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Author Topic: Dipole SWR?  (Read 1703 times)
OLLIEOXEN27
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« on: June 10, 2009, 01:43:49 PM »

I built a 20 meter dipole that has a 1:5 1 SWR at 5 watts output, a 1:8 1 SWR at 20 watts and a 2:2 1 SWR at 100 watts output. What causes the SWR to increase with higher outputs? I don't want to use a tuner. Thanks in advance.

oli
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WB2WIK
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« Reply #1 on: June 10, 2009, 05:07:19 PM »

It cannot, and I doubt it's changing at all.

What kind of SWR meter are you using, exactly?
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K5END
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« Reply #2 on: June 10, 2009, 05:23:54 PM »

something is wrong, more likely with the meter, but it could be other things, like a bad PA.
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OLLIEOXEN27
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« Reply #3 on: June 10, 2009, 05:33:08 PM »

I'm using an El Cheapo Radio Shack analog SWR meter - probably not very accurate. My Ten Tec Triton transceiver also has an SWR reading on it's meter and it shows 1:8 1 but it's only accurate at 100 watts. I suspect my actual SWR is about 1:8 1 - ok for my purposes. I don't think the PA is bad but who knows?

oli
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AD5X
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« Reply #4 on: June 10, 2009, 07:20:00 PM »

Are you re-setting the forward reference to full scale when you change your power?  If you don't do this, your SWR will appear to increase as you increase power.

Phil - AD5X
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OLLIEOXEN27
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« Reply #5 on: June 11, 2009, 10:58:31 AM »

WA8MEA,

No threats here. I think I have experienced that before but I never paid attention because I always used a tuner. I wounder how reactance either inductive or capacitive would effect SWR levels vs power. So maybe it's not my meter after all. Someone should do a write up on the subject here on Eham.

oli
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OLLIEOXEN27
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« Reply #6 on: June 11, 2009, 11:01:11 AM »

AD5X,

Yes, pardon but I'm not that green - I know how to use an SWR meter lol. In fact I have a extra license.

oli
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N5LRZ
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« Reply #7 on: June 11, 2009, 01:53:55 PM »

SWR is a ratio of power, a calculation.  The SWR is the same regardless of power.


What you are seeing is the effect incrased power has on detecting little flaws in the antenna system.  Also, almost all SWR meters are not exactly lab quality instruments calibrated to the highest standards.

As long as your SWR does not fluctuate very much and stays in about the same general area do not worry.  

Without going into all the details and reference to ref books just know that absolutely no one will ever notice a few watts of difference.  Do not get bogged down into the mentality of the so called perfect SWR of 1 to 1 must be achieved at all costs.
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K7KBN
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« Reply #8 on: June 11, 2009, 02:00:49 PM »

SWR stands for "Standing Wave RATIO" -- remember, it's a RATIO between forward power and reverse/reflected power.

When you adjust a Monimatch-type SWR meter for full scale at a given power level (say 50 watts) and then switch to "REFL" or "SWR", what you see is the relative amount of reflected power for that forward power level of 50 watts.  

If you crank up the power to 100 watts, the amount of reflected power will almost certainly increase, BUT the RATIO between the forward and reflected power will not change.

Get a cross-needle meter, or one of the various ones available that show the SWR digitally.  I use an old Palomar Laboratories M-840, which very clearly shows no increase in actual SWR from 5 watts to 1500 watts on any of my antennas.

Remember:  it's a RATIO.
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73
Pat K7KBN
CWO4 USNR Ret.
WB6BYU
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« Reply #9 on: June 11, 2009, 02:45:01 PM »

It doesn't have anything to do with the radio or the
antenna, but is a characteristic of the SWR meter circuit.

The meter taps off some RF proportional to the forward
and reflected power and applies it to a diode to drive
the DC meter.  The diode has a finite forward voltage
drop - about 0.6V for a silicon diode or 0.3V for germanium.


Let's say that you have 100 watts forward and 20 watts
reflected.  Depending on the meter circuit, this might
give you 7V across the meter for forward power and 3.2V
for reflected power.  Subtracting 0.6V from each of these
will make the SWR slightly lower, but not enough to worry
about.

But at 10 watts forward and 2 watts reflected (the
same SWR, but at 1/10 of the output power) we now have
2.2V forward and 1V reflected.  Now the 0.6V drop through
the diode causes the reflected voltage to be less than
half what it should be, which corresponds to about 20%
of the actual power.

So at low power levels and reasonably low SWR the reflected
power will measure low, making the SWR look better than
it actually is. As the power is increased the difference
gets smaller and the measured SWR becomes more accurate.

If you are simply adjusting a tuner for minimum SWR, the
minimum will still be at the same point when using low
power for tuning (generally a good idea) so this isn't
a problem.  But almost all SWR meters will under report
the SWR when the reflected power is too low to overcome
the voltage drop of the diode detector.  

This is a particular problem with CB-style SWR meters
on 80 and especially 160m.  They require higher power
to drive to full scale on the lower bands.  One that
I measured was about 5W for full scale on 40m and 25W
on 80m.  The minimum power required for and accurate SWR
reading goes up accordingly.
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KB9CRY
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« Reply #10 on: June 11, 2009, 04:45:13 PM »

El Cheapo



Any more questions?
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OLLIEOXEN27
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« Reply #11 on: June 15, 2009, 06:19:42 AM »

As usual a wide array of opinions. I like what WB6BYU said about the voltage drop across the diode being more of a factor at low power. The writer who made a big deal about SWR being a ratio said things that are self evident - not sure what he was trying to say except power loss in watts is greater at higher powers. We all know that.

Anyway thanks for the the effort. El Cheapo? The RS El Cheapo SWR meter is now $50 - a couple of tanks of gas - not so cheap. I'm not going to replace my current flaky MFJ tuner with a more expensive flaky MFJ tuner or a worthy expensive tuner by another brand. It's resonant antennas from now on.

oli
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OLLIEOXEN27
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« Reply #12 on: June 15, 2009, 06:40:15 AM »

Regarding the prices of ham radio equipment: Ham radio is just a hobby so is actually an extravagance for many of us - much like pouring money into a whole in the ground for a few cheap weekend thrills (people promoting ham radio businesses will disagree of course and many post here).

I guess if I was a doctor, lawyer, government employee, or in a similar predatory occupation like many out there I would be less concerned about spending $500-$2000 for a tuner but since I'm not I'll continue to improvise and make due with low cost solutions. Pardon my French.

oli
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N5LRZ
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« Reply #13 on: June 15, 2009, 10:43:26 AM »

I agree with KBN....

I find that the cross neegle meters offer three vital pieces of information in one quick glance--AND you do not have to calibrate them each time you use it.

You immediately see forward transmit power, reflected power, and SWR approx where the needles cross.  Just tune antenna to the lowest reflected power and the rest takes care of itself.
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OLLIEOXEN27
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« Reply #14 on: June 16, 2009, 10:12:38 AM »

I have an MFJ Cub 15 meter transceiver. When I tune it to a 1:1 SWR the output is a little less than one watt. When I tune it to a 1:1.2 SWR varying capacitance on one side it shows a little less and on the other side about one watt (an increase in power).

By this simple example power out seems to vary depending if a circuit is capacitivly reactive vs inductivly reactive vs purely resistive. This is what I really wanted to know more about. I don't think it's the meter since as others have mentioned it's a ratio.

Flamers don't bother - I won't lower myself to that level. If you are dignified or can act so and intelligent and know this subject please chime in.

oli
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