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SWR ON A IMAX 2000
- Thread starter cominatyalive
- Start date
cominatyalive,
"I am going with Qty 3, 300 Ohm, 5 watt carbon composition Resistors.
100 Ohms at 150 Watts power handling. I am also thinking of using the Paint can method with mineral oil."
I figure that was a goof in typing, but that's 15 watts, not 150 watts.
That aluminum heatsink is still going to be fun. It won't be a matter of just soldering one end of those resistors to it, but attaching the bodies of those resistors to the heatsink to get rid of the heat. If those resistors happen to have large "bunns" so that there's lots of area for that heat transfer, things should work okay, but don't count on it. That metal heatsink should have no current traveling through it at all, should be at ground potential. Otherwise you end up with a burn, either thermal or RF, not good. Which says nothing about if there's current flowing through it then it radiates. Not as well as an antenna, but it'll still radiate more than you'd like. @#$! There's always something else, ain't there? Oh well.
- 'Doc
"I am going with Qty 3, 300 Ohm, 5 watt carbon composition Resistors.
100 Ohms at 150 Watts power handling. I am also thinking of using the Paint can method with mineral oil."
I figure that was a goof in typing, but that's 15 watts, not 150 watts.
That aluminum heatsink is still going to be fun. It won't be a matter of just soldering one end of those resistors to it, but attaching the bodies of those resistors to the heatsink to get rid of the heat. If those resistors happen to have large "bunns" so that there's lots of area for that heat transfer, things should work okay, but don't count on it. That metal heatsink should have no current traveling through it at all, should be at ground potential. Otherwise you end up with a burn, either thermal or RF, not good. Which says nothing about if there's current flowing through it then it radiates. Not as well as an antenna, but it'll still radiate more than you'd like. @#$! There's always something else, ain't there? Oh well.
- 'Doc
HiDef,
If you were to change two words in that last post I'd agree with you. Otherwise, it's just not right. Change "best SWR" to "resonance". SWR has nothing to do with resonance, and resonance is the thing you should be shooting for.
- 'Doc
If you were to change two words in that last post I'd agree with you. Otherwise, it's just not right. Change "best SWR" to "resonance". SWR has nothing to do with resonance, and resonance is the thing you should be shooting for.
- 'Doc
Here is my bandwidth curve for my Imax. There are three different SWR scans that use the notations noted in the legend at the top-left of the page. Also note that I had to exaggerate the BW curve in order to see the bowl curve shown. My normal BW forms would show the curve to be almost flat across the range around 11 meters.
WA2fdu, a chart in my Imax manual indicates that 28.372 should have 85" remaining for the top section with 11" cut off for a starting point to cut the antenna for 28.450 mhz you want to work. See: View attachment Antenna Work Sheet 122309 Imax 2000.pdf
Didn't you get a manual with your new Imax?
I can't guarantee that these numbers are correct, but they are in the manual.
The frequencies in my handwritten BW curve for an Imax may be hard to read, but 28 mhz starts at 28.105 and follows with
.155 .255 .405 .555 .705 .805. The small decimal numbers beside each scan point are meter wattages with 12 watts fed to the antenna during the scan. The Imax is very good at controlling the feed point resistance steady over such a wide range. It may cost a bit in performance, but if you need BW then this is the antenna for you.
See below:
http://i617.photobucket.com/albums/tt251/Marconi_39zero/MarconisAntennaNotes060809BWforImax.jpg?t=1261572836
Check out these reviews on eham. These 6 pages talk a lot about what you are trying to do and most seem to indicated no need to adjust the antenna length. This also might help support what I told comintyalive : http://www.eham.net/reviews/detail/1376
Good luck on 10.
Very good point 'Doc.
WA2fdu, a chart in my Imax manual indicates that 28.372 should have 85" remaining for the top section with 11" cut off for a starting point to cut the antenna for 28.450 mhz you want to work. See: View attachment Antenna Work Sheet 122309 Imax 2000.pdf
Didn't you get a manual with your new Imax?
I can't guarantee that these numbers are correct, but they are in the manual.
The frequencies in my handwritten BW curve for an Imax may be hard to read, but 28 mhz starts at 28.105 and follows with
.155 .255 .405 .555 .705 .805. The small decimal numbers beside each scan point are meter wattages with 12 watts fed to the antenna during the scan. The Imax is very good at controlling the feed point resistance steady over such a wide range. It may cost a bit in performance, but if you need BW then this is the antenna for you.
See below:
http://i617.photobucket.com/albums/tt251/Marconi_39zero/MarconisAntennaNotes060809BWforImax.jpg?t=1261572836
Check out these reviews on eham. These 6 pages talk a lot about what you are trying to do and most seem to indicated no need to adjust the antenna length. This also might help support what I told comintyalive : http://www.eham.net/reviews/detail/1376
Good luck on 10.
Very good point 'Doc.
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Thank You everyone for the responses
Marconi - Thank You for the references. No, I did not get a manual with my antenna. I purchased a 6 month old antenna with unused GPK for about $105 including shipping. The only manual I have seen has been from the web and it did not include the chart you sent me. Once again - Thank You
I tried very hard to read the handwritten chart and got most of it - but still it was very difficult.
I have read the eham postings and did gain quite a bit of insight in the past.
For all of you - Here is my plan (with at least one question)
1. Isolate antenna mount from the mast. I will be using Central Vacuum tubing - this stuff is great!! We use it on our repeater antenna.
2. Use the GPK
3. Trim off approx. 11.5" from top element. (I found a post somewhere where someone swapped a radial with the top element - neat idea???)
4. Add a coil just below antenna - THIS IS WHERE I NEED HELP!! I have seen many references but no real definition of the coil specifications.
5. Acquire a few cold 807's and invite the gang over when the temperature is below zero and winds are at least 20mph (I live in Central New York) to raise the beast.
Thanks Again
Stephen
Marconi - Thank You for the references. No, I did not get a manual with my antenna. I purchased a 6 month old antenna with unused GPK for about $105 including shipping. The only manual I have seen has been from the web and it did not include the chart you sent me. Once again - Thank You
I tried very hard to read the handwritten chart and got most of it - but still it was very difficult.
I have read the eham postings and did gain quite a bit of insight in the past.
For all of you - Here is my plan (with at least one question)
1. Isolate antenna mount from the mast. I will be using Central Vacuum tubing - this stuff is great!! We use it on our repeater antenna.
2. Use the GPK
3. Trim off approx. 11.5" from top element. (I found a post somewhere where someone swapped a radial with the top element - neat idea???)
4. Add a coil just below antenna - THIS IS WHERE I NEED HELP!! I have seen many references but no real definition of the coil specifications.
5. Acquire a few cold 807's and invite the gang over when the temperature is below zero and winds are at least 20mph (I live in Central New York) to raise the beast.
Thanks Again
Stephen
Are you referring to a 2 liter plastic bottle with 8 winds of coax mounted just below the feedpoint - to remove common mode current from the coax?
I think so - I don't know for sure.
I have seen many specs for both diameter and number of windings. but nothing concrete.
TY
I have seen many specs for both diameter and number of windings. but nothing concrete.
TY
Ammendum 07/18/2010 (I did some editing here to correct a little bit of bad manners on my behalf).
Stephen, my apologies and welcome.
Welcome! In case you didn't notice, I am dealing with an IMAX myself here. So if your not offended by my present irritable state of mind from my months of stray RF, perhaps we could share ideas if you'd like. I'm always open to learning from others as well.
Yes Doc, I was tired & figured the wattage wrong. Your right, 15 Watts it is. Although I am wanting quite more than that.
It would be nice to have a 100 Ohm dummy load at 100 Watts or so, but how many resistors would that take with 5 Watt resistors being so scarce. In fact, wouldn't I need something like Qty 10, 10 Watt resistors to give me that sort of wattage? And you know we can't find anything more than 5 Watts.
Looks like it's back to the drawing board for me.
Also, getting back to my unanswered question, how does one go about calibrating the watt meter in my PAL KW-1 ? It shows 10 watts, but my newer meter says only 4 to 5 Watts, so I'm suspecting it's off by 5 watts. Any ideas? Should I just trash it?
Hey Gramps, Yeah, some people trim their antennas to obtain better resonance, but wouldn't that also change it from being a 5/8th wave? Actually, the Imax's are a .64 wave in reality. Isn't that due to the exact length that it is? Or does it not matter that much if your just trimming a little bit off?
Am I on the right track? or have I lost my mind? From what I gathered from your info, Some trim their verticals depending on what range of frequency they are using, To obtain the best performance, correct? Same as a dip pole antenna. Some are trimmed for 11 meters, some for 10 meters, I know that much.
As I mentioned, right now, since I've raised my tuning rings, my SWR is pretty good. 1.2:1 SWR on channel 20 & on up to 40. 1.3 to 1.5:1 channel 10 & lower. I don't think I'm going to mess with it at this point. I've seen a few of your charts on your antennas in your profile. I respect your work. I too will eventually chart my antenna. But remember now, I'm just a little Cb'r guy here now & it's going to take time.
And with my job, I'm looking at months, perhaps years.
Stephen, my apologies and welcome.
Welcome! In case you didn't notice, I am dealing with an IMAX myself here. So if your not offended by my present irritable state of mind from my months of stray RF, perhaps we could share ideas if you'd like. I'm always open to learning from others as well.
Yes Doc, I was tired & figured the wattage wrong. Your right, 15 Watts it is. Although I am wanting quite more than that.
It would be nice to have a 100 Ohm dummy load at 100 Watts or so, but how many resistors would that take with 5 Watt resistors being so scarce. In fact, wouldn't I need something like Qty 10, 10 Watt resistors to give me that sort of wattage? And you know we can't find anything more than 5 Watts.
Looks like it's back to the drawing board for me.
Also, getting back to my unanswered question, how does one go about calibrating the watt meter in my PAL KW-1 ? It shows 10 watts, but my newer meter says only 4 to 5 Watts, so I'm suspecting it's off by 5 watts. Any ideas? Should I just trash it?
Hey Gramps, Yeah, some people trim their antennas to obtain better resonance, but wouldn't that also change it from being a 5/8th wave? Actually, the Imax's are a .64 wave in reality. Isn't that due to the exact length that it is? Or does it not matter that much if your just trimming a little bit off?
Am I on the right track? or have I lost my mind? From what I gathered from your info, Some trim their verticals depending on what range of frequency they are using, To obtain the best performance, correct? Same as a dip pole antenna. Some are trimmed for 11 meters, some for 10 meters, I know that much.
As I mentioned, right now, since I've raised my tuning rings, my SWR is pretty good. 1.2:1 SWR on channel 20 & on up to 40. 1.3 to 1.5:1 channel 10 & lower. I don't think I'm going to mess with it at this point. I've seen a few of your charts on your antennas in your profile. I respect your work. I too will eventually chart my antenna. But remember now, I'm just a little Cb'r guy here now & it's going to take time.
And with my job, I'm looking at months, perhaps years.
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Sorry Stephen, perhaps it is me. Not a good week for me & several people fired where I work this week.
Say, here is a URL I came upon with a computer vertical antenna software calculator. You enter in the frequency and it gives you the length your antenna needs to be. Check it out, it may help you, it may not
Amateur Vertical Antenna Calculator by dxzone.com
James
Say, here is a URL I came upon with a computer vertical antenna software calculator. You enter in the frequency and it gives you the length your antenna needs to be. Check it out, it may help you, it may not
Amateur Vertical Antenna Calculator by dxzone.com
James
I was also wondering, if we cut our antennas to a certain length for a certain frequency, would we be somewhat restricted in being within range, or tune of other frequencies if we wanted to venture out of our band? Or would one still be within usable tolerance within a small range of frequencies?
Just a thought.
Just a thought.
B
BOOTY MONSTER
Guest
what you are refering to is bandwidth . its how wide a band of frequencys a antenna can tune to within a certian vswr range . our antennas are actually tuned to one frequency perfectly and thats usually a frequency in our band of choice , for example CB or 11 meters . as you start going above or below that perfect frerquency your vswr starts to rise . the points at the end/edge where the vswr becomes unacceptable is refered to as the bandwidth of the antenna . bandwidth can be affected by the diameter of the antenna elements and the matching network and probally other things that im not aware of .
on the cb band bandwidth isnt a issue because cb covers a small frequency span . trying to cover a wider frequency range or multiple bands with a single antenna will require some sort of manipulation of the antenna or a antenna tuner .
thats my basic understanding of it .
on the cb band bandwidth isnt a issue because cb covers a small frequency span . trying to cover a wider frequency range or multiple bands with a single antenna will require some sort of manipulation of the antenna or a antenna tuner .
thats my basic understanding of it .
Thank you Booty,
I realized that after I started doing some calculations.
So say I was tuning for CB band, 40 channels. 26.965 Mhz through 27.405 Mhz, depending on what a person was going to do, talk mainly on one channel only, then better to trim your antenna to that particular frequency.
but if using all 40 channels, trim the antenna to channel 20 or halfway between the above range. In which I figured out was approximately 27.185 Mhz, or just use channel 20 ( 27.205 Mhz ) as the halfway point.
And like Gramps was stating, (and I did get it) the Imax's manual does state that it is already pre-tuned to 27.205. But it was nice to do the calculations myself and see it come right out on the money.
The manual also goes on to state that any excess coiled coaxial cable will raise one's SWR. Well if that's the case, should I remove my homebrew RF Balun Choke? or keep it since my SWR is pretty good right now. All the better right? Since the Balun will keep stray RF from going back down my feedline.
You see, it's getting quite complicated. Everything ties into the other, so I have to be careful in whatever I do & do it one step at a time if anything. Right now, since I'm still in CB land, I think I'm sittin pretty good with my IMAX. But once I get my technician's license and start to dabble in 10 meters, I may have to trim the length, or adust my tuning rings?
Isn't that why they have tuning rings in the first place so one does not have to physically cut their nice antenna all to h_ll? lol! From what I've read, the Imax antenna is already tuned close enough for 10 meter usage with no problems also.
You know the saying, if it isn't broke, don't F_&@! with it.
BTW, Hows the Homebrew 5/8th wave working for you lately? I dug the tuning ring you fabricated. Have you worked any bugs out of it?
I realized that after I started doing some calculations.
So say I was tuning for CB band, 40 channels. 26.965 Mhz through 27.405 Mhz, depending on what a person was going to do, talk mainly on one channel only, then better to trim your antenna to that particular frequency.
but if using all 40 channels, trim the antenna to channel 20 or halfway between the above range. In which I figured out was approximately 27.185 Mhz, or just use channel 20 ( 27.205 Mhz ) as the halfway point.
And like Gramps was stating, (and I did get it) the Imax's manual does state that it is already pre-tuned to 27.205. But it was nice to do the calculations myself and see it come right out on the money.
The manual also goes on to state that any excess coiled coaxial cable will raise one's SWR. Well if that's the case, should I remove my homebrew RF Balun Choke? or keep it since my SWR is pretty good right now. All the better right? Since the Balun will keep stray RF from going back down my feedline.
You see, it's getting quite complicated. Everything ties into the other, so I have to be careful in whatever I do & do it one step at a time if anything. Right now, since I'm still in CB land, I think I'm sittin pretty good with my IMAX. But once I get my technician's license and start to dabble in 10 meters, I may have to trim the length, or adust my tuning rings?
Isn't that why they have tuning rings in the first place so one does not have to physically cut their nice antenna all to h_ll? lol! From what I've read, the Imax antenna is already tuned close enough for 10 meter usage with no problems also.
You know the saying, if it isn't broke, don't F_&@! with it.
BTW, Hows the Homebrew 5/8th wave working for you lately? I dug the tuning ring you fabricated. Have you worked any bugs out of it?
Quote: trying to cover a wider frequency range or multiple bands with a single antenna will require some sort of manipulation of the antenna or a antenna tuner .
thats my basic understanding of it .
So it's either cut our antenna to the frequency & band of our choice, or slap down a couple hundred on a decent antenna tuner/matcher. Am I correct or no?
Back in my hey day, we never had any RF balun Chokes or antenna tuners. Just my old original Avanti Astroplane, a Vanco SWR meter & my simple Pace mobile radio and it was a perfect 1.1:1 on all 23 channels. I'd kill to have that again.
J
thats my basic understanding of it .
So it's either cut our antenna to the frequency & band of our choice, or slap down a couple hundred on a decent antenna tuner/matcher. Am I correct or no?
Back in my hey day, we never had any RF balun Chokes or antenna tuners. Just my old original Avanti Astroplane, a Vanco SWR meter & my simple Pace mobile radio and it was a perfect 1.1:1 on all 23 channels. I'd kill to have that again.
J
Almost any antenna can be 'made' to work on almost any frequency. That doesn't mean that it will work well, just that it will work there. The way to go about that is to match impedances so that your transmitter will 'like' it. One easy way to do that is with a tuner. That tuner does not change the antenna's resonance frequency, it does not 're-tune the antenna. It only matches impedances. For an antenna to work well on a frequency it should be resonant there, and provide a radiation pattern that satisfies your requirements. That's how you get the 'most' out of the thing. The further away from that resonace point, the less you can expect from an antenna (any antenna). If you use SWR to determine how far an antenna is usable, it means that the thing has to be resonant at the frequency you start from. If it isn't, then that usable SWR 'range' is meaningless. Who cares if there's a real nice SWR if the antenna doesn't radiate worth a hoot? SWR deals with getting power to the antenna. Resonance deals with how well that antenna radiates. Without both, then you will always end up with less than what's possible. If that "less than possible" satisfies your particular requirement, then good. If not, then 'fix' it.
If you can't adjust for -both- resonance and impedance matching (SWR) then an antenna is not going to be very usable across a wide range of frequencies.
- 'Doc
If you can't adjust for -both- resonance and impedance matching (SWR) then an antenna is not going to be very usable across a wide range of frequencies.
- 'Doc