| Author |
Message |
    
Mikefromms
Intermediate Member
Username: Mikefromms
Post Number: 154
Registered: 6-2003
| | Posted on Saturday, May 01, 2004 - 10:43 am: | 
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Has anyone replaced the top section of their Imax 2000 with a steel whip? How did it work for you? Do your really feel like this made the total antenna system stronger and able to withstand high winds better?
Mikefromms |
Marconi390 (Unregistered Guest)
Unregistered guest
| | Posted on Saturday, May 01, 2004 - 4:10 pm: |
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Mike, the SS whip is about 8% longer than the top section of the Imax. That will lower the center frequency to the lower side of 25 mHz. Is that where you want to talk? You may be right, it probably will be a bit stronger considering the wind, but you will have to cut the whip.
I am registered as Marconi, but I forgot my password. Is that against the rules?
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Tech808
Moderator
Username: Tech808
Post Number: 2269
Registered: 8-2002
| | Posted on Saturday, May 01, 2004 - 4:22 pm: |
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Marconi,
Nope Not against any Rules.
If the Forummaster reads this he may be able to find your password and send it to you again or issue a New Password for you.
Or drop him a Note to: forummaster@copperelectronics.com
Glad to see you back posting again. I always enjoyed reading your helpful post's. 
Hope this help's.
Lon
Tech808 |
Yankee
Intermediate Member
Username: Yankee
Post Number: 113
Registered: 7-2003
| | Posted on Saturday, May 01, 2004 - 9:19 pm: |
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Wonder what a 102 inch fiber glass whip that I have would do over all on the top of the I-Max. I'm thinking of using it for HF ham bands, I know of a fellow ham using one with a tuner from 10 meter to 20 meter, Although the I-Max is listed as a 10 meter antenna in the amateur radio catalogs. Yankee |
Tech808
Moderator
Username: Tech808
Post Number: 2275
Registered: 8-2002
| | Posted on Saturday, May 01, 2004 - 9:35 pm: |
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Mikefromms,
Run a search on Tech833,
I am sure he answered this Exact Question within the last month posted by someone else.
If not maybe he will see this post.
Hope this help's.
Lon
Tech808 |
Kc0gxz
Advanced Member
Username: Kc0gxz
Post Number: 784
Registered: 3-2003
| | Posted on Sunday, May 02, 2004 - 3:33 pm: |
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Yankee
It will work.
Jeff, kc0gxz. |
Cbblackbeard
Member
Username: Cbblackbeard
Post Number: 52
Registered: 12-2003
| | Posted on Monday, May 03, 2004 - 8:49 pm: |
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What about using a Francis fiberglass whip to replace the top section of the IMAX? |
Mikefromms
Intermediate Member
Username: Mikefromms
Post Number: 157
Registered: 6-2003
| | Posted on Tuesday, May 04, 2004 - 11:06 am: |
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Would the 102" whip have to be cut to get cb and 10 meter frequencies resonant?
mikefromms |
Tech808
Moderator
Username: Tech808
Post Number: 2310
Registered: 8-2002
| | Posted on Tuesday, May 04, 2004 - 11:12 am: |
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Mikefromms,
The IMAX 2000 has Tunning Ring's to Adjust for the Frequencies / Coverage you want to use.
Lon
Tech808 |
Mikefromms
Intermediate Member
Username: Mikefromms
Post Number: 159
Registered: 6-2003
| | Posted on Tuesday, May 04, 2004 - 5:06 pm: |
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I forgot about that. So if a whip changed it's center frequency then you just retune the antenna with the rings. I may just get one to play around with now.
mikefromms |
Tech833
Moderator
Username: Tech833
Post Number: 655
Registered: 8-2002
| | Posted on Tuesday, May 04, 2004 - 10:07 pm: |
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Negative on the Francis whip.
Positive on the 102 inch whip.
Adding length via the top section, then detuning it with the rings will work, but will also narrow the bandwidth. |
Tech548
Moderator
Username: Tech548
Post Number: 4
Registered: 5-2004
| | Posted on Wednesday, May 05, 2004 - 6:03 am: |
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Tech833. I have one for you.
If you cut 6 inches off the 102 SS whip, that will bring it down to the same 8 foot level that the original was. By doing this, will the band width still narrow up some or will "everything" be back to normal with this shortened 102?
Jeff. |
Tech833
Moderator
Username: Tech833
Post Number: 670
Registered: 8-2002
| | Posted on Wednesday, May 05, 2004 - 10:05 am: |
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Back to normal. |
Marconi
Intermediate Member
Username: Marconi
Post Number: 294
Registered: 11-2001
| | Posted on Wednesday, May 05, 2004 - 11:51 pm: |
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Mike, a few days ago I posted something about your idea of using a 1/4 wave SS whip in the top of an Imax. I cautioned about lowering center frequency if you did so. I got to wondering about that so I remeasured the top section and it is 95" which is 7 1/2" shorter than the whips I have around. I built a little simple gig with a ground plane using two whips screwed into a threaded 5" spacer. I attached this GP to the mirrow bracket with vice-grips. I checked the Imax tip and a 1/4 wave that was 102 1/2" long.
The 1/4 wave was resonant at 27.250 and the Imax tip was resonant at 26.940. That surprised me. As things generally go I was expecting the Imax tip to show to be somewhat higher in frequency since it was shorter. This tells me that the tip is loaded a bit, similar to the shorter Francis type 1/4 antennas. That may account for why it is so limber in the air.
So if you use a SS whip in an Imax instead of the tip, you may find the center frequency has in fact gone up instead of down. My error.
I will be testing this idea of yours in the next few days on an Imax and will get back. If the Imax responds as the A99's that I have checked in the past did, then I believe I will find that moving the rings up and down probably will have only a minimal affect on the true resonance. To my thinking this coil seems to have a sweet spot in the middle and thus it presents a good control to the match over a rather wide frequency range.
As we move the rings away from the center the antenna no longer seems to respond as effectively as it did when rings are set to the middle. Therefore I personally believe the rings have little or no measurable affect on the true resonance. |
Tech833
Moderator
Username: Tech833
Post Number: 673
Registered: 8-2002
| | Posted on Thursday, May 06, 2004 - 11:50 am: |
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Marconi,
The reason for the physically shorter fiberglass whip looking 'longer' to your signal than the SS whip is due to velocity factor.
The fiberglass coating on the radiator slows the RF down as it travels over the skin of the radiating element. In the SS whip, the velocity factor is nearly 100% because it is in air. The fiberglass may be around 95% or so.
The rings change the reactance at a factor of nearly 3:1 coupled with feedpoint resistance. The length of the antenna element changes the resistance at a factor of approx 2:1. If you raise the resistance by shortening the antenna and try to compensate by retuning with the rings, you will lose bandwidth. |
Marconi
Intermediate Member
Username: Marconi
Post Number: 295
Registered: 11-2001
| | Posted on Thursday, May 06, 2004 - 2:16 pm: |
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Well Mike, I setup the Imax on the jig at about 11" above earth on a 20' pushup mast. I installed the original tip first, then a 102 1/2" SS whip, and then an old Francis Wheeler Dealer that I believe was called a 3/4 wave mobile whip in those days. It was cut for the higher end of 11 meters so it had been modified a little.
The following were the results. I did not test for resonance in this chart. I did do some resonance checking at the end with the Francis whip installed while moving the rings about from top to middle to bottom.
Original Imax tip
Freq---Meter---X----R----SWR
21.000--1.6---29---54---1.74
25.000--1.3---18---59---1.47
28.000--1.4---12---40---1.40
29.000--2.5---58--103---2.83
SS whip
21.000--1.6---28---54---1.70
25.000--1.3---18---59---1.44
28.000--1.3---13---45---1.31
29.000--1.9---48---97---2.51
Francis
21.000--1.6---26---49---1.68
25.000--1.2----8---58---1.22
28.000--1.4---12---40---1.40
29.000--2.1---55---98---2.73
Moved the feed point to 20' and nothing remarkable happened with the readings. Isolation at 11' was adaquate for operations.
Moved the rings to the bottom and scanned for resonance. The results at the antenna feed point using analyzer were: 26.850 X=0 - R=50 - SWR=1.00
Input 100 watts into antenna thru other end of 50' RG 213 and it showed basically flat SWR for all of 11 meters and more. I did not plot much here due to time.
I move the rings to the top and could not find good numbers using analyzer. Looks like the sweet spot is in 11 meters and resonance favors the lower side as well. I saw similar results to those noticed when testing the A99.
Looks to me like just about any stinger that is resonant near 11 meters will work up there.
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Mikefromms
Intermediate Member
Username: Mikefromms
Post Number: 160
Registered: 6-2003
| | Posted on Thursday, May 06, 2004 - 6:09 pm: |
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Interesting test! You mean the Imax can be used without a tuner on 21 mhz? 29 mhz? Wow. Now, would the steel whip stay up there longer without breakage to the antenna? I can't get over the broadbandedness of the antenna. That is incredible.
Thanks for taking the time to do the test.
mikefromms |
Creator
Member
Username: Creator
Post Number: 86
Registered: 1-2004
| | Posted on Thursday, May 06, 2004 - 8:48 pm: |
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Does any manufacturer make an aluminum whip? Less lightning prone.. |
Marconi
Intermediate Member
Username: Marconi
Post Number: 296
Registered: 11-2001
| | Posted on Thursday, May 06, 2004 - 10:47 pm: |
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I don't know about the life expectations thing, but I would not have any problem talking on the low end of the band. The high end has a much sharper curve and I was not able to find a good match up there so I would be a little cautious going up. I am going to try some shorter elements and see if the upper area can be made better in doing so. I will keep in touch.
BTW, I ran a similar test using my A99 at about 70' and believe it or not it showed an almost similar response band for band. I have not scanned it with the analyzer yet, but maybe I will do that also before I tear down the jig. I also pushed the Imax up to about 30' and noting receive signals tonight, both the Imax and the A99 are showing the same signal, station to station. If I had them reversed I am confident that I would see about the same. |
Tech548
Moderator
Username: Tech548
Post Number: 23
Registered: 5-2004
| | Posted on Friday, May 07, 2004 - 10:12 pm: |
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Creator
An aluminum whip is still a metal antenna. ANY metal in the air is lightning prone. Even the metal couplings on the I-Max and A-99 are lightning prone.
I have never lost a fiberglass antenna do to the common wind storms we get here in Nebraska of 50 to 70 and more MPH. What glass antennas I have lost was always due to lightning strikes at the couplings.
Jeff. |
Creator
Member
Username: Creator
Post Number: 87
Registered: 1-2004
| | Posted on Saturday, May 08, 2004 - 12:34 am: |
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Re:What glass antennas I have lost was always due to lightning strikes at the couplings.
Tech548,
What if you coated the metal couplings with Plasti Dip, its the liquid rubber you would use to coat a pair of plyers that did'nt have a rubber coating on the handles (like to use for electrical work).. Would'nt that stop the contuctivity in the air? Obviously the lightning isnt hitting the fiber glass so the next best thing is to make contact with the metal couplings, but what if there were Plasti Dip protecting them..... Hmmmmmm.....
What do you think?
Creator |
Tech833
Moderator
Username: Tech833
Post Number: 679
Registered: 8-2002
| | Posted on Saturday, May 08, 2004 - 9:38 am: |
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I use heat shrink tubing. Not only do you insulate the metal fittings, but you also help keep them from unscrewing themselves.
West Marine sells various sizes of heat shrink tubing that also has hot melt glue inside. The glue seals the coupling better than silicone. Just don't ever plan on disassembling the antenna again. |
