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Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 12
Registered: 12-2009
| | Posted on Saturday, April 24, 2010 - 1:22 am: | 
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This is a stumper to me. I have a recently installed 3 element flat side under an Imax 2000. The issue is, I show more watts out on the 3 element than the ground plane. I have RF isolation baluns on both antennas, LMR 400 on the flat side and 9913 on the GP. Both antennas show SWRs of 1.3 on CH20. The disparity is on the order of 20 percent it appears.
The wattage output rating for the radios is consistent with the output of the beam while the GP is less than the rated output. I am stumped. I can get reactance readings if that is the culprit. Maybe the Imax isn't as efficient?
Thanks in advance for any insight. |
Tech237
Moderator
Username: Tech237
Post Number: 1333
Registered: 4-2004
| | Posted on Saturday, April 24, 2010 - 1:54 pm: |
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1) are both antennas using the same size, length and type of coax?? These factors can all effect output power. As stated in you message, you are using different types of coax with different characteristics. Swap the coax at the antennas and see what happens..
2) Where are you measuring the RF Output power?? IF measured at the radio you can get inaccurate readings due to the length of coax, type of, and condition of coax. If measured at the antenna, the coax length and loss factors then come into play.
3) Antenna efficiency SHOULD NOT affect the output power from the radio, but an incorrect (and possibly higher than displayed) SWR can.
4) If you feed, say 10W into each coax, how much power do you measure at the antenna end?? This is a test of coax quality and signal loss.
These are just some of the factors that may be involved, and I am sure Tech833 can add others.
Tech237
N7AUS
God only made some many perfect head, on the rest he put hair.
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Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 13
Registered: 12-2009
| | Posted on Saturday, April 24, 2010 - 10:36 pm: |
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I'll try these things and see what happens; specifically, I'll swap coax cables and check, measure at the antenna and check power and SWR levels there.
Thanks for the tips... |
Tech833
Intermediate Member
Username: Tech833
Post Number: 165
Registered: 12-2001
| | Posted on Sunday, April 25, 2010 - 9:41 pm: |
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More important, what kind of watt meter are you using to read this?
If the beam has a resistance of 54 ohms and the ground plane has a resistance of 47 ohms, both will show 1.3:1 SWR, but ground plane will show higher wattage on a consumer grade watt meter.
best bet, if you are worried about this, use a Bird or similar meter before you get too concerned.
Your radio 'Mythbuster' since 1998
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Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 14
Registered: 12-2009
| | Posted on Monday, April 26, 2010 - 6:14 pm: |
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Ah, ok...that makes sense. The vertical is an Imax 2000 which has the lower wattage reading. The beam shows the wattage for which the radio(s) are rated. Apparently, the Imax has a slightly higer ohm reading according to your example.
Thanks for the insight. |
Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 15
Registered: 12-2009
| | Posted on Tuesday, April 27, 2010 - 12:19 pm: |
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I just calibrated my meter against a Bird and now have much more accurate readings. I also checked both antennas with an antenna analyzer and found a slightly different ohmic and reactance readings than I anticipated. Now I have a much more accurate picture of what's going on.
Again, thanks for the insight. |
Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 18
Registered: 12-2009
| | Posted on Monday, May 10, 2010 - 10:59 pm: |
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I purchased a LP-100A computing SWR/Watt meter that displays reactance in real time. I found that the Beam's R=48 and the SWR=1.1. The Imax's R=54 and the SWR=1.3. Today I noticed that the Imax's R now equals 64 with the same SWR. It rained pretty hard yesterday and today. Perhaps that has something to do with it as I did not touch a thing. The Beam was unchanged. It does have an impact on the wattage output.
I don't know quite what to make of that. What can cause reactance to go up? If anything, this is a good learning experience.
Thanks in advance for any input. |
Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 20
Registered: 12-2009
| | Posted on Tuesday, May 11, 2010 - 11:02 pm: |
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I changed out the coax on the beam to 9913 and I now have R = 49.5 and X = 0. I can't get the GP down for some reason but good enough is good enough I suppose. R = 65 and X = 2.0. I think it has something to do with being above the beam. It works well so Im leaving it alone. |
Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 24
Registered: 12-2009
| | Posted on Sunday, April 03, 2011 - 10:38 pm: |
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After having lived with the LP-100a for some time now and reading more about reactance, impedance and SWR, I'd like to ask a question:
At the operating frequency range, is it better to have the lowest reactance even if the impedance is up at 70; or, is it better to have an impedance that is closer to 50 with a higher reactance. (SWR is 1.5 or lower in either of the scenarios).
I believe lower reactance is the better number showing the antenna resonant at the frequency; however impedance match means the transmitter is seeing the load for which it is designed.
Which is the better number for performance? I believe this is critical when cutting an antenna. Achieving a 1.1:1 SWR does not always equal a low reactance or 50 ohm impedance.
Thanks for shedding light on this. |
Tech833
Moderator
Username: Tech833
Post Number: 1946
Registered: 8-2002
| | Posted on Monday, April 04, 2011 - 2:05 pm: |
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Getting your reactance to zero is by far the most critical factor.
Impedance can dance around all it wants, little effect. Reactance is your radio killer. Run your transmitter into a capacitive load for a few minutes and you can fry an egg on it.
Your radio 'Mythbuster' since 1998
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Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 26
Registered: 12-2009
| | Posted on Monday, April 04, 2011 - 5:37 pm: |
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Ah, ok, I thought so. My reactance is 7 down to 1 on the IMAX and 16 down to 4 on the beam. Is 16 acceptable or should I fiddle with it more?
I really appreciate this answer. I have been learning alot with the LP-100a; specifically that SWR is not the end-all-be-all of measurements. |
Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 28
Registered: 12-2009
| | Posted on Monday, April 04, 2011 - 6:44 pm: |
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I was able to get the reactance down to 5. The SWR is 1.6 at its highest and 1.3 at its lowest. Based on the reactance importance, I will assume these SWR readings are acceptable. I was always lead to believe that it would hurt a radio or amp if the SWR was over 1.3. In this instance, if I got it down to 1.3, my reactance is way out of whack.
This has been very interesting indeed. |
Dale
Senior Member
Username: Dale
Post Number: 1696
Registered: 12-2002
| | Posted on Tuesday, April 05, 2011 - 7:03 pm: |
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interesting point .most amps DO say
1.3.1 or less.i guess in my own stupidity
i assumed the reactance would be 0 or 1
if swr was 1.1.1 or 1.2.1
dale/a.k.a.hotrod
cef426
cvc#64
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Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 29
Registered: 12-2009
| | Posted on Tuesday, April 05, 2011 - 8:15 pm: |
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If this is indeed true, lower reactance is more important than SWR, then it changes my whole way of measuring. I have a reactance across 11 meters of 10 or less and a SWR of 1.5 or less. When I see a low SWR, I don't always see a low reactance. As a matter of fact, it is upwards of 20 to 25. |
Tech833
Moderator
Username: Tech833
Post Number: 1950
Registered: 8-2002
| | Posted on Wednesday, April 06, 2011 - 1:43 pm: |
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The difference is in the reactance. SWR is a measurement of currents forward vs. reflected on the same conductor (in this case, your coax cable). Nothing more.
Let's say you have 50 ohms impedance with an inductive reactive component of 10 ohms. Transmitter will still be OK, like is good. Let's say you have a capacitive reactive component of 10 ohms, even though your SWR still looks acceptable, your transmitter will still run hotter than it should.
So, it isn't just about knowing the reactance, it's also about knowing whether the reactance is inductive or capacitive.
SWR just happens to be simple and easy, not just from an understanding point of view, but mostly from a hardware point of view. You can but an SWR meter for 10 bucks. You cannot buy a bridge that will measure and display reactance for anywhere near that. And, just try to explain how resistance and reactance works to John Q. Public. SWR measurement is easier and more marketable. That is why it is so common.
Don't let this scare you. CB radios running 4 watts carrier aren't going to "blow up" if there is a slightly capacitive component at the feedpoint. But when running several kilowatts (or sometimes megawatts) like I do at work, reactance becomes quite critical. And, these days with IBOC digital and DRM digital modes, even rotation becomes important.
For your CB, get the SWR low and be happy. Since you mentioned an amplifier, I have to assume this is ham radio we are talking about, and if running the maximum legal power of 1500 watts, you may want to pay some attention to reactance.
Your radio 'Mythbuster' since 1998
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Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 30
Registered: 12-2009
| | Posted on Wednesday, April 06, 2011 - 5:02 pm: |
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\How does one know if it is capacitive? I don't think I saw that as a measurement on the LP-100a. I think that is a measurement on the MFJ. On the LP-100a, there is resistive, reactive, phase and impedance. What started this is the change in measuremens when adding a simple jumper between the meter and the antenna. This spawned several arguments.
1. One can indeed 'tune' and antenna or better put the 'load' the amp sees by cutting coax according to the meter.
2. Getting the lowest impedance match between the amp and the antenna does not necessarily mean the lowest SWR. Add reactance to this cocktail and things get messy.
Thank you for your time in answering. I am indeed talking about a ham amplifier on ham bands. (I am one of those who took all three exams in one month and vowed to learn the theory behind the questions).
I appreciate your help. |
Tech833
Moderator
Username: Tech833
Post Number: 1954
Registered: 8-2002
| | Posted on Thursday, April 07, 2011 - 2:59 pm: |
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Nice job on passing all 3 at one whack, OM!
It took me 5 tried to pass the extra exam. Yes, I freely admit it- I'm dumb, why hide it?
As for tellign if reactance is inductive or capacitive, the lower cost meters (like the MFJ) will not tell you one or the other, only that there is an inductance component present (in ohms).
You have to spend a bunch more to tell if the reactance is one or the other.
There are other methods, but it is real wordy and complex. Best bet is to get that reactance to zero on your meter and call it good!
Your radio 'Mythbuster' since 1998
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