Just put up I-Max 2000

[Marconi]

Wonder if put it over a tin roof how high or how low above it should one go to make it act as a ground plane.

An old radio buddy put his Imax above his Dental Design Service Shop which was the last suite on the South of the strip center building about >100' feet long North and South, and 40' feet deep East to West. This may be larger than you noted above HM, but it is the only example I'm sure about. He had terrible results with his Imax to the South based on his mount being close to the end of the building and very close to the metal roof.

He set the antenna up in the center of the building about 5' feet from the South end above his shop. The antenna worked fine to the North, East, and West, but it failed miserably to the South.

The building was a flat built-up roof design with corrugated metal supporting the tar and gravel roofing material. He mounted the antenna on a heavy 2' foot high cast iron restroom vent pipe. The antenna was dead toward the south where most of the traffic was located around him in Houston at the time. He said he could hardly hear 5 miles to his buddies to the South.

He ended up moving the antenna North to the next suite about 40' feet away and it worked a little better, but was still noticeably off, so he went to a 20' mast...and then it seemed to work fine in all directions.

IMO, the Imax does not need a ground plane...so you should be OK if you get well above the tin roof.

I'll try and do an Eznec model of this latter, and post it for you to check out. Maybe it will or maybe it won't help us to see the effects I talk about above...where the very large ground plane, in such an example, tends to dominate the RF...and thus ill-effects the pattern's RF higher up than desirable.

I've done something similar to this before and the guy came back and said I was wrong...so you may find different results.

 

[Heavy Metal]

An old radio buddy put his Imax above his Dental Design Service Shop which was the last suite on the South of the strip center building about >100' feet long North and South, and 40' feet deep East to West. This may be larger than you noted above HM, but it is the only example I'm sure about. He had terrible results with his Imax to the South based on his mount being close to the end of the building and very close to the metal roof.

He set the antenna up in the center of the building about 5' feet from the South end above his shop. The antenna worked fine to the North, East, and West, but it failed miserably to the South.

The building was a flat built-up roof design with corrugated metal supporting the tar and gravel roofing material. He mounted the antenna on a heavy 2' foot high cast iron restroom vent pipe. The antenna was dead toward the south where most of the traffic was located around him in Houston at the time. He said he could hardly hear 5 miles to his buddies to the South.

He ended up moving the antenna North to the next suite about 40' feet away and it worked a little better, but was still noticeably off, so he went to a 20' mast...and then it seemed to work fine in all directions.

IMO, the Imax does not need a ground plane...so you should be OK if you get well above the tin roof.

I'll try and do an Eznec model of this latter, and post it for you to check out. Maybe it will or maybe it won't help us to see the effects I talk about above...where the very large ground plane, in such an example, tends to dominate the RF...and thus ill-effects the pattern's RF higher up than desirable.

I've done something similar to this before and the guy came back and said I was wrong...so you may find different results.

Ok was just wondering I heard others in past using the tum room mount their super penetrators ect above it and getting excellent rests was just curious if it would work on a fiberglass type one.

 

[Marconi]

HM, like I said above...your mileage my vary, if you mount your Imax over a tin roof like I think you suggested...but now I'm not sure what you meant saying "...on a fiberglass type one."

 

[doctor]

I put one up today, on a 18 foot pole out from any obstructions. I checked the swr and it is 1.5 to 1.6 on the 40 channels. What swr is anyone getting, and where did you set the adjustment, mine looks like 4 to 5 turns or about in the middle.
DOCTOR/795

 

[wavrider]

Should come factory preset for 11 meters no adjustment needed.

 

[Marconi]

I put one up today, on a 18 foot pole out from any obstructions. I checked the swr and it is 1.5 to 1.6 on the 40 channels. What swr is anyone getting, and where did you set the adjustment, mine looks like 4 to 5 turns or about in the middle.
DOCTOR/795

Doctor below is what I got with my first Imax bandwidth matching results back in 1999, at 10' feet and at 49.5' feet at the tip. I measured with my VA1 analyzer and my inline meter, both at the TX end of the feed line.

 

[Heavy Metal]

HM, like I said above...your mileage my vary, if you mount your Imax over a tin roof like I think you suggested...but now I'm not sure what you meant saying "...on a fiberglass type one."

Back in the late 70's a few would put their aluminum ground plane antennas up on tim roof and would make the tin roof act as a solid ground plane.

 

[Oldme]

Back in the 70's I had a small cottage style house with a tin roof. Placed an original Starduster on the middle of the roof on a chimney mount and let the ground wire lay across the metal roof then down to the ground. No noise or RFI problems,
Worked great.

 

[Marconi]

Back in the late 70's a few would put their aluminum ground plane antennas up on tim roof and would make the tin roof act as a solid ground plane.

HM, I've heard similar claims and I tend to question them. This idea may work fine for a 1/4 wave radiator, even though I think the larger size ground plane in such cases may have some effects, maybe good or maybe not. I have seen the effects of radials larger than a tuned 1/4 wave seem to act badly, while others are claiming 1/2 & 5/8 wave long radails work super.

Why don't you just try some of your ideas and tell us what you see.

It is alright to change the subject during a discussion, but I was curious what you meant when saying, "...on a fiberglass type one." What does that mean?

 

[Heavy Metal]

HM, I've heard similar claims and I tend to question them. This idea may work fine for a 1/4 wave radiator, even though I think the larger size ground plane in such cases may have some effects, maybe good or maybe not. I have seen the effects of radials larger than a tuned 1/4 wave seem to act badly, while others are claiming 1/2 & 5/8 wave long radails work super.

Why don't you just try some of your ideas and tell us what you see.

It is alright to change the subject during a discussion, but I was curious what you meant when saying, "...on a fiberglass type one." What does that mean?

Like Antron iMax Shakespeare ect type antennas? Some have said they act differently complainers to all metal aka iMax 2000 vs super penetrator ect...

 

[Heavy Metal]

Back in the 70's I had a small cottage style house with a tin roof. Placed an original Starduster on the middle of the roof on a chimney mount and let the ground wire lay across the metal roof then down to the ground. No noise or RFI problems,
Worked great.

What about an antennas like star duster or 4 ground radials at base and place say like yours above a tin roof? What effects happens some have said nothing some wow and other did not do anything positive note negative?

I know when I had an 8 element beam I had it dual polorized so only needed one run of coax and on back elements I put copper wire making it diamond like every inch or so around it was amazing how it did absolutely nothing to swr readings or bandwidth but anything behind almost shut hearing down to nada compared to not having the copper wire around it and also improved forward gain and hearing? Was curious if having solid tin roof would do anything and if so upwards and out or what... Just a bit curious.

 

[Marconi]

Yes you are right HM...we see stories that differ on just about any subject you want concerning antenna performance. That is why I suggested to you, "Why don't you just try some of your ideas and tell us what you see."

That is what I did many years ago.

 

[Heavy Metal]

Yes you are right HM...we see stories that differ on just about any subject you want concerning antenna performance. That is why I suggested to you, "Why don't you just try some of your ideas and tell us what you see."

That is what I did many years ago.

Oh trust me I do just cannot fork out a lot at every whim lol wish I could lol... But it's what it is and others they do this and that and together someone might have and may chime in.

 

[Captain Kilowatt]

HM, I've heard similar claims and I tend to question them. This idea may work fine for a 1/4 wave radiator, even though I think the larger size ground plane in such cases may have some effects, maybe good or maybe not. I have seen the effects of radials larger than a tuned 1/4 wave seem to act badly, while others are claiming 1/2 & 5/8 wave long radails work super.

Why don't you just try some of your ideas and tell us what you see.

It is alright to change the subject during a discussion, but I was curious what you meant when saying, "...on a fiberglass type one." What does that mean?

It has been my observation that tuned 1/4 wave radials work best if the objective is to actually alter the tuning of the antenna a bit to get a perfect SWR. If you want performance then use as long of radials as you can get. Think about how good vertical antennas work over salt water. The salt water acts as in infinitely long radial in ALL directions. I know lots of hams that have installed multiband trap verticals over a metal roof and have outstanding performance from them.

 

[Marconi]

Oh trust me I do just cannot fork out a lot at every whim lol wish I could lol... But it's what it is and others they do this and that and together someone might have and may chime in.

It has been my observation that tuned 1/4 wave radials work best if the objective is to actually alter the tuning of the antenna a bit to get a perfect SWR. If you want performance then use as long of radials as you can get. Think about how good vertical antennas work over salt water. The salt water acts as in infinitely long radial in ALL directions. I know lots of hams that have installed multiband trap verticals over a metal roof and have outstanding performance from them.

HM, IMO if you set a 1/4 wave radiator without radials directly on a metal roof using a good mount, and the roof span is near being 1/2 again as large or larger as 1/4 wave radials would present...you are going to have matching problems, like Captain Kilowatt suggest, and no amount of trimming the radiator will fix the mismatch. See the models below.

If you set a Starduster with its radials above a metal roof a few feet higher or enough to clear...I don't think it will ill-effect the antenna much so as you can tell just using your radio, but I'm not sure it will help much either. I'll bet however, if I did such a project this way...I too would likely report I saw a big difference...this is the nature of human bias.

Below are two models.

One model is a resonant tuned 102" inch 1/4 wave radiator with a suitable ground plane attached at 27.205 mhz. The model is over real Earth and 36' feet high. I also show this model in free space. Note the maximum gain and the low angle gain near the horizon for all these models. Also check the feed point match noted in the source data report as well.

The second model is also the same as #1, except I made the radial 2x the length. Compare these two models. If you will look close you will see that the longer radials have more current (red lines) flowing than the shorter radials in #1. To me this shows, among other things, these very long raised type radials are dominant over the radiator...and such results might not always be desirable.

HM, I think CK might be right about the multiband antennas he notes, but generally such radials are ground mounted, and that alone may be a difference to be considered regarding your idea.