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Gonzo
Intermediate Member
Username: Gonzo
Post Number: 117
Registered: 9-2004
| | Posted on Monday, November 29, 2004 - 6:38 pm: | 
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I re-posted this, I hope it helps. Can you effectively measure TRUE antenna gain? The answer is surprisingly Yes. There is the cheap somewhat effective way, and the more accurate way. Here is the shortest descriptions I could come up with:
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1. You can measure gain in reference to a dipole antenna, such as a Antron 99..this is a coaxial dipole.
2. You will need another antenna, to measure it against.
3. You will need a CB buddy on a base station 20+ miles of groundwave away. Someone who you gives you about a steady 3 or 5 "S" meter reading. The looser his "S" meter the better.
4. You will need a Variable Linear amp, something that you can vary from 0-100 watts, and an ACCURATE bird or equivalent watt meter.
5. A scientific calculator
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Procedure:
1. Note accurately your current SWR, coax lenght, and antenna height.
2. Push 100 watts through your antron 99 and ask the far station what you are doing on his "S" meter very EXACTLY.
3. Change antenna's... If this is another omni..such as a 5/8 wave Imax2000 or similar, this should be relatively easy.
4. Make sure your SWR and antenna height is exactly the same, as the first antenna, if not adjust it to what your first antenna was. DO NOT adjust for best SWR, just adjust to equal what your first antenna's SWR was.
5. Push 100 watts through your new antenna and ask the far away station what you are doing on his "S" meter. Then LOWER your wattage so that you read EXACTLY the same on his "S" meter as the first antenna. Note this EXACT wattage you are now using to obtain the same "S" meter reading
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How to measure gain:
1.Take the lower (presumably second) wattage number, lets say: 50 watts and divide this into the first wattage you used 100 watts.... = 2
2. Take the number 2 (in this example) and take the "log" of it on your scientific calculator. Enter 2 then press "log" this will give you .30 as an answer
3. take the .30 (or whatever number you got)and times this number by 10....this equals 3DB gain!!
This is the amount of gain from one antenna over the other, in this case a dipole. If you want to find the gain versus an isotropic radiator add 2.1...this is how most antennas are rated. |
Gonzo
Intermediate Member
Username: Gonzo
Post Number: 119
Registered: 9-2004
| | Posted on Monday, November 29, 2004 - 10:33 pm: |
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Problems with this scenario:
1. By design an antenna radiates from the bottom, where the vertical radiator is nearest the ground system.
2. This means that a Standard dipole or Antron 99, Shakespeare big stick, or similar Starduster or AstroPlane, are already radiating 9 feet higher than an antenna such as a traditional 18 foot long 1/2 wave antenna, with horizontal ground radials. IE: Maco Alpha V, 10K Interceptor,etc.
3. So to be an accurate test these antennas would have to be raised 9 feet higher.
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Beam antennas:
1. A Beam such as a 3 ele vertical beam (Maco M-103), are really 3 dipoles arranged in a parasitic pattern. They have 3X the ground reflection/absorbtion problems that Omni's have.
These antennas work best when they are mounted at least 36 feet above the ground, or else the gain figures you come up with will be somewhat inaccurate. This may place them higher than your first measured original Omni antenna. And therefore use more coax as well.
2. You may also not be able to place the Beam at the same location as the Omni, and this can also (depending upon surrounding objects & terrain) influence your results.
3. Although these differences are minor...they are things to consider when measuring gain.
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Better way to measure gain, Use the same scenario as above with these differences:
1. Get a Digital Field Strenght meter such as a I.C. Engineering meter, with variable sensitivity and a output for a PL-259 standard antenna connection.
2. Instead of talking with an station 20 miles away, you can use your cb buddies a block or two away.
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How to measure:
1.Connect your Field Strenght meter up to your buddies CB antenna.
2. While on the phone with him, push 100 watts out through your first antenna.
3. Tell him to adjust the Field Strenght meter so that he has a reading of 50 or 100, while you are transmitting. Then do not touch the Field Strenght meter at all.
4. Change antennas, just as before, and when transmitting through the new antenna, have your buddy note the new reading. adjust your wattage down so that, your new antenna reads 50 or 100 or EXACTLY the same on the field strenght meter as it did before.
5. again divide as before the lower wattage on the new antenna into the 100 watts you used on the first antenna, and do the same calculations as before.
6. You now have a very accurate measurement of the difference in wattage and the gain implied by the second antenna.
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This scenario has the same problems as the first measurement system however is more accurate than a "S" meter reading. In this scenario you also have to be careful that the signal you are reading from the Field Strenght meter is indeed YOUR signal, as these meters are very sensitive. |
Tech833
Moderator
Username: Tech833
Post Number: 898
Registered: 8-2002
| | Posted on Tuesday, November 30, 2004 - 10:13 am: |
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Nope. Sorry, you got this one way off.
I will have to go point by point, so please follow as best you can.
"1. You can measure gain in reference to a dipole antenna, such as a Antron 99..this is a coaxial dipole. "
** Nope. An A99 is an end fed 1/2 wave. The old style Shakespeare Big Stick is a coaxial dipole.
"3. You will need a CB buddy on a base station 20+ miles of groundwave away. Someone who you gives you about a steady 3 or 5 "S" meter reading. The looser his "S" meter the better. "
** That is way too far away for any accuracy. Measurements should be made no more than 1 mile away.
"4. You will need a Variable Linear amp, something that you can vary from 0-100 watts, and an ACCURATE bird or equivalent watt meter."
** NO! Not only is this against the law for CB bands, the high signal density will cause all sorts of distortions in your findings! Normally, power levels measured in milliwatts are used to measure gain accurately.
"1. By design an antenna radiates from the bottom, where the vertical radiator is nearest the ground system. "
** Not all antennas radiate from the bottom. In fact, some antennas radiate from the top (so to speak). On an end fed 1/2 wave antenna, the whole antenna radiates. The current and voltage nodes vary along the element and most of the current distribution (loss along the radiator if you will) is from the center to top. On a 1/4 wave ground plane, most of the current is at the bottom, so your statement could be partially true. In the Top One, most of the current is at the top x2.
"2. This means that a Standard dipole or Antron 99, Shakespeare big stick, or similar Starduster or AstroPlane, are already radiating 9 feet higher than an antenna such as a traditional 18 foot long 1/2 wave antenna, with horizontal ground radials. IE: Maco Alpha V, 10K Interceptor,etc.
3. So to be an accurate test these antennas would have to be raised 9 feet higher. "
** This is exactly why a vertical antenna is measured while laying on its side. Height above ground is now a non-issue in the measurment field.
Oh well...
One of these days, I should really spell out the procedure for measuring antenna gain and follow with photos. It's just a severe lack of time. Sorry guys. If the Copper Forum is still active after I retire, I can really go to town on this stuff for you.
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Gonzo
Intermediate Member
Username: Gonzo
Post Number: 121
Registered: 9-2004
| | Posted on Tuesday, November 30, 2004 - 4:02 pm: |
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1.Yes you are correct about the A-99, meant the Big Stick, which I mentioned in next section, sorry.
3. "The farther away the better", this is true. Ever notice that almost everybody in your local area does a 9 or a 10? I have personally found this is the best way using the less accurate "S" meter way.
4."Not only is this against the law for CB bands"
Yes,you are correct. OF COURSE I should have said this in my description. I know NO ONE here would knowingly EVER break the law.
"power levels measured in milliwatts are used to measure gain accurately" Yes this is true, however most people here do not have access to the test equipment necessary to measure this way.I posted this as a way for the average person here to test their own antennas relatively accurately with other cb/ham, friends.
1."Not all antennas radiate from the bottom" Yes I should have eliminated this sentence and just put where the vertical radiator is nearest ground, this would have been more accurate.
3. "a vertical antenna is measured while laying on its side." Any measurements made while the antenna is not in the intended position of use, at the intended height used, would be erroneous, that include laying antennas on their side.
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"I should really spell out the procedure for measuring antenna gain and follow with photos."
I am sure that would be great, too bad we have to wait until you retire for that possibility. |
Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Tuesday, November 30, 2004 - 5:26 pm: |
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An easy way to measure VERY small signal differences on a CB is to connect a digital multimeter to the s-meter circuit and measure the voltage.
I read a comparison of mobile antennas over a decade ago by Bill Cheek where he calibrated his mobile radio (Cobra 148GTL)to a certain power level into a dummy load (I think it was 2 watts on a Bird 43) and did the same with his base radio (Cobra 2000GTL) into a groundplane antenna. Both radios were modified to allow voltage readings from the s-meter circuit with a DMM. He drove to a location with direct line of sight about a mile away. Then he took readings both transmitting and receiving with various mobile antennas and used the information to compare the antenna performance.
While not a way to accurately measure antenna gain, it does give a good relative comparison under the same circumstances. |
Gonzo
Intermediate Member
Username: Gonzo
Post Number: 123
Registered: 9-2004
| | Posted on Tuesday, November 30, 2004 - 6:34 pm: |
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Yes Racer X excellent,I have used the same thing many times as a rough estimate.
Since we are apparently going to get a bit technical, here is a another more LEGAL way:
Using a Digi-Field meter, send power through the antenna that is being tested, enough to get a average reading of 100 on the field strenght meter. For the signal strength data, use a non-conductive tripod and a string can be used to keep the same radius around the antenna. Walk around the antenna 360 degrees. Take measurements every 15 degrees. Plot the readings in a circular graph around the antenna. Do this at varying distances and varying heights above the radiating plane of the antenna. Note the frequency and any environmental fields that may effect the pattern. You must have a level flat area for this to be accurate with little if any obstructions. This is what I do in Vermont on my antenna farm.
Change antennas and do exactly the same procedure, note differences in readings and apply more or less power as needed with the new antenna to get exactly same readings as first antenna. then follow the calculations described above to get differences in gain. |
Tech833
Moderator
Username: Tech833
Post Number: 902
Registered: 8-2002
| | Posted on Wednesday, December 01, 2004 - 3:23 pm: |
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The reason you want to measure gain with lower power levels is to reduce ground effects and multipath cancellation or multiplcation which will throw your readings way off. Distance between the probe and measured specimen will make your readings inaccurate for variable path losses.
Also, measuring an antenna on its side when you rotate it for the gain pattern is the only way to do it accurately. I believe you may be missing the point. Gain is not just measured in signal strength. Gain is a formula which takes into account mag/elec field losses, antenna structure losses, inefficiencies in the materials, matching network, heat loss, capacitive loss, inductive loss, skin effect velocities, and other factors. A simple field measurement would only be accurate if every antenna installation was exactly the same. Raise or lower your antenna in increments and watch the ground capacitivity losses and reflections vary your measurements by 6 dB or more!
Gotta go again. Sorry., |
Gonzo
Intermediate Member
Username: Gonzo
Post Number: 128
Registered: 9-2004
| | Posted on Wednesday, December 01, 2004 - 10:09 pm: |
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Gain is the measurement of power increase between one antenna source and another. This is expressed in Decibels. Decibels of gain are the RATIO between 2 power levels. This is why antenna manufacturers quote things like "multiplies your power by 10X"
Decibel gain = power gain.
3db's = 2X your power gain.
That is all the formular (which I stated above)for gain is.
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The gain formular does not take into account "mag/elec field losses, antenna structure losses, inefficiencies in the materials, matching network, heat loss, capacitive loss, inductive loss, skin effect velocities" These are things that can affect your measurements.
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As you and I both correctly stated: field measurements between antennas must be as close as possible to be accurate.
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If you do not have an antenna farm-like space, with a flat area and no obstructions, and good test equipment. Then the procedure described above using an "S" meter between stations of considerable distance will have to do. As I mentioned this is a "cheap somewhat effective way" for the average Cb'er to do a test. |
Tech833
Moderator
Username: Tech833
Post Number: 907
Registered: 8-2002
| | Posted on Tuesday, December 07, 2004 - 12:04 pm: |
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Gonzo,
I always encourage experimentation and homebrewing. I am a strong supporter of cheaper, easier ways to do things for the hobby radio community. Where I put my foot down is when someone comes on this forum and gives erroneous information disguised as 'facts'.
In order to state the gain of one antenna over another, all factors must be included. You should contact Jampro and ask for a tour of the antenna test range sometime. It is an amazing place and will certainly be an education for you. Once you have seen it done 'right', you will understand why I believe you are making mistakes with your 'facts'.
I will not argue, I would applaud you if you found out all the facts before making assumptions and posting them as facts. You are obviously interested in antennas, and I would be excited to see you write something better researched. I think it would make a good read on the forum. Besides, I could use some help out here! |
Gonzo
Intermediate Member
Username: Gonzo
Post Number: 133
Registered: 9-2004
| | Posted on Wednesday, December 08, 2004 - 7:44 am: |
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"in order to state the gain of one antenna over another, all factors must be included."
I don't see where we are arguing here. And I do have an antenna farm, that is fairly well known here in the Northeast.
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Gain is the measurement of power increase between one antenna source and another. This is expressed in Decibels. Decibels of gain are the RATIO between 2 power levels.
Whats wrong with this fact???? |
Bruce
Senior Member
Username: Bruce
Post Number: 1850
Registered: 9-2003
| | Posted on Wednesday, December 08, 2004 - 7:08 pm: |
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Intresting seems like the chicken and egg which is first .... My gripe has always been there is no standard to which antemmas's have to comply.
gonzo my firewall ate your 8 elm antenna data
Could you resend it to wa4gch@juno.com
Thanks
Bruce |
Crafter
Senior Member
Username: Crafter
Post Number: 1057
Registered: 12-2002
| | Posted on Wednesday, December 08, 2004 - 10:10 pm: |
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Bruce feed it more often!! |
Gonzo
Intermediate Member
Username: Gonzo
Post Number: 134
Registered: 9-2004
| | Posted on Thursday, December 09, 2004 - 7:57 am: |
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OK, Bruce No Problem, I am going to send you as bunch of cool stuf today |
Bruce
Senior Member
Username: Bruce
Post Number: 1853
Registered: 9-2003
| | Posted on Thursday, December 09, 2004 - 1:39 pm: |
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Gonzo it's been a bad day i ve been working on the local court house radios ( It's amazing how non-teckies can mess them up ) anything has to be a step up from that mess ...... send lots of cool stuff.
Bruce
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