Hi
I am building an rf transmitter for a short range data link at 433MHZ
and am almost done, but I would like to understand better exactly what
I am seeing with regard to antenna performance.

All technical notes I have read recommend a 1/4 wave whip over ground
plane as offering the best performance, statements like: "Best range
is achieved with either a straight piece of wire, rod or PCB track @
1/4 wavelength over a ground plane", I understand many factors effect
performance however I have found that a "bent" 1/2 wavelength length
of wire offers better performance.

If I use a 1/4 wavelength I need (due to case requirements) to have
two 90 degree bends in it (feed - up, across, up).
If I use a 1/2 wavelength I need to run it once around the (plastic)
case (feed - up, around the case, up).

I hope this makes some sense, anyway I have found the 1/2 wave is less
effected by polarisation and offers generally better performance.
However while more ground plane may help a 1/4 wave it seems to hinder
the 1/2 wave, I guess because it shields the loop around the case?

Regards

"Nug" wrote in message
m...
Hi
I am building an rf transmitter for a short range data link at 433MHZ
and am almost done, but I would like to understand better exactly what
I am seeing with regard to antenna performance.

All technical notes I have read recommend a 1/4 wave whip over ground
plane as offering the best performance, statements like: "Best range
is achieved with either a straight piece of wire, rod or PCB track @
1/4 wavelength over a ground plane", I understand many factors effect
performance however I have found that a "bent" 1/2 wavelength length
of wire offers better performance.

If I use a 1/4 wavelength I need (due to case requirements) to have
two 90 degree bends in it (feed - up, across, up).
If I use a 1/2 wavelength I need to run it once around the (plastic)
case (feed - up, around the case, up).

I hope this makes some sense, anyway I have found the 1/2 wave is less
effected by polarisation and offers generally better performance.
However while more ground plane may help a 1/4 wave it seems to hinder
the 1/2 wave, I guess because it shields the loop around the case?

A 1/4 wavelength antenna really needs to be straight and at right angles to
the ground plane. That is probably why the 1/2 wavelength antenna works
better in your case.

Leon
--
Leon Heller, G1HSM
http://www.geocities.com/leon_heller

On Sun, 20 Feb 2005 22:09:15 -0800, Nug wrote:

Hi
I am building an rf transmitter for a short range data link at 433MHZ
and am almost done, but I would like to understand better exactly what
I am seeing with regard to antenna performance.

All technical notes I have read recommend a 1/4 wave whip over ground
plane as offering the best performance, statements like: "Best range
is achieved with either a straight piece of wire, rod or PCB track @
1/4 wavelength over a ground plane", I understand many factors effect
performance however I have found that a "bent" 1/2 wavelength length
of wire offers better performance.

If I use a 1/4 wavelength I need (due to case requirements) to have
two 90 degree bends in it (feed - up, across, up).
If I use a 1/2 wavelength I need to run it once around the (plastic)
case (feed - up, around the case, up).

I hope this makes some sense, anyway I have found the 1/2 wave is less
effected by polarisation and offers generally better performance.
However while more ground plane may help a 1/4 wave it seems to hinder
the 1/2 wave, I guess because it shields the loop around the case?


A 1/4 wave antenna will match to a low impedance, unbalanced. A 1/2 wave
dipole will match to a low impedance, balanced. A 1/2 wave piece of wire
fed at the end will match to a high impedance.

What kind of circuit are you using for your output?

Thanks,
Rich

I would make the grandiose statement that since you are bending the wire
it no longer exhibits the performance of a "standard" 1/4 or 1/2 wave
antenna. I would suggest that if you indeed made a 1/4 wave GP that
protruded from the box surface (with a suitable counterpoise) it would
outperform the 1/2 wave bent one..

Assuming you have to put the antenna inside the box or wrapped around it
I suggest you look into tuning it with some C and/or L. In that case you
would construct the antenna to fit your case parameters and adjust the
matching for best radiation. Keep in mind that the C/L tuning components
could be lengths of coax and open feeder/wire. (because of the high
operating freq)

Cheers Bob VK2YQA (Sydney Australia)



Nug wrote:
Hi
I am building an rf transmitter for a short range data link at 433MHZ
and am almost done, but I would like to understand better exactly what
I am seeing with regard to antenna performance.

All technical notes I have read recommend a 1/4 wave whip over ground
plane as offering the best performance, statements like: "Best range
is achieved with either a straight piece of wire, rod or PCB track @
1/4 wavelength over a ground plane", I understand many factors effect
performance however I have found that a "bent" 1/2 wavelength length
of wire offers better performance.

If I use a 1/4 wavelength I need (due to case requirements) to have
two 90 degree bends in it (feed - up, across, up).
If I use a 1/2 wavelength I need to run it once around the (plastic)
case (feed - up, around the case, up).

I hope this makes some sense, anyway I have found the 1/2 wave is less
effected by polarisation and offers generally better performance.
However while more ground plane may help a 1/4 wave it seems to hinder
the 1/2 wave, I guess because it shields the loop around the case?

Regards

In article ,
Nug wrote:
Hi
I am building an rf transmitter for a short range data link at 433MHZ
and am almost done, but I would like to understand better exactly what
I am seeing with regard to antenna performance.

[.. 1/4 wave and 1/2 wave ...]

An antenna looks like an LC tuned circuit loaded by the radiation
resistance. Your output stage has some impedance that correctly matches
to it (there are exceptions we will ignore) and it is this impedance you
want the antenna system to have. When the correct matching is done, the
antenna works as an impedance mathcing network that matches the output
stages impedance to the radiation resistance.

The normal (90 degrees to) 1/4 wave whip over a ground plane is one half
of a dipole that is 1/2 wave length. The ground plane operates like a
mirror. The electrostatic lines of force follow the same path with the
mirroring as they would if the other 1/2 of the dipole was there. This
lets you use a smaller (1/4 wave) antenna to get the same effect as the
1/2 wave.

In your case, you are not using a whip antenna. If I've read what you
wrote correctly, the antenna spends more of its length parallel to the
surface of the PCB than it does running 90 degrees away from it. You
have some circuit with a ground plane and a limitted sized box to work
with, so the mechanical shape is constained by the box and not the ideal
electronics.

Since the box is small: If you have the equipment to do so, I suggest you
measure (estimate) the impedance of the longest single loop of wire that
will fit within the case. ie: connect to both ends. You have to have the
electronics PCB in the case when you do this. If you are very lucky, its
impedance will not be too hard to match to the output stage.


--
--
forging knowledge

Ken Smith wrote:

In article ,
Nug wrote:
Hi
I am building an rf transmitter for a short range data link at 433MHZ
and am almost done, but I would like to understand better exactly what
I am seeing with regard to antenna performance.

[.. 1/4 wave and 1/2 wave ...]

An antenna looks like an LC tuned circuit loaded by the radiation
resistance. Your output stage has some impedance that correctly matches
to it (there are exceptions we will ignore) and it is this impedance you
want the antenna system to have. When the correct matching is done, the
antenna works as an impedance mathcing network that matches the output
stages impedance to the radiation resistance.

RF transmitters are not impedance matched to antennae in the sense of maximum
transfer of power. "Maximum transfer of power" is a small signal (ideal linear
parameters) issue, not a large signal issue. That is, the antenna/load are not
conjugately matched. What is said, is that a TX'er will deliver some given
power into, for example, 50 ohms. This says nothing about the output impedance
of the PA.

Power amplifiers are concerned with DC input power to RF output power
efficiency, thus they are load-line "matched," not impedance matched. The
concept of "output impedance" breaks down for large signal devices. For
example, what is the output impedance of a class C or D amp taken when the
transistor is on or off? I suppose one could consider the time-averaged
impedance, but I'm not sure of the utility (to be fair, the time-averaged
reactive output component is tuned out as best possible). The vague output
impedance is a problem even for large signal class A devices. Again, RF PA's
should be load-line matched. Output-Z is irrelevent.

On Wed, 23 Feb 2005 19:08:20 GMT, gwhite wrote:

RF transmitters are not ....

Sorry OM,

This was all nonsense.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On Wed, 23 Feb 2005 19:08:20 GMT, gwhite wrote:

RF transmitters are not ....

Sorry OM,

This was all nonsense.

Nice articulation. I don't know who OM is, but RF transmitter power amps are
not "impedance matched." Neither are audio power amps for that matter.

In article , gwhite wrote:
Richard Clark wrote:

On Wed, 23 Feb 2005 19:08:20 GMT, gwhite wrote:

RF transmitters are not ....

Sorry OM,

This was all nonsense.

Nice articulation. I don't know who OM is, but RF transmitter power amps are
not "impedance matched." Neither are audio power amps for that matter.

"OM" is an amateur radio term. It is short for "Old Man". It is a
respectful term for all other males that is quick to transmit via Morse
code. Richard Clark appears to be an amateur radio operator or the like.

RF transmitter power amps are certainly "impedance matched" to the
intended load. Take a look in the ARRL "The radio amateur's handbook".
If you have the 1944 addition, you will need to start reading at page 96
in the lower right column. If you don't have that, try Motorola's AN-721.

As for audio amp, you are 1 for 3 my friend.

--
--
forging knowledge

On Fri, 25 Feb 2005 03:17:12 GMT, gwhite wrote:

RF transmitter power amps are
not "impedance matched." Neither are audio power amps for that matter.

Hi OM,

You seem to be shy of facts and long on claims. Got any experience at
the bench, or is this all arm-chair philosophy?

73's
Richard Clark, KB7QHC