| Author |
Message |
    
Slim1
| | Posted on Wednesday, October 02, 2002 - 12:04 am: | 
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i am installing my imax 2000 above my tv ant.
tip of tower is at 38'. what would your suggestions be for distance between tv ant and
the imax.(no groudplane for the imax either)
i thought maybe the tv ant would help the SWR
and act as a groundplane for the imax if i kept them 2'-3' apart or so? or should i just push the imax up as far as i can and put the tv ant at the tip of the tower? oh,it'll be mounted to a 20'
mast pipe.(galvanized water pipe 1 1/4)thanks all
SLIM CDX285 |
Swamper
| | Posted on Wednesday, October 02, 2002 - 11:25 am: |
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hmm i didnt think anyone used the old tv antennas anymore.but if i were you id get your i max 2000 as far away from your tv antenna as possible. |
Tech833
| | Posted on Wednesday, October 02, 2002 - 12:21 pm: |
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If the T.V. antenna doesn't have one of those 'corner reflectors' for the the UHF elements (it is completely flat) then mount the Imax 2000 as close to the T.V. antenna boom as you can. Yes, the T.V. antenna's boom will act like a ground plane and may possibly improve the performance. |
Scrapiron63
| | Posted on Wednesday, October 02, 2002 - 2:06 pm: |
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Speaking of radials on ground plane antennas, one of my old antenna books from the early 70s, written by David Wells, has an article about adding another set of radials for "increasing performance" in his words. Of course back then all the groundplanes were metal, and most had radials. His instructions were to add another set on the mast pipe or tower, the distant from the other set being the same as the driven element of the groundplane. I never tried adding radials, but I did mount a hygain penatrator on top of my metal shop, and made sure the distant from the radials to the metal top was the same as the driven element. It was a real talker, but I never tried it any other way, so I had no comparison. I had it in the middle of a 50 X 60 metal roof, lots of groundplane there. |
Slim1
| | Posted on Tuesday, October 08, 2002 - 11:46 pm: |
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well i got her up last saturday afternoon.i put
the imax about 3' above the channel master tv ant
and then about 3' down from there to the tip of
the tower.also,i did the little trick of 8-9 coil winds just below the imax connector w/the coax.
came out pretty good.running barefoot,i get so
little interference it's hardly worth the mention!
i cut 4" off the top section and put the tuning
rings all the way to the top and got these SWR's.
23 mhz-1.1-1
26 mhz-1.6-1
27 mhz-1.1-1
29 mhz-1.5-1
30 mhz-1.7-1
all tests @ 10-watts AM.
i think i kinda defeated my purpose cutting off
the 4" since i had to tune rings up so high but
oh well. it worked out well i think! the worst thing is there are no locals on around here anymore for me to make a comparison to the A-99's
readings. oops,alsmost forgot;from the ground to bottom of the imax is approx.45'. just thought
ya'll would like to know the results.thanks for the input too guys!!catch ya on the airwaves!!!
from the land of the worthless nut..SLIM CDX285 |
Marconi
| | Posted on Wednesday, October 09, 2002 - 11:36 am: |
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833, considering that Slim had to raise the rings to the top after cutting the tip, what do you think that would or should have done for the match and the bandwidth curve?
From my simple investigation of the insides of an A99, I concluded that this end fed matcher was actually a sweet spot device and not specifically made for tuning. I noted that moving the rings was not really effective at moving resonance, unless you cut the tip for the ten meter area and then, as the doc's state, move the rings to the top of the coil. I figured this simply moved the rings to another sweet spot on the coil that rematched the change. I seem to recall that the matcher did control the imput impedance near 50 ohms pretty well all across the range of ring adjustment, but as you moved away from the center the reactance began to increase substantially. It was a while ago and I did not look back at my notes, but that is what I recall.
Even in the A99, the bandwidth is such that moving the rings seems to be relatively inaffective at making positive resonance changes that you can see using an SWR meter. I think Solorcon more or less states this in their section of the docs covering the subject when they tell you not to move the rings.
Slim, I believe your report confirms other reports about the Imax being very broadbanded. Specifically, what were you thinking when you decided to cut off 4" of the tip?
Marconi |
Tech833
| | Posted on Wednesday, October 09, 2002 - 10:46 pm: |
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Marconi,
Good question. From what I can tell about the matching scheme, I don't think he really harmed his bandwidth or gain any. All he did was shorten the radiator and then re-lengthen it (sorta).
He just cancelled his chances of using the antenna at 20 MHz. is all. |
Slim1
| | Posted on Wednesday, October 09, 2002 - 11:58 pm: |
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well,i cut off the 4" in anticipation of getting
a better SWR at bottom the 10-meter band along
with a better SWR at the top of the 11-meter band.
in actuality i should have done it by just mov-
ing the rings i believe.i'm not real fond of heights and didn't have a way of experimenting
without being too close to the ground(affecting
SWR).so i went by the owners manual and it said cut-off 4" for 28.016Mhz. i figured it would be
close enough at that but it was a little high(SWR)
at my goal freq. of 27.475 so i moved the rings up and brought the SWR back down at that freq.
probably wasn't worth the effort initially as i
said before but i wanted to try it. i still have
the original top section "uncut" just in case i
should decide to put it back on the ant.
i am really amazed at how broadbanded this ant
is and i am very happy w/it. my biggest concern is
with it breaking under windy conditions. my
location is kinda "down in a holler" so to speak
so i don't usually get too strong of winds,thank-fully! but that also hurts my antenna systems
performance-tv and cb and whatever else i use.
enough rambling from me guys,c-ya SLIM CDX285 |
Taz
| | Posted on Thursday, October 10, 2002 - 10:56 am: |
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My imax has stood up in 70+mph winds. Some even gusting to 100mhp over here in phoenix. The monsoons are brutal. |
Alsworld
| | Posted on Thursday, October 10, 2002 - 2:27 pm: |
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Slim1,
in the past month I have gone through two tropical storms. At the base of my antenna, i easily see over the neighbors houses, across the intercoastal waterway, barrier island into the Gulf of Mexico.
The bottom line is my antenna has no protection from the wind when these blow through.
I took it down after both cleared and thoroughly inspected it, not a problem. The last one was rough whipping it back and forth like a fly rod. The IMAX's are really flexible, but over time will truly tell.
I think you'll be okay.
Alsworld |
Marconi
| | Posted on Thursday, October 10, 2002 - 6:35 pm: |
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Well Slim, I understand what provoked you to cut the top of the Imax, but wasn't that old Imax telling you "...I can make it to 28.000 from here easy, it's just a hop, skip, and a jump. Just about everyone who as spoken on this subject and having an Imax, has told us they are almost flat from 23.000 to 32.000. Your own results probably suggested that your goal was almost there without cutting.
Glad you still have a stock top element. I guess it is too late now, because you have it installed. But if you ever take it down, run a bandwidth curve on it the way it is and then put the rings in the middle and see what it does for the curve.
It may be different from my work with an A99 but the old 99 made a very uneaven curve with the rings at the top and the bottom. It was nice an smooth with the rings in the middle. I already mentioned what my analyzer showed.
Have fun, it should give you good service if the wind doesn't get you. If that old Moonraker can take it, I'll bet the Imax can too.
Taz is brutal monsoon testing his Imax for Solarcon as we speak, and I bet you thought he was in Arizona. Taz, you're a mess.
Marconi |
Taz
| | Posted on Thursday, October 10, 2002 - 9:26 pm: |
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haha, im no mess im just messed with. |
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