that the exposed length was about 1/2 wavelength. The wire antenna was
isolated from the metal airfame with a small nylon penetrator and
connected to the transmitter via about a 12" length of RG-178 coax.
The coax was terminated at the antenna via two small jumpers (soldered
to the center conductor and shield). The shield was grounded on the
metal airframe transforming the entire rocket into an artificial
ground plane (the antenna was also swept back at about a 45 degree
angle to reduce drag.

So you designed for a half wave antenna fed with 12" of RG178.

Lets suppose for a moment that the antenna has a feedpoint Z of say,
2000
+j0 ohms. The line will transform that to 5+j14 at the tx end, and with a
loss of 4.8dB (ie 33% efficiency). The tx is unlikely to develop is rated
output power into such a load, so there will be some further reduction.

Yes, an antenna of half the size (ie a quarter wave fed against the
metal
rocket body) might well work ten times as good.

Owen

I have been reading the ARRL antenna book and while there is much
information, a lot of it is over my head. But one point I think a
chapter makes is that a 1/4 wave monopole will work better than a 1/2
wave using an artificial ground plane because of the way the voltage
peaks at the end of the antenna; or that is how it seems.

You are right. Antenna radiate from the end.

look, if you want to discuss your harebrained ideas please keep them in your
own threads. don't try to confuse someone who has a real world problem with
your idiotic theories.

On Sun, 24 May 2009 19:22:24 +0200, Szczepan Bia?ek
wrote:
So it should be thick as possible.
Absolute nonsense, confirmed by the utter lack of experience:
How the length should be I do not know.

73's
Richard Clark, KB7QHC

mr1956 wrote in
:

....

I have been reading the ARRL antenna book and while there is much
information, a lot of it is over my head. But one point I think a
chapter makes is that a 1/4 wave monopole will work better than a 1/2
wave using an artificial ground plane because of the way the voltage
peaks at the end of the antenna; or that is how it seems.

Your transmitter is designed for a particular load impedance, almost
certainly 50 ohms... check the specs.

The load presented to the transmitter terminals depends on the feed point
impedance of the antenna, and impedance transformation that may occur on
the coaxial feedline.

In the case you describe, the *antenna system* design was most unsuited
to a 50 ohm transmitter, hence the considerable loss predicted.

Your newer proposal for a quarter wave wire perpedicular to the rocket
body with the coax shield connected to the metallic rocket body is not
exactly 50 ohms, but probably close enough that antenna system efficiency
will be fairly good and the transmitter should deliver most of its rated
power.


Owen

On May 24, 5:49*pm, Owen Duffy wrote:
mr1956 wrote :

...



I have been reading the ARRL antenna book and while there is much
information, a lot of it is over my head. *But one point I think a
chapter makes is that a 1/4 wave monopole will work better than a 1/2
wave using an artificial ground plane because of the way the voltage
peaks at the end of the antenna; or that is how it seems.

Your transmitter is designed for a particular load impedance, almost
certainly 50 ohms... check the specs.

The load presented to the transmitter terminals depends on the feed point
impedance of the antenna, and impedance transformation that may occur on
the coaxial feedline.

In the case you describe, the *antenna system* design was most unsuited
to a 50 ohm transmitter, hence the considerable loss predicted.

Your newer proposal for a quarter wave wire perpedicular to the rocket
body with the coax shield connected to the metallic rocket body is not
exactly 50 ohms, but probably close enough that antenna system efficiency
will be fairly good and the transmitter should deliver most of its rated
power.

Owen

What I have been finding out is developing a matching circuit for the
output of the transmitter is not easy, at least for me and at that
frequency. It could be done if I was a cell phone manufacturer. I
think much will be gained by using the 1/4 wave as opposed to the 1/2
wave on the airframe and a yagi or helix antenna on the ground
station.

I will post a link to some images of the project when I have time.
Information on the flight can be found at www.lokiresearch.com and
look for information about the Proteus 6 flight at the Balls 17 event,
Black Rock, NV September 2008.

I'll also add some thoughts about how I intend to proceed based on
everyone's great input from this forum. One quick question: Am I
correct in assuming that the ground lead of the rocket mounted antenna
should be as short as possible? i.e., the connection between the
shield of the RG-178 and the metal airframe.

Thanks,

Curt

On Mon, 25 May 2009 09:48:23 -0700 (PDT), mr1956
wrote:

One quick question: Am I
correct in assuming that the ground lead of the rocket mounted antenna
should be as short as possible? i.e., the connection between the
shield of the RG-178 and the metal airframe.

Very much so - mm instead of inches.

73's
Richard Clark, KB7QHC

mr1956 wrote in
:

....

What I have been finding out is developing a matching circuit for the
output of the transmitter is not easy, at least for me and at that
frequency. It could be done if I was a cell phone manufacturer. I
think much will be gained by using the 1/4 wave as opposed to the 1/2
wave on the airframe and a yagi or helix antenna on the ground
station.

The easy solution is the one I suggested.


I will post a link to some images of the project when I have time.
Information on the flight can be found at www.lokiresearch.com and
look for information about the Proteus 6 flight at the Balls 17 event,
Black Rock, NV September 2008.

I'll also add some thoughts about how I intend to proceed based on
everyone's great input from this forum. One quick question: Am I
correct in assuming that the ground lead of the rocket mounted antenna
should be as short as possible? i.e., the connection between the
shield of the RG-178 and the metal airframe.

Loss on the coax can be much worse when it is mismatched. I gave you
figures for a 2000 ohm load (representing your end fed half wave antenna).

The loss for 0.3m with a 30 ohm load is 0.5dB, it is 90% efficient. If you
shorten it, you will reduce the loss, but I wouldn't obsess over it. The
problem you have right now is due principally to high standing waves on the
line and unsuitable transmitter load, and you solve much of that using a
quarter wave antenna.

Without a diagram, we are at great risk of misunderstanding. If you cannot
affort the time to invest in making a drawing, you take the risk of
misinterpretation of your words. The connection from the shield to the
metal air frame and presumably metal skin should be short. Regard that the
effective length of the antenna in that case is the distance from the
shield to the tip of the whip.



Owen