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1/4 vs 1/2 wavelength antenna



 
 
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  #1  
Old February 21st 05, 06:09 AM
Nug
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Default 1/4 vs 1/2 wavelength antenna

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
  #2  
Old February 21st 05, 08:07 AM
Leon Heller
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"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


  #3  
Old February 21st 05, 09:12 AM
Bob Bob
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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

  #4  
Old February 21st 05, 09:37 AM
Rich Grise
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Default

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


  #5  
Old February 21st 05, 03:52 PM
Ken Smith
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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

  #6  
Old February 21st 05, 07:13 PM
Richard Harrison
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Nug wrote:
"I am building an rf transmitter for a short range data link at 433 MHz
and am almost done, but I would like to understand better exactly what I
am seeing with regard to antenna performance."

Your wavelength is about 0.69 meter or about 2.3 feet. In antennas
everything depends on wavelength. If you use a transmitter housing as
the ground plane for your antenna, it needs to be a sizeable part of a
wavelength or the salient part of your antenna must be longer to
compensate for the small ground plane.

If you had an infinite ground plane, a 1/2-wave wire perpendicular to it
would produce up to 50% more volts per meter field strength than a
1/4-wave wire perpendicular to the ground plane. It`s not something for
nothing. Total radiation is the same in both cases. More of the
radiation is perpendicular to the wire in the 1/2-wave and less goes off
at some other angle to the wire.

50% more field in some particular direction is realy not very
significant in most cases, and there are other consequences of using a
1/2-wave wire instead of a 1/4-wave wire.

An end-driven 1/2-wave wire presents a very high impedance. It is
equivalent to a parallel-resonant circuit. It would match a direct
connection to a parallel resonant tank circuit perhaps.

An end-driven 1/4-wave wire presents a very low impedance when worked
against a ground plane, maybe about 30 ohms.

How well you are able to radiate a signal from a wire is likely to
depend on how well it is matched to the transmitter and less about the
bends in the wire. In any case the complete antenna must be resonant to
eliminate reactance which opposes the signal`s entry into the wire.

For a small transmitter operating at a very short wavelength, the size
of the antenna is not onerous and it would be possible to use a
center-fed 1/2-wave antenna. Each half would be just a little over a
half foot in length. Drivepoint impedance is in the 70-ohm range.
Another possibility is a full-wave loop, about 2.3 feet in perimeter
with a drivepoint impedance of about 120 ohms.

Performance of all the suggestions is probably about the same. You can
find the best by trying them.

Best regards, Richard Harrison, KB5WZI

  #7  
Old February 22nd 05, 06:43 AM
[email protected]
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Default

Thanks for all your responses, good stuff here.

Rich, I am using TX modules from Liapac which specify a 50 ohm load.

So putting it all together (I think) by bending the antenna (both 1/4
and 1/2) I am messing with its impedance and subsequent matching? So I
would be better off with a straight 1/4 over ground plane if I had the
choice.

From reading I've done I think that a bend from vertical to horizontal

(closer to ground) would be decreasing the antenna impedance, does that
sound correct? Would I benefit from adjusting (extending) the antenna
length slightly (I don't think so because it would no longer be at
wavelength, but I am not sure?).

Ken said ... "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"... Actually it's about half and half
(feed up 15mm , loop around the case, up - {clear of case} 130mm).
Note the horizontal loop does not cross over itself.

Sort of like this (~~ is horizontal loop) side view.

|
|
|
~~~~
|
feed

I understand that more of the radiation is perpendicular to the wire in
the 1/2-wave (than 1/4 wave), broadly speaking how will the bend('s) in
the aerial effect radiation pattern (I understand this is tough to
answer)? I can't fit a full loop inside the case.

I have no rf test equipment so can only use trial and error, thanks
again to all who responded. As I said it actually works fine it's just
I don't like not understanding the reason its working, and would like
to make any small changes that may improve performance.

Thanks Again

  #8  
Old February 23rd 05, 07:08 PM
gwhite
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Posts: n/a
Default

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.
  #9  
Old February 23rd 05, 10:27 PM
Richard Clark
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Default

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
  #10  
Old February 23rd 05, 10:32 PM
Richard Harrison
Guest
 
Posts: n/a
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G. White wrote:
"Output Z is irrelevant."

This is an old argument in this newsgroup. I became convinced long ago
that there are cases in which impedance is very important.

"Transmission Lines, Antennas, and Wave Guides" by King, Mimno, and Wing
make a clear and concise case for the princuple of conjugates in
impedance matching on page 43:

"If a dissipationless network is inserted between a constant-voltage
generator of impedance Zg, and a load of impedance ZR such that maximum
power is delivered to the load, at every pair of terminals the impedance
looking in opposite directions are conjugates of each other.

To secure maximum power output from a generator whose emf and internal
impedance are constant the load must have an impedance equal to the
conjugate of the generator`s internal impedance."

Radio transmitters don`t produce significant harmonics. It`s the law.
They are linear power sources. We can and do tune them for all the power
they will produce under their particular operating conditions of drive
and d-c power supply. They operate at more than 50% efficiency which
means that they don`t take power 100% of the time, but are switched-off
during part of the r-f cycle. Output impedance is thus an average over
the entire cycle. It`s OK. We have no harmonics. Gaps are filled by the
tank circuit and other filters. The radio is a proper source. The
impedance added by off-time is called "dissipationless resistance"
because no power is lost in the radio while it is switched-off.

Best regards, Richard Harrison, KB5WZI


 




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