Is it possible for a VSWR meter to read 1 to 1 with reactance at the
antenna?
Ex. If the antenna Z was R= 30 and X= 40 would the meter read 1 to 1?
Or is there a combination of R and X that would give 1 to 1?
Thanks for your answer and
explanations,
Mike

amdx wrote:
Is it possible for a VSWR meter to read 1 to 1 with reactance at the
antenna?
Ex. If the antenna Z was R= 30 and X= 40 would the meter read 1 to 1?
Or is there a combination of R and X that would give 1 to 1?

The SWR at the source is lower than the SWR at the
antenna due to losses in the transmission line. 200
feet of RG-58 used on 440 MHz will indicate an SWR
very close to 1:1 at the source no matter how reactive
the load at the antenna. On a normalized Smith Chart,
the SWR "circle" is really a spiral with an end limit
of Z = 1.0 + j0 in the center of the chart.
--
73, Cecil http://www.w5dxp.com

"amdx" wrote in message
...
Is it possible for a VSWR meter to read 1 to 1 with reactance at the
antenna?
Ex. If the antenna Z was R= 30 and X= 40 would the meter read 1 to 1?
Or is there a combination of R and X that would give 1 to 1?
Thanks for your answer and
explanations,
Mike

Mike

The VSWR meter is showing power reflected back down the line to the meter.
If the line is very lossy, like a long length of coax that has been
penetrated by water, the reading may well show 1:1 but the radiated power
will be zero. On its own, the VSWR meter provides a very useful indication
that the antenna system is matched to the output of the transmitter. It is
not a guarantee that the system is actually radiating a signal. A simple
field strength meter will give an indication that the power supplied via the
feeder to the antenna is actually being radiated rather than absorbed.

Cheap VSWR meters can be quite inaccurate when presented with loads which
differ from their design impedence. Used with an understanding of their
limitations, they are fine for everyday ham use, but something a bit more
sophisticated is needed for serious antenna system measurements.

Mike G0ULI

amdx wrote:

Is it possible for a VSWR meter to read 1 to 1 with reactance at the
antenna?...

The vswr is determined by the absolute value of the reflection
coefficient, being defined by:

1 + |ro|
vswr = ----------
1 - |ro|

where the greek letter "ro" indicates the complex reflection coefficient

Z - Zo
ro = ---------- .
Z + Zo

So the one and only case in which we can have unity value for vswr is
when |ro| = 0, or perfect match. In a * lossless * transmission line,
the vswr is the same in every point of the line (this is an easily
demonstrable general result, it comes right down from the above
expression), so (with normal lines) the answer is definitely no

Of course, as correctly pointed out by Cecil, in a lossy line the
reflection coefficient decreases as you move along the line away from
the termination, and the same happens to the vswr, so that a long lossy
line may simulate a good matching also when the matching isn't even near
to good (btw, using a lossy coax is an often used trick (especially in
microwaves), when you need to give to the generator a seemingly good load).

--
73 es 51 de i3hev, op. mario

Non è Radioamatore, se non gli fuma il saldatore!
- Campagna 2006 "Il Radioamatore non è uno che ascolta la radio"

it.hobby.radioamatori.moderato
http://digilander.libero.it/hamweb
http://digilander.libero.it/esperantovenezia

On Aug 15, 7:09?am, "amdx" wrote:
Is it possible for a VSWR meter to read 1 to 1 with reactance at the
antenna?
Ex. If the antenna Z was R= 30 and X= 40 would the meter read 1 to 1?
Or is there a combination of R and X that would give 1 to 1?
Thanks for your answer and
explanations,
Mike

As others have pointed out, unless the line is lossy, the answer is
no.

Here is a way to calculate SWR from R and X without the complex
arithmetic. It is taken from 'Reflections: transmission lines and
antennas' by Walt Maxwell, 1st ed, page 23-5, or 2nd ed page 'appendix
3-1'.

Let r = R / line impedance
Let x = X / line impedance

b = (1 + r*r + x*x) / r

SWR = (b + SQRT(b*b - 4)) / 2

The minimum value of b is 2 when r = 1 and x = 0

The minimum value of SWR is 1 when b = 2

Hence it can be readily seen that any reactance will cause the the SWR
to be greater than 1 at the load..

Just to complete the maths,the modulus of the relection coefficient,
ie mod(p),
= SQRT( (b-2) / (b+2) )

HTH and 73

John KC0G

On Aug 15, 5:09 am, "amdx" wrote:
Is it possible for a VSWR meter to read 1 to 1 with reactance at the
antenna?
Ex. If the antenna Z was R= 30 and X= 40 would the meter read 1 to 1?
Or is there a combination of R and X that would give 1 to 1?
Thanks for your answer and
explanations,
Mike

Contrary to what I see in other postings, yes, sure it could. This is
very easy to see on the Smith chart. Consider that the meter is
calibrated to 50+j0 ohms (which, by the way, it's not guaranteed to be
"out of the box" when you first get it!). Consider that connecting
the 30+j40 ohm load to the meter are two sections of lossless line.
The section connected to the load is 42.1 electrical degrees of 45 ohm
line, and the section from there to the SWR meter is 87.7 degrees of
82.5 ohm line. That gives you 50.03-j.01 ohms presented to the
meter. Note that 45 ohms is within 10% tolerance of 50 ohms, and 87.7
is a poorly-made 75 ohm line that's out of tolerance. Hey, copper has
gotten expensive, and the manufacturer is scrimping to save a few
bucks.

Likely the meter is NOT calibrated to 50 ohms, and that would allow a
different set of line impedances to achieve a match. With some loads,
you only need a single section of line the right length. For example,
if you said your meter was calibrated to 50 ohms, and the load was
67+j28 ohms, you'd get a 1:1 SWR reading with 42.6 degrees of 75 ohm
line. Note that the actual SWR on that line is NOT 1:1. SWR meters
tell you the line SWR only if they are calibrated to the line
impedance. Reg Edwards always used to rail against calling them SWR
meters, but to me, the name is fine if you understand the need for
calibration, the need to apply them properly.

Cheers,
Tom

"amdx" wrote in message
...
Is it possible for a VSWR meter to read 1 to 1 with reactance at the
antenna?
Ex. If the antenna Z was R= 30 and X= 40 would the meter read 1 to 1?
Or is there a combination of R and X that would give 1 to 1?
Thanks for your answer and
explanations,
Mike

The reason the question arises.
I've build a 2.4Ghz Patch antenna. It has gain. I move the feedpoint to
change the
input impedance. Now it has more signal strength, so I move in the same
direction again
and it shows even more signal strength, 2 more adjustments more signal, 1
more move
and signal strength looks like it's starting to go down in signal strength.
(These moves were about .060" each.)
Now I see a VSWR meter at http://pe2er.nl/wifiswr/
I wonder if this can help me get a handle on adjusting for proper impedance?
The reason I want 50 ohms is because I want to build an array of 4 Patches.
The 50 ohms is important to impedance matching with 1/4 wavelength coax.
I'm using this site as info for the matching and connections.
http://www.darc.de/distrikte/g/T_ATV...d-Array-GB.pdf
Yes, I'm using Patches instead of Biquads.

Thanks, Mike

PS. What do you think about the antenna spacing?







So now the problem I don't know what the input impedance is.

On Wed, 15 Aug 2007 14:38:46 -0500, "amdx" wrote:

I've build a 2.4Ghz Patch antenna. It has gain. I move the feedpoint to change the
input impedance. Now it has more signal strength, so I move in the same direction again
and it shows even more signal strength, 2 more adjustments more signal, 1 more move
and signal strength looks like it's starting to go down in signal strength.

Hi Mike,

Looks like you found away to get a low SWR (does it really matter how
much it is if you can't improve the output?).

(These moves were about .060" each.)
Now I see a VSWR meter at http://pe2er.nl/wifiswr/
I wonder if this can help me get a handle on adjusting for proper impedance?

It looks like a pretty good design. Remember to observe the power
limit.

The reason I want 50 ohms is because I want to build an array of 4 Patches.
The 50 ohms is important to impedance matching with 1/4 wavelength coax.
I'm using this site as info for the matching and connections.
http://www.darc.de/distrikte/g/T_ATV...d-Array-GB.pdf
Yes, I'm using Patches instead of Biquads.

The element lengths will vary (as will the pattern to some degree).

PS. What do you think about the antenna spacing?

You should stick to THOSE shown on the previous link.

So now the problem I don't know what the input impedance is.

You've got the plans for a good SWR meter, let it go at that. You
could probably build it faster than waiting for the final benediction
here.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Wed, 15 Aug 2007 14:38:46 -0500, "amdx" wrote:

I've build a 2.4Ghz Patch antenna. It has gain. I move the feedpoint to
change the
input impedance. Now it has more signal strength, so I move in the same
direction again
and it shows even more signal strength, 2 more adjustments more signal, 1
more move
and signal strength looks like it's starting to go down in signal
strength.

Hi Mike,

Looks like you found away to get a low SWR (does it really matter how
much it is if you can't improve the output?).

Oh, there's always a but,
I cut the foil to the antenna in a wifi adapter and soldered a 2" piece of
coax to the
foil and ground, my Patch is at the other end. The wifi adapter is a dongle
mounted
at the antenna. See Picture at;
http://www.tp-link.com/products/product_des.asp?id=47
Now the question is: Is the wifi adapter output really 50 ohms? If it's
not 50 ohms
then I need not attempt impedance matching with 1/4 Lambda length coax.
I searched for several hours looking for specs on the RF chip in the wifi
adapter
without any success.

(These moves were about .060" each.)
Now I see a VSWR meter at http://pe2er.nl/wifiswr/
I wonder if this can help me get a handle on adjusting for proper
impedance?

It looks like a pretty good design. Remember to observe the power
limit.

The reason I want 50 ohms is because I want to build an array of 4
Patches.
The 50 ohms is important to impedance matching with 1/4 wavelength coax.
I'm using this site as info for the matching and connections.
http://www.darc.de/distrikte/g/T_ATV...d-Array-GB.pdf
Yes, I'm using Patches instead of Biquads.

The element lengths will vary (as will the pattern to some degree).

PS. What do you think about the antenna spacing?

You should stick to THOSE shown on the previous link.

So now the problem I don't know what the input impedance is.

You've got the plans for a good SWR meter, let it go at that. You
could probably build it faster than waiting for the final benediction
here.

73's
Richard Clark, KB7QHC

I don't mind waiting, I don't have any parts for the VSWR meter.
(I was looking for confirmation that it would be useful.)
Regarding the spacing; The literature I've seen shows spacing of
0.2, 0.3 or 0.4 Lambda before grating lobes form. But the Patch is 0.5
Lambda wide so you can't get them closer than 0.5 Lambda.
Here's one source;
http://www.orbanmicrowave.com/The_Ba...nna_Arrays.pdf
At this point I have a working Patch with good signal strength, so now I'm
just trying to see if I can improve on it, The project has got complicated
because
the only measurement tool I have is the software signal strength meter that
came
with the wifi adapter card.
Thanks for your input,
Mike

On Wed, 15 Aug 2007 16:12:29 -0500, "amdx" wrote:

Now the question is: Is the wifi adapter output really 50 ohms? If it's
not 50 ohms
then I need not attempt impedance matching with 1/4 Lambda length coax.

Hi Mike,

There are very, very, very few antenna designs that arrive out of the
box with 50 Ohms Z. You usually have to manipulate something (feed
point or linear loading, or coupling between elements or other
radiators) to get it.

The good thing about microwaves is that you can build and test so
quickly - and throw away what doesn't work. This isn't like you are
making jewelry after all.

The scale of construction is one you can get your arms around at the
kitchen table. If someone reliable built one and documented it enough
(both of these provisos are, admittedly, built on a LOT of trust);
then you don't need any instrumentation at all.

I searched for several hours looking for specs on the RF chip in the wifi
adapter without any success.

It sometimes takes some deep digging.

Regarding the spacing; The literature I've seen shows spacing of
0.2, 0.3 or 0.4 Lambda before grating lobes form. But the Patch is 0.5
Lambda wide so you can't get them closer than 0.5 Lambda.
Here's one source;
http://www.orbanmicrowave.com/The_Ba...nna_Arrays.pdf

Pretty tedious stuff, that. EZNEC can do it all in real time - even
the free version is sufficient for your purposes. For one, don't
confuse matching with the radiation lobes - those issues are separable
and you can ignore matching to examine lobes.

At this point I have a working Patch with good signal strength, so now I'm
just trying to see if I can improve on it,

Give yourself the benefit of the doubt and just move forward - the
rest is analysis paralysis.

73's
Richard Clark, KB7QHC