| Author |
Message |
    
Crab_apple
New member
Username: Crab_apple
Post Number: 1
Registered: 5-2014
| | Posted on Monday, May 05, 2014 - 5:51 pm: | 
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Hi all! I just mounted my new I max on a tripod-mast on the my roof top. The base of the antenna is about 7 foot off the roof top and is about 23 feet from the ground to the base of the antenna. There are no structures near the antenna! There is 50 foot of new coax! No matter what i do with the tuning rings on the I max, nothing happens with my SWR! My SWR readings are 1.9 on ch.1 and 1.6 on ch.40. What is my problem? Do i need the I max ground plane kit? Thanks for any and all help!
Crab-apple |
Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 338
Registered: 10-2002
| | Posted on Wednesday, May 07, 2014 - 6:38 am: |
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What type of coax do you use? Have you tried to cut the coax to try to change the swr? If I remember , you need to be at least 9 foot from any object. so you might want to raise the antenna to get it away from the roof. Also check all connection's. |
Crab_apple
New member
Username: Crab_apple
Post Number: 2
Registered: 5-2014
| | Posted on Sunday, May 25, 2014 - 9:24 am: |
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Update: After much headache and going up and down a ladder, come to find out that i got a bad antenna!! It went back!! Put my old A99 up and all is well! Thanks! |
Tech237
Moderator
Username: Tech237
Post Number: 1792
Registered: 4-2004
| | Posted on Monday, May 26, 2014 - 8:03 am: |
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Charliebrown - cutting the coax DOES NOT change the actual SWR, but causes an apparent change due to standing waves on the coax adding or subtracting to the reflected power.A multiple of an electrical half wave is the best length to use as this repeated the impedance at the end into the shack, thus giving a more accurate reading in the shack.
Also that 9ft is from PARALLEL objects, and a metal or any roof under the antenna will only be seen as a ground
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Dale
Senior Member
Username: Dale
Post Number: 2089
Registered: 12-2002
| | Posted on Tuesday, May 27, 2014 - 7:09 pm: |
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crab apple your swr is 1.9.1 on 1 and 1.6 on 40
that tells me ya need to lower your rings
more.tech 237 correct me if im wrong.
how ya know antenna is bad??if new send it back
heres a tip only use a 6 foot pipe saves from
going up/down ladders
dale/a.k.a.hotrod
cef426
cvc#64
454 [dx numbers]
38lsb
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Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 340
Registered: 10-2002
| | Posted on Thursday, May 29, 2014 - 11:05 pm: |
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Hi Dale, The antenna location from the ground will depend upon the type of location he has like hill's, mountain's or flat land and the type of communicating he would like to do. Like local only or skip or both. Also not to mention his ability to perform climbing. I live where we have very tall mountain's. So for me I put my antenna only 30 foot from the ground to the bottom of the antenna on the top of the mountain. That way the GPK will perform well. Now if he would like to talk skip get the antenna as high as possible but do not use the GPK for it will be useless on anything higher than 36 foot. Remember not every location will perform the same. What will work well for you or me might not work the same for him or any other person. Any person would do well to read all he can about antenna's and installation. The tech's here can do better than I on this subject. 833 is the expert on this. Dale what time of day or night do you get on 38 LSB?? Perhaps we could see if we can make contact. |
Possum_lodge
Junior Member
Username: Possum_lodge
Post Number: 21
Registered: 5-2014
| | Posted on Friday, May 30, 2014 - 10:32 am: |
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Without rehashing an old problem. Lets look at some statistics. IN ORDER FOR YOUR ANTENNA TO RADIATE PROPERLY YOU NEED TO GET THE ANTENNA AT LEAST ONE WAVE LENGTH ABOVE THE GROUND.
That is 36'
In order to get your antenna to radiate properly, you need to have at least 36 ground radials cut at least 1/4 wavelength long - 9' and have them spread radially around the base of the tower or antenna mast..
Or, you can invest in the radial kit that places 3 or 4 elevated radials beneath the antenna.
Next you need to own a true 50 ohm dummy load.
You need to calibrate the dummy load to the SWR meter.
SWR meters are not the last word when it comes to diagnosing a problem, in fact they can sometimes be more of a problem then a solution.
You see a SWR Bridge just compares the swr it see's on your line to a known variable, it doesn't exactly tell the whole story..
If you are going to put up and tune antenna's, you need to buy yourself an antenna analyzer and then you need to find someone to teach you how to use it properly and to be able to understand what the measurement means.
THERE IS TWO COMPONENTS TO A RADIO WAVE, THERE IS A ELECTRICAL COMPONENT AND THERE IS A MAGNETIC COMPONENT.
The radio is called the Generator and the antenna is called the Load. The purpose of the coax is to conduct the power from the Generator to the Load.
When you have a match - the load equals what the generator outputs - you have the most efficient use of power.
You want the antenna to be purely Capacitive - 50 Ohms, because that is what the Generator expects to see. This is the resistive component of the measurement. The Reactive component - the part that doesn't actually do anything except reflect the signal back towards the source - needs to be neutral.
If these two measurements are not 50/j0 then you have to add components to get it to balance out so that the antenna absorbs the load.
Any power that is reflected back towards the source is combined in the next radio wave and directed forward again.
The loss is the loss in the coax measured in Db per 100', each time the radio wave has to travel back and forth in the coax - the loss is subtracted from the final amount of radiated power.
The purpose of the antenna tuner is to change the components to trick the radio into believing that the antenna is resonant. If the antenna system is not resonant, the radio folds back it's radiated power to try to protect it's final transistors.
Because 11 meters is such a small portion of the band, all we have to do to make the antenna system resonant is add or subtract a portion of the length of the antenna.
Channel 1 being the lowest frequency - requiring a long antenna and channel 40 being the highest frequency requiring a shorter antenna.
We check the resonance at channel 19 since it is somewhere in the middle.
So if channel 1 has a high SWR and channel 19 has an ok SWR and channel 40 has a low SWR - we know that the antenna is too short.
If channel 1 has a low SWR and channel 19 is ok and channel 40 is high - we know that the antenna is too long..
All we are looking for is a compromise - something less then 2:1 on all frequencies used.
Any antenna with a high Q is going to suffer from these qualities...
The reason why you could not adjust the IMAX is because it has a high Q. That is the QUALITY of the antenna is such that at channel 19 - you can obtain a 1:1 match if installed properly, and everywhere else within the band it will have less than 2:1 match..
The Solorcon / Antron 99 is a low Q antenna, it does an OK job everywhere, but does not give as much apparent gain as does the IMAX.
This is what got you into trouble.
Note I said apparent gain, the only gain a vertical antenna can give is gain over a Dipole antenna or gain over an Isotropic antenna - since we know how a Isotropic antenna or a Dipole antenna performs. It does not mean that it actually provides some type of rejection in one or more directions or an improvement in other directions..
In theory, the more aluminum or antenna if you will - that you put up in the air, the better it is going to perform..
The IMAX just happens to be a longer antenna. |
Dale
Senior Member
Username: Dale
Post Number: 2090
Registered: 12-2002
| | Posted on Friday, May 30, 2014 - 12:56 pm: |
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i agree with everything ya said. tech 833
has a very good write up HOW TO CHOOSE A GROUNDPLANE
its a very good read.on im on 38lsb on/off
mostly on weekends fri,sat,sun. early mornings
and early-late nights. thru the week not so much
maybe 1-2 hrs from 7-9pm. all est. my numbers are below this post ,who knows maybe we already talked
before
dale/a.k.a.hotrod
cef426
cvc#64
454 [dx numbers]
38lsb
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Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 341
Registered: 10-2002
| | Posted on Monday, June 02, 2014 - 7:30 am: |
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Dale, Just got back. I will try to make contact this week end on the time you said in the evening. Listen for 55 old double nickel |
Tech237
Moderator
Username: Tech237
Post Number: 1796
Registered: 4-2004
| | Posted on Thursday, June 05, 2014 - 2:27 pm: |
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Possum lodge stated "Note I said apparent gain, the only gain a vertical antenna can give is gain over a Dipole antenna or gain over an Isotropic antenna"
Any gain over a dipole or isotropic is REAL gain, there is nothing apparent about it. It is the same gain that a horizontal antenna of the same type/size would show over a dipole or isotropic.
"In theory, the more aluminum or antenna if you will - that you put up in the air, the better it is going to perform.."
I know what Possum Lodge means, but as written it infers that an antenna cut for a low band, would work better on a higher band than one cut for that band, and we know that with a few exceptions this is NOT the case. I.E. an 80m antenna will not work too well on 2m. What was meant is a half wave antenna is better than a quarter wave one, and extended double-zepp (5/8 wave each leg) would be better again.
"You want the antenna to be purely Capacitive - 50 Ohms, because that is what the Generator expects to see."
In over 35 years working in the trade, I have yet to see a radio that expects to see a capacitive load. The radio expects to see a 50 ohm impedance with little (preferably no) reactive component. A capacitive load has reactance that needs to be tuned out for proper loading.
"IN ORDER FOR YOUR ANTENNA TO RADIATE PROPERLY YOU NEED TO GET THE ANTENNA AT LEAST ONE WAVE LENGTH ABOVE THE GROUND."
Actually a half-wave will work just as good. In both cases, that is above TRUE ground, which may or may not be the surface you are walking on. For several reasons, true ground can actually be BELOW the visible surface.
"Any power that is reflected back towards the source is combined in the next radio wave and directed forward again"
Actually it adds ALGEBRAICALLY, which means that a lot of the time it bucks the transmitted signal and actually reduces the power reaching the antenna. Rarely (only when coax electrical length, frequency etc. all coincide nicely) will the reflected signal be in phase with the transmitted signal.
I agree with respect to SWR Meters, and will only add that as far as radials are concerned - 4 will work 36 is better and more is even better
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 348
Registered: 10-2002
| | Posted on Wednesday, July 23, 2014 - 11:03 am: |
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Possum lodge, Tech 833 , ( I hope I quote him right ) that for the ground plane kit to have the max gain to help the antenna should be around 28 to 30 foot with a max of 36 foot. As you get higher with the antenna above 30 foot and above 36 foot the gain for the gpk will decrease. So for the max gain I have mine at 30 foot. It works well for me since my antenna is on top of a mountain. |
Tech833
Moderator
Username: Tech833
Post Number: 2291
Registered: 8-2002
| | Posted on Saturday, July 26, 2014 - 3:29 pm: |
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I never said anything about specific heights above ground as a minimum. I have printed how high an antenna was above ground for a test or when running a computer model on it.
The rule of thumb is to get the antenna as high as possible. Keep in mind there are some antenna height restrictions for Part 95.
Your radio 'Mythbuster' since 1998
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Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 350
Registered: 10-2002
| | Posted on Sunday, July 27, 2014 - 7:38 pm: |
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Somewhere in one of the older post not this one. I thought you had said the performance of the GPK would decrease as you increase the height from the ground. I am not speaking of the antenna by itself. Just the GPK performance. Do you not change and loose the angle of radiation and the performance of the GPK if the GPK is to far from the ground? Here where I live people will put their antenna in a tree as high as 100 foot. I thought that the max gain for the GPK was 30 to 36 foot from ground level |
Tech833
Moderator
Username: Tech833
Post Number: 2294
Registered: 8-2002
| | Posted on Thursday, July 31, 2014 - 1:43 am: |
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The GPK changes the pattern of the Imax. If the antenna is at least 20 feet above ground (to the feedpoint), the GPK should make a positive difference. If the Imax is mounted lower, then the GPK won't be much help, if any.
Your radio 'Mythbuster' since 1998
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Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 351
Registered: 10-2002
| | Posted on Friday, August 01, 2014 - 10:23 pm: |
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O.K. but, what about the pattern when the I max with the gpk is above 36 foot like 54 , 72 or even 90 foot in a tree or tower? I know there is a limit to the height that is allowed but here people put their antenna's in tree's that high or higher. We have a lot of mountain's. With the gpk at these height's above ground is it not about useless to have the gpk on the antenna for there would be little or no gain at those height's? |
Tech833
Moderator
Username: Tech833
Post Number: 2295
Registered: 8-2002
| | Posted on Tuesday, August 05, 2014 - 7:08 pm: |
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The GPK will lower the takeoff angle even if the antenna is mounted 1,000 feet AGL. SO, yes, in a tree, the GPK would help.
Your radio 'Mythbuster' since 1998
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Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 353
Registered: 10-2002
| | Posted on Thursday, August 07, 2014 - 9:58 pm: |
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Thank you 833 for the clarification on this matter. It had been my understanding that the higher the Gpk the less it would help. So I guess I am wrong and misunderstood. I just wish that there would be a chart on how well it would help at different height's. |
