horizontaly mounted imax 2000

[richie]

any idea's how an imax 2000 horizontaly mounted in the apex of an attic would perform?
i know about the confined spaces, metals and electrical wire's etc., but any idea's on how it would perform horizontaly would be greatly appreciated.................regards..........richie

 

[Marconi]

It should perform just as any horizontally polarized antenna with the same basic characteristics. In the vertical it will produce an omni pattern and in the horizontal it will produce a bi-directional wave pattern in the traditional form of the figure 8.

You likely will have to mount it using some appropriate insulated hangers of some sort that will keep it fairly straight and try to run the feed line as far in the horizontal as possible also. Even so be prepared for heavy RF on the feed line of this antenna which may show up as TVI in the shack. Accoring to my testing the Imax really likes a radiating feed line that is at least 1.2 wavelenght long in the same plane as the antenna.

Can you do that before turning down the feed line to the shack? However, don't take my word for it, that is something that must be tried for sure. Choking with a balun or RF choke will likely server little to no real advantage without a good ground plane for this bugger to work against. The impedance of the feed line is just to low and attractive to the RF to keep the Common Mode Current from flowing in the coax instead of the antenna where you might like as much of the available curren you have to flow and be radiated by the antenna instead of the random lenght of the feed line.

Something to consider, but a better deal might be a simple dipole installed.

 

[Highland Ranger]

That's 40 feet of straight cable run out the bottom of the antenna?

So mounted vertically, you need to get the antenna up such that there is 40 feet from the base of the antenna to the ground?

 

[Marconi]

HR that should not be hard to do and it does not all have to be completely vertical to the vertical install. As long as the feed line remains elevated several feet above the earth the line will radiate CMC RF just fine even if horizontal part of the way. You have to be careful with what material the feed line touches to some degree here or the RF pattern can be affected some, but you will never tell that in performance. If you don't like so much RF reaching the station or maybe it is causing some RF problems, then just allow the feed line to run along or below the surface for a few feet before entering the shack or ground the shield at this point.

I think that reference about the preferred feed line length of 1.2 WL for the Imax came from modeling examples. Examples of RF on the feed line that radiated similar to the antenna itself and not straight up into the air.

In my own testing I noted that the bandwidth curves developed at different heights for this antenna also seemed to support that somewhere around the first wavelenght things worked out best. This particular antenna uses the feed line below to effectively work and that is also evident in operations if you know how to test it using a simple FS meter. The Imax produces heavy feed line currents unless something is done to discourage that from happening.

Will this make a difference in performance? I do not know, but I believe the Imax works best when high with plenty of feed line below it in almost every case.

I have tried to sit and look at antennas and understand how they work, but I fail every time, so I figure you have to test them as best you can to try and develop this understanding. It doesn't always work, but it is better than listening to the BS of CB forums.

I also test in only one location and that too can make a considerable difference. My results are not always universal in affect. I believe the composition of the earth below and near the antenna has more of an affect than we want to consider. Of course conditions and stuff around on the surface of the earth always rule when doing anything in testing an antenna too.

 

[DTB Radio]

The dipole idea, in my opinion, is the best one. There's no complicated matching needed, and they can be built out of just about any copper or aluminum tubing you have lying around. Whetehr you'llr un itno reflections in theat confined space taht will riase your SWR, I can't say for sure. I run a simple dipole, verticaly, in my attic as a 2-meter antenna, connected with alligator clips, and I can activate repeaters over 50 miles away.

 

[richie]

firstly..........thank you for your replies
one thing is puzzleing me, it has been said that the imax likes a radiating feedline.........the imax in this case will be fed by 50 ohm low loss coax........to my understanding this feedline should not radiate as its designed to cancel each other out and rf losses should be limited by quality sheilding, but dont quote me, this is just my understanding.
secondly..........as i hoped when mounted horizontally, it will perform and radiate as a dipole........the only reason i wish to try a horizontally mounted imax is the gain factor of this antenna compaired to a normal dipole..............im thinking, will it retain its gain values?????????
im also thinking of adding artificial ground along the base of the attic, this should help eliminate any rf entering the shack.
any more ideas and coments about this idea will be appreciated
thanks

 

[Master Chief]

My gut says it won't work as expected. The coax shield is actually part of the antenna system with an A-99 and iMax. At least run 9' or so in the same plane as the antenna (straight out the bottom) before making any turns.

An artificial ground as you say, may be too close to the antenna and cause problems. Try it without the artificial ground first.

The gain of the iMax comes from its low angle of radiation in the vertical plane, BY DESIGN. Laying it on it's side will not give you the desired pattern.

Experiment and come back here and let us know how it worked out for you! But, I think you are better off with a regular dipole in this situation.

I've often said that if it is such a good idea, why aren't the manufacturers doing it? Like everything else in this world, look at the professionals and see how THEY do it; it's probably the best way!

"I have this square shovel and I want to dig a round hole. Will it work?" "Sure", I say. "But why not buy a round shovel and make it easier on yourself?"

 

[richie]

ha ha.......point taken MC
after reading the posts on this topic im now ready to start experimenting with this antenna........i wish to retain some of the gain value and omni-directional properties so im going to mount it at 45 degrees.
the whole point of this idea is that ive had great success with this antenna in the past as a vertical, which is what it is obviously designed for ha ha.........i cannot erect an antenna outside my home so ive been experimenting with wires etc.
i am now looking to find the most effective antenna system which i can errect in my loft, eliminate tvi as much as possible and have no rf entering my shack..................i will keep this topic posted on my findings.........thanks to all who posted.............richie

 

[richie]

just one more thing to add..........Marconi, i have noted your comments about the feedline with this antenna and i shall monitor this closley while testing.
any more idea's from anyone regarding effective loft antennas would also be appreciated..................richie

 

[richie]

ive now worked with this antenna in the 45 degrees angle in my loft............10m low loss 50ohm coax............furthest contact was 150 miles (approx.), groundwave ssb.................reports were 4/4................switched to dipole and the contact was completley lost..........switched to vertical 5/8 firestick type with 1/4 wave groundplanes and the contact was barely readable..............so far this is the best performing antenna ive had in the loft........ even the swr was very good with quite a large bandwidth and no tvi untill 300 watts........but the downside............way too much rf in the shack @ only 50 watts, unable to use any external speech processing and my f.s. meter i monitor with went wild............obviousley i cannot continue to use this antenna as it is.........does anyone have any idea's how to cure this problem????????? ive tried all the usual things now im thinking of how to change the radiating pattern???????any idea's?....................im also going to try and use different feeder cable, ribbon etc....................regards.............richie